Nuclear Instruments and Methods in Physics Research A

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1 Nuclear Instruments and Methods in Physics Research A 635 (211) Contents lists available at ScienceDirect Nuclear Instruments and Methods in Physics Research A journal homepage: Gas jet studies towards an optimization of the IGISOL LIST method M. Reponen a, I.D. Moore a,, I. Pohjalainen a, T. Kessler a,1, P. Karvonen a, J. Kurpeta b, B. Marsh c, S. Piszczek b, V. Sonnenschein a,j.äystö a a Department of Physics, University of Jyväskylä, P.O. Box 35 (YFL), FI-414 Jyväskylä, Finland b Faculty of Physics, University of Warsaw, ul.hoża 69, -681 Warsaw, Poland c CERN, CH-1211 Geneve 23, Switzerland article info Article history: Received 2 August 21 Received in revised form 24 January 211 Accepted 24 January 211 Available online 11 February 211 Keywords: Gas cell Gas jet Nozzle LIST abstract Gas jets emitted from an ion guide have been studied as a function of nozzle type and gas cell-tobackground pressure ratio in order to obtain a low divergent, uniform jet over a distance of several cm. The jet has been probed by imaging the light emitted from excited argon or helium gas atoms. For a simple exit hole or converging-diverging nozzle, the jet diameter was found to be insensitive to the nozzle shape and inlet pressure. Sonic jets with a FWHM below 6 mm were achieved with a background pressure larger than 1 mbar in the expansion chamber. The measurements are supported by the detection of radioactive 219 Rn recoils from an alpha recoil source mounted within the gas cell. A Laval nozzle produced a well-collimated supersonic jet at low background pressures with a FWHM of 6 mm over a distance of 14 cm. Direct Pitot probe measurements, on-axis, revealed a non-uniform pressure distribution in the gas jet of the Laval nozzle, supporting the visual observations. All measurements are motivated by the requirement of a good geometrical overlap between atoms and counter-propagating laser beams in the gas cell-based Laser Ion Source Trap (LIST) project. Computational fluid dynamics gas flow simulations were initiated to guide the future development of the gas jet system. & 211 Elsevier B.V. All rights reserved. 1. Introduction The helium-jet transport system has been used as a mechanism for the extraction of radioactive nuclides since the 196s and has developed over the years to become a standard technique at several facilities [1 3]. Later, the conventional ion source could be eliminated in the development of the helium-jet ion guide technique [4,5]. Of particular note was the discovery of the favourable charge-state distribution brought about by the thermalisation process within the gas, with a large fraction of nuclear reaction products settling in a 1 + charge state [6]. This characteristic formed the basis of the Ion Guide Isotope Separator On-Line (IGISOL) principle [7], allowing sub-millisecond conversion of reaction products into a low energy radioactive ion beam. In the IGISOL technique reaction recoils are stopped in a buffer gas chamber filled with helium or argon at a pressure of typically a few hundred mbar. The thermalized reaction products are guided by the gas flow towards an exit hole with a diameter of approximately 1 mm. The flow within the buffer gas cell has been Corresponding author. Tel.: ; fax: address: iain.d.moore@jyu.fi (I.D. Moore). 1 Present address: Physikalisch-technische Bundesanstalt, Bundesallee 1, D Braunschweig, Germany. studied both experimentally and by simulation [8 1] and is well understood. Upon exit from the gas cell the pressure drops by orders of magnitude and the gas flow becomes either sonic or supersonic and is highly expansive. Although the theory of supersonic flow required to describe the gas jet downstream from the exit hole is generally well-established [11,12], the computational simulation of supersonic flow at boundaries is still an active field of research [13,14]. Considerable effort has been made towards creating mathematical models and computer codes for the development of internal supersonic jet targets which utilize novel nozzle designs and show considerable promise for a variety of experiments at storage rings [15,16]. Additionally, an exploratory study into the visualization of expanding gas flow from helium-jet and ion guide nozzles has been conducted by Rasi et al. [17]. In the traditional IGISOL approach a skimmer system was used to separate the ions of interest from the neutral buffer gas environment. The resultant ion trajectories are governed by a combination of the electrostatic skimmer potential and gas flow. More recently, multipole structures replacing the skimmer device have successfully improved the transmission efficiency and beam emittance of the gas cell-based mass separator system [18 2]. The development of the radiofrequency (rf) sextupole ion beam guide (SPIG) at the Accelerator Laboratory of the University of /$ - see front matter & 211 Elsevier B.V. All rights reserved. doi:1.116/j.nima

2 M. Reponen et al. / Nuclear Instruments and Methods in Physics Research A 635 (211) Jyväskylä (JYFL) was motivated by the so-called Laser Ion Source Trap (LIST) project [21]. This is a variant of the Ion Guide Laser Ion Source concept, IGLIS [22,23], where pulsed lasers are used to selectively ionize neutral atoms within the gas cell volume [24,25]. Such a principle is motivated by the lack of Z-selectivity inherent in the standard ion guide technique. The LIST was originally proposed to improve the beam quality from a hot cavity laser ion source [26] by decoupling the hot production and evaporation regions from the ionization volume. It has since been adapted for coupling to a gas catcher system [27]. In this latter approach, the reaction products are allowed to neutralize within the gas cell and upon exit they are selectively re-ionized with counter-propagating lasers within the supersonic gas jet. The photo-ions are captured by the RF-field of the SPIG located immediately after the gas cell and are transported to the mass separator by a voltage gradient. A positive DC voltage applied to the first electrode of the SPIG acts to repel any non-neutral fraction from entering the RF guide, ensuring the highest possible beam purity is maintained for subsequent experiments. There are two parameters which determine the overlap efficiency between the atoms and the laser photons. The first is related to the duty cycle of the laser system. In order that each atom has the chance to interact with the laser beam at least once over a collimated interaction length of 1 cm, the needed repetition rate is 1 khz for a jet velocity of 1 m/s. The second factor, the geometrical overlap, is directly affected by the jet collimation and can be optimized using a special nozzle design or specific background pressure outside the gas cell. In the present work, we investigate the gas jet with different background pressure conditions and exit nozzle shapes. In a similar manner as Rasi et al. [17] we utilize an arc discharge source to ionize and excite helium and argon buffer gas atoms. In the process of relaxation, afterglow light is emitted which can be seen by the naked eye downstream from the nozzle. In order to support the visual observations of the jet we have utilized the transport and detection of recoils from a 223 Ra source. In addition to these qualitative measurements we have used thin pressure probes made from hypodermic needles in order to quantitatively explore the stagnation and static pressure distributions as a function of distance, on-axis, from the exit hole of the gas cell. Finally, the measurements are supported by gas jet simulations performed at the University of Warsaw which are summarized in this article. 2. Theory of supersonic gas flow The properties of the gas jet after the ion guide depend strongly on the shape of the nozzle and the pressure boundaries. The theory associated with gas jet expansion is treated in detail in Refs. [11,3]; however, we provide a short introduction here for clarity. The main features of the transition from subsonic to supersonic flow in a converging-diverging nozzle can be described in a quasi-one-dimensional symmetry, assuming a constant velocity perpendicular to the symmetry axis of the boundaries. In the case of an adiabatic flow, the change in velocity in terms of a Mach number M is described by the so-called area-velocity relation [11], da A ¼ðM2 1Þ du ð1þ u where A is the cross-sectional area of the enclosing structure and u the gas velocity. Eq. (1) states that the velocity of the gas is completely determined by the boundary surface. A direct result of this relationship is that the speed of sound (M¼1) can only be reached in the throat of the nozzle where da¼. Inside the ion guide, the gas accelerates as the area is decreased until a velocity of Mach 1 is reached at the minimal radius. After the throat, the gas accelerates during the expansion phase as da4 and a supersonic flow is attained. For a calorically perfect gas, Eq. (1) can be solved explicitly to obtain the area-mach number relation: A 2 ¼ 1 2 g 1 ðg þ 1Þ=ðg 1Þ 1þ A n M 2 gþ1 2 M2 ð2þ where A n oa is the area of the nozzle throat and g the ratio of specific heats (5/3 for monoatomic gases such as helium and argon). For every value of A4A n, two solutions of M fulfil Eq. (2), the subsonic and the supersonic solution. A visualization of the Mach number as a function of area ratio can be seen in Fig. 1. Once the variation in the Mach number through a nozzle is known, parameters such as the temperature, density and pressure can be calculated for a calorically perfect isentropic gas. The pressure in the jet P is calculated using the relationship P ¼ 1þ g 1 g=ðg 1Þ P in 2 M2 ð3þ where P in is the static pressure for M¼, in other words, the ion guide pressure. In a similar manner, the density and temperature can also be calculated [11]. Eqs. (2) and (3) can be combined to obtain an expression for the pressure ratio as a function of the area ratio, illustrated in Fig. 2. The behaviour of the isentropic solution for a supersonic jet is usually referred to as the design condition. At the nozzle throat a value of M¼1 is reached and Eq. (3) reduces to P 2 g=ðg 1Þ P ¼ : ð4þ in throat gþ1 For monoatomic gases such as He and Ar a value of :49 is reached at the throat. In reality, the gas jet does not follow the isentropic design solution as it is forced to a fixed pressure boundary at the nozzle exit given by the pumping capacity of the vacuum system. As an example of the dependence of the gas behaviour under different boundary conditions one may consider a standard IGISOL ion guide with a simple converging-diverging nozzle. If no pressure difference is applied between the gas cell and the vacuum Machnumber M Supersonic flow Nozzle throat A/A n Subsonic flow 1 1 Fig. 1. Velocity in the transition from a subsonic to a supersonic gas jet as a function of the area ratio.

3 26 M. Reponen et al. / Nuclear Instruments and Methods in Physics Research A 635 (211) P/P in condition leading to a spatial compression of the jet. In this case the nozzle is said to be overexpanded. Finally, if the pressure P bg is reduced to below the isentropic value at the exit, P bg ¼ P under o P design, the jet can only reduce its pressure by forming an expansion wave. In this instance the nozzle is said to be underexpanded. In both overexpanded and underexpanded nozzle configurations complex shock patterns occur downstream from the nozzle which are characteristic for the reflection of the jet on the free pressure boundary A/A n Fig. 2. Pressure ratio as a function of area ratio in the transition from a subsonic to a supersonic flow regime. Fig. 3. Behaviour of the gas pressure ratio in a shaped nozzle for different background pressures. chamber no flow occurs. Only when the background pressure P bg in the IGISOL chamber is decreased to a value of P bg =P in o:49 a supersonic flow condition is obtained. A mismatch between the pressure boundary at the nozzle exit and the pressure of the jet is compensated by the formation of shock structures in the jet. The impact of the background pressure on the behaviour of the gas jet exiting a converging-diverging nozzle is depicted in Fig. 3. The subsonic and supersonic isentropic design solutions are indicated as guidelines. Let us first consider the situation for a background pressure P bg ¼P 1. As P 1 o:49, the nozzle is choked, however, the pressure at the exit is higher than the pressure P design calculated from the isentropic solution for the freely expanding jet. The only way to fulfil the required higher pressure boundary is to form a normal shock within the nozzle as indicated by the vertical line in Fig. 3. Following the shock an immediate increase in pressure is seen in the nozzle. The flow becomes subsonic and the pressure continuously increases during the expansion phase to converge to the given pressure boundary P 1 at the exit. By decreasing the background pressure P bg further, the shock disc moves downstream towards the exit of the nozzle. At a pressure of P bg ¼P 2, the shock is positioned at the nozzle exit and the jet is supersonic throughout the whole volume of the nozzle. If the background pressure is reduced further to a value P design op bg ¼ P over op 2, an oblique shock occurs outside the nozzle area which increases the pressure towards the boundary 3. Experimental studies of gas jets In the 199s, an exploratory study into the nature of gas flow and expansion outside a gas cell was performed by Rasi et al. [17]. This comprises visual observations made possible through excitation, by electrical discharge, of the gas within a chamber. The faint fluorescence visible from the gas following the excitation is a result of relaxation, finally to the ground state. For metastable states, this relaxation is of the order of several milliseconds and so, at typical gas cell vacuum chamber pressure differentials, allows imaging of the gas jet far into the expansion region. The gas-jet photographs obtained allowed qualitative information about the complete jet expansion and identification of the distinct shock-wave structures described by the fluid dynamics of a freely expanding gas. Additionally, observations were made with gas flow obstructions, in the form of circular discs with a central aperture, placed downstream of the exit nozzle. This was intended to simulate the interaction of the gas jet with a skimmer electrode, usually placed a few cm from the ion guide. The study concluded that for large apertures, the gas flow remains largely unperturbed. However, as the aperture diameter is reduced to less than the visible gas-jet width, a shock wave is observed and the gas flow after the aperture becomes more diffusive and expansive. The work of Rasi et al. was conducted considering the IGISOL method to be used as an ion guide technique, whereby the ion trajectories after the gas cell are primarily governed by electric fields. The properties of the gas flow, including shock waves, were not considered to be of importance. The present work, however, is motivated by the development of the laser ion source and the use of the gas cell as an atom guide. Furthermore, the local environment within the jet is highly attractive for in-jet laser spectroscopy, with detrimental effects to the atomic linewidth due to temperature and pressure broadening substantially reduced [27]. We have revisited the method of Rasi et al. and have used the gas jet fluorescence as a means to probe the effect on the jet as a function of nozzle design, gas cell pressure and variation in the background extraction chamber pressure. It should be noted that in all experiments the ratio of P bg =P in o:49, and therefore, we are within the regime of choked flow conditions. All studies discussed in this article have been performed in a vacuum chamber designed for a new ion-guide quadrupole mass spectrometer system. This provides more flexibility and ease of use than the IGISOL target chamber Gas cell exit nozzle shapes Initially, two main types of nozzle design have been compared, a converging-diverging nozzle and a simple exit hole. The exit hole is typical for all ion guides in use at IGISOL and cannot be shaped. However, the converging-diverging nozzle can be shaped to match the expected pressure conditions between the gas cell and the extraction vacuum chamber such that it operates at the design condition. In reality we have been limited to practical designs which can be produced by the technical workshop. In the IGISOL technique one critical parameter is the size of the exit hole

4 M. Reponen et al. / Nuclear Instruments and Methods in Physics Research A 635 (211) or in a shaped nozzle, the throat diameter, which determines the conductance of the gas cell. The typical exit hole diameter used is 1.2 mm, however, when operating the gas cell under conditions suitable for the laser ion source the nozzle diameter is a factor of two smaller. We have used the solutions to Eqs. (2) and (3) as a means of guiding our choice of nozzle sizes. When the quadrupole mass spectrometer chamber was first being constructed, the pumping capacity was rather limited and a typical pressure ratio between the ion guide inlet and extraction chamber was 1%. Eq. (3) is solved at this pressure ratio to determine that a Mach number of 4 is required to satisfy the design condition at the exit of the nozzle. With the design Mach number, Eq. (2) is then solved indicating that the ratio of the nozzle exit area to the throat area should be 5:6, thus a diameter ratio of 2:4. The convergingdiverging nozzle tested in this work had a throat diameter of 1.4 mm and an outlet diameter of 4 mm, reasonably close to the design condition requirements set by the earlier pumping capacity (for which the nozzle had been designed). More recently, with a larger pumping capacity available for the studies described in this work, the corresponding pressure ratio has decreased to :2%. Fig. 2 illustrates that such a decrease in the background pressure can be compensated by an increase in the diameter of the exit of the nozzle in order to maintain the design condition. As mentioned previously, the design pressure condition for a standard exit hole is.49 (under the assumption that the throat diameter is equivalent to the exit hole diameter). With our experimental pressure conditions the jet following the exit hole is consequently highly underexpanded, and an expansion wave is formed. Although this condition is not preferred, as an exit hole is in typical use at the IGISOL facility we have studied this design with two different diameters,.7 and 1.34 mm. Both nozzle types are illustrated with appropriate dimensions in Fig Experimental set-up and gas jet photography Fig. 5 illustrates, among other details, a schematic of the gas cell used for the gas-jet observations. Helium or argon gas, regulated using a needle valve, was injected into the inlet of the gas cell. A Leybold capacitive pressure sensor (model DI2) attached to the gas line was used to measure the inlet gas pressure. The extraction vacuum chamber was pumped with one Roots blower with a specified pumping speed of 4 m 3 /h. A Balzers Pirani gauge (model TPR 1), positioned on one side flange of the chamber approximately 2 cm from the gas jet region, allowed monitoring of the background pressure. In practice, the minimum pressure achieved within the extraction chamber (without gas injection) was of the order of 1-3 mbar. The background pressure could be adjusted in a crude manner by manually closing the valve to the roots pumping line. The gas cell and extraction region were visible from the side of the vacuum chamber through a glass window (see photograph in Fig. 6). A single electrode inside the gas cell was connected to a 5 ma current-limited DC power supply. To initiate the discharge, a voltage in excess of 3 V was required. With an increase to 7 V, the gas emerging from the nozzle exhibited a glow easily visible to the naked eye. In off-line experiments at IGISOL, spark discharge source conditions are rather sensitive to the ion guide pressure. In this work a variety of gas cell pressures were used depending on the stability of the discharging electrode. The excited states and the reactions in which they are populated and subsequently quenched are difficult to determine without spectroscopic measurements of the afterglow, which is beyond the scope of this work. For example, the population of metastable states can occur via inelastic scattering of electrons, ion atom impact excitation, or in electron ion recombination. The relaxation of states can happen by a number of reactions, whose rates Spark electrode Sidetector Gasin 1.34 or.7 4, 1,4 6 mm Fig. 5. Schematic set-up of the gas cell for use with either a spark electrode or a 223 Ra a-recoil source. The moveable silicon detector is used to count the alpha recoil products. 1,9 2, 6, Fig. 4. Schematic drawings of the standard exit hole (A) and converging-diverging nozzle (B) tested in this work. All dimensions are in mm. The gas cell would be attached to the right-hand side of nozzle B in this figure. Fig. 6. Photograph of a gas jet measurement set-up. The gas cell is situated on the right and the perspex SPIG structure on the left. The distance between the gas cell and the SPIG structure has been arbitrarily chosen in this picture. The nozzle of the gas cell is interchangeable.

5 28 M. Reponen et al. / Nuclear Instruments and Methods in Physics Research A 635 (211) can be affected by impurities in the gas. In the analysis of the gas jet fluorescence detailed in the following section we, therefore, neglect possible quenching of excited states. For more details regarding the production and relaxation of metastable states we refer to Refs. [28,29]. The gas jet was photographed using a Casio Exilim Ex-F1 PRO digital camera in manual mode with the sensitivity set to ISO 1, an aperture of F7.5 and an exposure time of 1 s. The low sensitivity and relatively long exposure time, combined with the camera mounted on a fixed support ( 2 cm from the gas jet), resulted in low noise photographs. The photographs, of 6 Megapixel resolution, were saved in a RAW format which contains the RGB intensity data of each pixel allowing, for example, a more comprehensive analysis afterwards. When coupling the gas jet into a SPIG structure the dimensions of the jet are particularly important [13,14]. The entrance to the JYFL SPIG is defined by the first electrode aperture of 6 mm in diameter [2]. The typical operating distance between the SPIG and the exit hole of the ion guide is 5 mm. As discussed by Rasi et al., if the gas jet diameter is larger than the aperture the jet experiences a normal shock and becomes subsonic after the nozzle leading to an almost spherical distribution of the gas. In this work we have utilized a perspex SPIG structure of similar dimensions to the first sextupole used at IGISOL [2], shown in the photograph of Fig. 6. In this manner, the interaction and coupling of the gas jet to the structure can be visually studied. Radial distance (mm) Longitudinal distance from ion guide (mm).2 mbar 5.6 mbar 4. Analysis of the gas jet fluorescence A number of photographs were taken using the nozzle types discussed in Section 3.1, for a constant gas cell pressure of either helium or argon. Greyscale images were then created by summing the RGB colour channels of the unmanipulated raw images. In order to represent changes of intensity, reduced noise contour maps were made from the greyscale images. Noise reduction was performed using the pixelwise adaptive Wiener filter of MATLAB s image processing Toolbox [31]. An area of 2 2 pixels corresponding to :42 mm 2 was used in the smoothing procedure. Fig. 7 is an example of a contour map which illustrates the gas jet fluorescence profile at a background pressure of.2 and 5.6 mbar, respectively. The converging-diverging nozzle with a throat diameter of 1.4 mm was used and the gas cell was operated with helium at a pressure of 56 mbar. The contour lines represent exponentially increasing intervals of intensity, such that each line corresponds to about a 26% increase of brightness radiating outwards from the gas jet. Due to the expansion of the fluorescent jet, the ion guide head (on the right) and the perspex SPIG (on the left) are visible in the figure. Given the ratio of the nozzle exit to throat diameter (Fig. 4), an isentropic design pressure ratio of :5% can be deduced. In order to achieve this pressure ratio, for a given inlet pressure of 56 mbar the background pressure should be :28 mbar. Consequently, the two jet structures in Fig. 7 correspond to an underexpanded and overexpanded regime, respectively. In both situations the nozzle is choked. With a background pressure of.2 mbar, less than the design pressure of the nozzle, the jet freely expands to supersonic velocities in the extraction chamber. A shock disc is not visible, however, which may be due to the increasingly diffusive nature of the gas downstream from the nozzle caused by the interfering presence of the SPIG structure. At a background pressure of 5.6 mbar, significantly larger than the design pressure, a shock is formed within the nozzle and the gas downstream is moving at a subsonic velocity. It is clear that the increase in the background pressure improves the coupling to the SPIG structure. Fig. 7. Contour map following the image analysis of a gas jet photograph taken at two background pressures,.2 mbar (top) and 5.6 mbar (bottom). The SPIG is visible on the left and the ion guide on the right. The dark regions visible above and below the exit nozzle are the screws connecting the nozzle to the gas cell. Intensity (arbitrary units) Radial position (mm) Fig. 8. Gas jet profiles 2 cm from a 1.34 mm diameter exit hole as a function of background pressure. The gas cell was operated with helium at a pressure of 56 mbar. Colour online. In order to understand the effect of the background pressure on the gas jet diameter, Fig. 8 shows intensity plots of gas jet cross-sections at a distance of 2 cm from the ion guide. In this instance the gas cell was again operated with helium at a pressure of 56 mbar; however, a standard exit hole of 1.34 mm diameter was used. The thickness of the cross-sectional slice used to extract the information was 4 pixels or 2.5 mm wide. The gas jet intensity and its corresponding error were calculated as a function of radial distance from the average and standard deviation of

6 M. Reponen et al. / Nuclear Instruments and Methods in Physics Research A 635 (211) each 4 pixel row. In order to characterize the gas jet more quantitatively, the Full Width at Half Maximum (FWHM) of the profiles was determined from either Lorentzian or Gaussian fitting. We note that the choice of distribution has not been motivated by any physics of the gas jet and simply provides an appropriate metric of the jets profiles. Due to the diversity in the overall shape of the profiles, particularly at low background pressures in which the presence of the SPIG may have an influence on the tails of the jet (as suggested by the contour distribution in Fig. 7), each individual intensity plot was checked for overexposed areas or other artifacts not directly related to the jet (for example illuminated areas of the chamber seen in the raw images). Fig. 9 illustrates the change of the gas jet FWHM as a function of background pressure for all the nozzle types and two different gas cell pressures, as indicated in the figure legend. A number of conclusions can immediately be drawn from the figure. Firstly, the FWHM of the gas jet shows a lack of sensitivity to the two helium inlet pressures. Secondly, the gas jet diameter is insensitive to the type of nozzle used. The most striking dependence, however, is that all results show a rapid reduction in the FWHM from close to 4 mm at the lowest background pressures ( :1 mbar), followed by a slow levelling off at.5 mbar, wherein the FWHM has reduced to 6 mm. We note that although the gas jet can couple into the SPIG (aperture 6 mm in diameter) with minimal interference at background pressures greater than 1 mbar, this is not an ideal scenario for IGISOL operating conditions due to an increase in the probability of discharge between electrodes of the ion guide system. In addition to the gas jet measurements as a function of background pressure, we also studied the profile of the gas jet as a function of longitudinal distance from the exit nozzle, without the perspex SPIG. An example of such a measurement is shown in Fig. 1 for the case of the 1.4 mm converging-diverging nozzle attached to the ion guide with an inlet pressure of 5 mbar of argon. In this instance the maximum pumping capacity of the extraction vacuum chamber was utilized and the corresponding background pressure was mbar. The most interesting observation occurs within a longitudinal slice taken 17 mm from the nozzle, in which a rabbit-eared structure is seen. The contour map in Fig. 11 illustrates the reason for such a structure in the gas jet profile, a rarefied region of expanding supersonic gas 1 Intensity (arb. units) Radial position (mm) Fig. 1. Gas jet profiles as a function of axial distance from the 1.4 mm converging-diverging exit nozzle. Colour online. Radial distance (mm) Longitudinal distance from ion guide (mm) Fig. 11. Contour map of an argon jet expanding from the converging-diverging nozzle. The two dark curved bands extending from the ion guide represent the oblique shocks. The normal shock, or Mach disc, is located at a distance of 22 mm from the nozzle. The white region below the nozzle screw was overexposed in the original photograph. Jet FWHM (mm) Background pressure (mbar) Fig. 9. Gas jet FWHM as a function of background pressure for the different nozzle types and two gas cell pressures. surrounded by oblique and normal shock waves. In this instance the pressure in the ion guide had been increased to 25 mbar and the corresponding background pressure mbar. Fig. 11 indicates that the converging-diverging nozzle behaves in a similar manner to the converging-only nozzle (exit hole), probably because the diverging duct is relatively short. The pressure difference between the inlet chamber and extraction chamber is now considerably larger ( 6) and thus the type of jet formed for the nozzle in use is said to be strongly underexpanded. Under such conditions the gas exits the nozzle and starts to accelerate to high speeds (Mc1) while the density drops. Due to radial acceleration the jet cross-section increases (as seen from the two visible oblique shocks whose outer surfaces define the jet boundary). The jet pressure reduces until it becomes less than the background pressure, at which point the jet moves into an over-expansion region. Due to the radial gradient of pressure, the radial cross-section of the jet reduces as the gas is driven back towards the nozzle axis. The normal shock, or Mach disc,

7 3 M. Reponen et al. / Nuclear Instruments and Methods in Physics Research A 635 (211) surrounds the first region of fast, low density gas expansion and is immediately upstream of a compression wave in which the gas density is considerably greater. The Mach disc appears to be rather more diffuse than the oblique shocks (which are also defined by the term barrel shock), illustrated by the density of the contour lines. In our pressure regime, the shock wave is less of a sudden discontinuity and can be thought of as a band in which the changes in pressure, mass density, temperature and flow velocity are rather gradual. For a monoatomic gas such as argon, the shape and size of the Mach disc and barrel shock, as well as their location with respect to the nozzle, is simply determined by the ratio of the inlet to background pressure and the nozzle diameter d [32]. The position of the disc x M can be calculated for the condition 15rP in =P bg r17 (which is satisfied for all pressure ratios in this work): x M ¼ :67dðP in =P bg Þ 1=2 : According to this relationship, as the ratio between the inlet pressure of the gas cell and the background pressure becomes larger, the Mach disc moves downstream from the nozzle. We have estimated the Mach disc location from the jet photographs as a function of inlet pressure. Using Eq. (5) and the known throat diameter of the converging-diverging nozzle, we have extracted the background pressure from the Mach disc location and compared it to the measured pressure from the Pirani gauge, located 2 cm from the gas jet region. The results of the comparison are shown in Fig. 12. The error on the pressure inferred from the position of the shock disc is representative of the uncertainty of the location, whereas the Pirani gauge measurements have a 2% error according to the model specifications. We note that at the lowest inlet pressures the Mach disc is barely Extraction chamber pressure (mbar) Gas cell pressure (mbar) Fig. 12. A comparison between the extraction chamber pressure measured using a Pirani gauge and that determined via Eq. (5) and measurements of the Mach disc location, vs the ion guide pressure. ð5þ visible as the gas jet is no longer strongly underexpanded, thus the location becomes more difficult to determine. The corresponding variation in the position of the Mach disc is 7mm over the range of inlet pressure. In general, the results are in good agreement, supporting our understanding that the Mach disc represents the location, where the jet pressure rapidly changes to match the background pressure. The diameter of the Mach disc D M is related simply to the location of the disc x M [32]: D M ¼ :42x M for P in =P bg ¼ 2 D M ¼ :48x M for P in =P bg ¼ 1: ð6þ These relationships have been found for air, however, an accurate value depends on the ratio of specific heats, g, and the ratio of inlet to background pressure. We have extracted values for the diameter from the jet photographs, and for a change of inlet pressure from 3 to 1 mbar, D M varies from 1 to 4 mm. We note that the gas jet profiles of Fig. 1 and the contour plot of Fig. 11 illustrate the rather wide radial distribution of the jet at the lowest operated background pressures. Although a low background pressure is desirable at IGISOL, the figures clearly demonstrate a detrimental behaviour towards coupling the jet into the SPIG, thus resulting in a poor laser-atom overlap efficiency. The key to providing a highly uniform, low divergent expansion at low background pressure is to design the contour of the nozzle such that the flow at the exit has a constant Mach number, hence the density and temperature at all points within the subsequent postnozzle flow are fixed. Such a nozzle design is called the Laval nozzle. It is known that the design of the convergent and throat areas are less critical to the performance of the nozzle, although in general the convergent section should have a relatively large radius of curvature near the throat region (five times the throat radius) [33]. However, the shape of the divergent section has to be precisely calculated and manufactured in order to minimize the thickness of the boundary layer [34]. Such a layer describes the region where the nozzle wall influences the flow (heat transfer and viscous effects become important) and can be calculated using computational fluid dynamics methodology. A Laval nozzle has some disadvantages when compared to other supersonic nozzle designs. Firstly, there are complications associated with the fabrication of the nozzle and, furthermore, it is designed to work best only at one pressure, which means that if the ion guide pressure is tuned the jet shape suffers due to a change in background pressure. Although the effect is small it could be avoided by using so-called altitude compensating nozzles such as the aerospike nozzle [35]. Nevertheless, we recently manufactured a proof-of-principle Laval nozzle based on designs we found in the literature [34,36] with the practical criteria that the throat diameter be similar to the other nozzles tested in this work ( 1:4 mm) and the exit of the nozzle equivalent to the diameter of the entrance to the SPIG (6 mm). By solving Eq. (3), the design Mach number for this nozzle is 6:2 and thus the isentropic design pressure ratio is :14%. Fig. 13. Schematic of the Laval nozzle tested in this work (right) followed by a contour map of the resulting argon gas jet (left). A series of expansion and compression waves are visible until viscous effects finally dissipate the jet energy. The background pressure was.33 mbar.

8 M. Reponen et al. / Nuclear Instruments and Methods in Physics Research A 635 (211) In comparison to our previous converging-diverging nozzle (Fig. 4), the distance from the nozzle throat to the exit is considerably longer in the Laval nozzle as illustrated in Fig. 13. In addition to the nozzle schematic, Fig. 13 shows one example of the resultant jet structure visualized as a contour map. The ion guide was operated with argon at a pressure of 25 mbar. At this pressure the background pressure should be.35 mbar to meet the design requirements and in reality it was very close,.33 mbar. Unlike the single dominant shock structure in Fig. 11, the Laval nozzle exhibits a series of expansion and compression waves which, following reflection from the jet boundary, intersect to form a curved shock (seen at distances of 4 and 85 mm from the ion guide). The process repeats itself until viscous effects finally dissipate the jet energy. The initial results of Fig. 13 are very encouraging, the gas jet maintains a reasonably collimated structure over a considerable distance of 14 cm while remaining fully supersonic. Following the first expansion, the radial profile of the jet has a maximum diameter of 6 mm, a promising development towards an improved overlap efficiency in a future LIST experiment. 5. Direct pressure measurements The previous section detailed our extensive studies of the gas jet fluorescence in order to gain insight into the behaviour of the jet as a function of background pressure and nozzle type. To gain a more quantitative understanding of the pressure of the jet it is very useful to determine the impact pressure at a given point in the flow. The pressure distribution of the gas jet was studied onaxis as a function of distance from the nozzle for a fixed gas cell pressure, and also as a function of pressure at the fixed location of the exit of the nozzle. Here we only studied the Laval nozzle, detailed in Fig. 13, using argon gas. The measurements were performed using thin stagnation and static pressure probes (Pitot probes) made from hypodermic needles of typically 1 mm in outer diameter. It is well known that results obtained with this method are of modest accuracy, nevertheless, they provide direct and quantitative information, unlike the qualitative measurements based on the analysis of the gas jet fluorescence. We note that all pressures are absolute and not relative to atmosphere. The needles were attached to 4 mm outer diameter aluminium tubes located 5 cm from the needle tip, which in turn were mounted to a skimmer plate drive, to be used in the future ion guide-based quadrupole mass spectrometer system. Such a set-up allowed the needles to be moved using computer control with sub mm precision. The pressure is measured via a low-cost silicon pressure sensor with an amplified voltage output (model PX138-15A5V from Omega Engineering), operated from a 8 Vdc regulated power source. The sensor was calibrated using a Pfeiffer CMR 261 capacitance gauge. A simple Pitot probe with a hole facing the gas jet measures the so-called stagnation pressure, or total pressure. This is a sum of the static pressure and dynamic pressure (or impact pressure). The static pressure probe is generally measured via a hole on the side of the needle. Both measurements are required in order to derive the dynamic/impact pressure which is extremely sensitive to the Mach number. The Pitot tube induces a standing shock wave in the flow and by assuming that the slowing down of the flow downstream of the shock wave is isentropic, the static pressure p s and impact pressure p i are linked to the Mach number through the Rayleigh Pitot relationship [37]: p i ¼ gþ1 ðg þ 1Þ=ðg 1Þ M 2g=ðg 1Þ gþ1 1=ð1 gþ : ð7þ p s 2 2gM 2 gþ1 If we assume once more an isentropic expansion within the nozzle we can extract the relationship between the impact pressure, the ion guide pressure p in and the Mach number: g=ðg 1Þ p i ¼ ðgþ1þm2 gþ1 1=ðg 1Þ : ð8þ p in ðg 1ÞM 2 þ2 2gM 2 gþ1 Fig. 14 shows the results of the stagnation pressure probe as a function of distance from the nozzle. This measurement was repeated for several ion guide pressures, listed in the figure legend. Measurements were not restricted to regions of the gas jet downstream from the nozzle but also included the diverging region of the nozzle. A striking first maximum can be seen with a typical FWHM of between 6 and 8 mm, which moves downstream as the ratio of the ion guide to background pressure increases. This behaviour is qualitatively similar to that of the Mach disc resulting from the converging-diverging nozzle discussed in the previous section. The break in the data points at 2 mm in Fig. 14 is where the needle had to be physically moved due to the limited travel of the skimmer drive mechanism. Following this, a second, smaller amplitude and broader peak is visible. Note that fewer ion guide pressures were used to study this second peak. Fig. 14 can be compared with the gas jet fluorescence analysis of Fig. 13. The overexposure of the photograph in the first 1 mm from the nozzle limits the comparison; however, the structure between 4 and 5 mm is clearly supported by the direct pressure measurements. Measurements of the static pressure were also performed. From the nozzle exit as a function of axial distance downstream, the pressure was rather constant at mbar. This is in reasonable agreement with the earlier work of Iivonen et al. who demonstrated that, after a sharp drop near the nozzle, the static background pressure in the channel between the target chamber and skimmer electrode was fairly constant and typically of the order of 1 mbar [38]. As the static pressure is a negligible fraction of the total pressure we can assume that the stagnation pressure is equivalent to the impact pressure in our conditions. Fig. 15 illustrates the ratio of the ion guide pressure to impact pressure as a function of the ion guide pressure, at the nozzle exit. At low pressures the ratio P in =P i sharply decreases as the gas flow approaches sonic conditions. As the shape of the nozzle has not been properly calculated in this first design, the supersonic flow is not smooth and the peak between 15 and 1 mbar Stagnation pressure (mbar) Longitudinal distance from ion guide (mm) Fig. 14. Stagnation pressure measurements as a function of distance from the exit of the Laval nozzle for different ion guide pressures. The vertical dotted line represents the nozzle exit. The nozzle throat begins at 15 mm in this figure.

9 32 M. Reponen et al. / Nuclear Instruments and Methods in Physics Research A 635 (211) P in / P i P in (mbar) Fig. 15. Measured ratio of the ion guide and impact (stagnation) pressures as a function of ion guide pressure. The inset shows the P in =P i dependence on the Mach number according to the supersonic Rayleigh Pitot formula (Eq. (7)). represents the first pressure maximum of Fig. 14 as it moves past the nozzle exit. The supersonic Rayleigh Pitot relationship of Eq. (7) is used to estimate the jet Mach number from the P in =P i ratio (see inset of Fig. 15). The deduced Mach number at the nozzle exit is for 1oP in o4 mbar. Using the isentropic relations, similar to that of Eq. (3), we can calculate fundamental parameters such as the jet temperature and density using the deduced Mach number. For an ion guide pressure of 3 mbar the Mach number is 4. The measured temperature within the gas cell (without discharge source) is 296 K, and therefore, the jet temperature at the nozzle exit is 46 K. The corresponding jet density for an ion guide pressure of 3 mbar is atoms cm 3. For completeness, the static pressure in the jet has been calculated using Eq. (3) to be 3 mbar at the nozzle exit. This is in reasonable agreement with the static Pitot probe measurement. One of the interests in the LIST project is to study the feasibility of laser spectroscopy inside the gas jet for exotic nuclei. The LISOL group of the University of Leuven have performed studies on the impact of the gas jet on the resonant linewidth of a specific element and have compared this with pressure conditions inside the gas cell and in a reference cell under vacuum [27]. In addition, using laser Doppler-shift velocimetry, the group were able to determine the flow velocity of moving nickel atoms in the jet. For a longitudinal ionization geometry a supersonic velocity of 56 m/s was quoted for an Ar gas jet. Similar studies are currently underway at the IGISOL facility, JYFL. In this work however, it is possible to obtain the gas jet velocity via the deduced Mach number and the isentropic relationship [33]: V 2 ¼ g 1 2 M2 1þ g M2 Vmax 2 : ð9þ Here, V max is the thermodynamic limiting velocity (the velocity the flow would reach at M ¼1) and is a function of the gas cell temperature T in, the ratio of specific heat capacities and the mass m of the flowing gas. It is constant for a given expansion and can be calculated using V max ¼ 2g g 1 k 1=2 bt in ð1þ m where k b is Boltzmann s constant. For argon gas, V max ¼ 555 m=s, and therefore, the jet velocity (at the nozzle exit and for a Mach number of 4) is deduced to be 59 m/s. A direct comparison cannot be made with the LISOL measurement as the nozzle in this work differs from the simple exit hole used in the work of Sonoda et al. [27]. However, a comparison will be made using the laser velocimetry technique in the near future. 6. Alpha decay recoil measurements An important question to be answered is whether the visible fluorescence is representative of not only the excited gas density but also neutral gas atoms. In order to provide a first insight an attempt was made to study the transport of particles within the gas jet. An a-recoil source of 223 Ra was collected on to the tip of an aluminium needle ( 5 mm long section) and mounted within the volume of the gas cell as shown in Fig. 5. The needle was positioned approximately 1 mm from the exit of the gas cell, defined by a simple exit hole. 223 Ra is a commonly used radioactive source for off-line IGISOL testing due to the suitable lifetimes of the daughter products 219 Rn (3.96 s) and 215 Po (1.8 ms). The source is originally collected as the granddaughter of the decay of a 2 kbq 227 Ac (21.8 yrs) source [39]. The ions recoiling into the gas cell following the alpha decay of the source were transported out of the cell and carried into the vacuum chamber within the gas jet. A small silicon detector (15 mm diameter active area) was placed on a movable mount within the vacuum chamber and could be positioned at various locations within the previously observed gas flow region. The probability of particle detection at a given location within the gas jet is proportional to the gas-jet density at this point. The normalized alpha recoil counts from four different detector positions (A D) are shown in Fig. 16 as a function of the background pressure. The carrier gas used in these measurements was helium and the gas cell was operated at a fixed pressure of 2 mbar. Close to and on the axis of the nozzle, an increase in the background pressure greatly improves the transport efficiency of particles as seen in detector position A. On the other hand, 15 mm off-axis, where a wide gas jet is required to transport particles to the detector (position B), the ion count rate is greatest for the lowest background pressures. The situation is strikingly similar at a distance of 8 cm from the nozzle. Although only one Fig. 16. Normalized a-recoil counts as a function of pressure for different Si detector positions. The detector was positioned either on-axis (A, C) or 15 mm off-axis (B, D) at two distances from the ion guide, 4 and 8 mm, as labelled in the figure. The solid lines are used to guide the eye.

10 M. Reponen et al. / Nuclear Instruments and Methods in Physics Research A 635 (211) measurement was made at a background pressure greater than 1 mbar, a consistent feature of apparent saturation of the a-recoil count rate is seen with increasing pressure. A possible explanation is that for a pressure greater than 1 mbar, the gas jet width is less than the diameter of the detector surface. For detector positions B and D, the gas jet completely misses the detector surface at this pressure whilst with the detector in position A or C, the jet is entirely within the detector surface. A further test was performed with the detector on-axis at a distance of 12 cm from the gas cell, outside the observable range of the gas jet fluorescence. Over a 2 s integration time, the count rate for a background pressure of.16 mbar was :4 a=s whereas for a background pressure of 1.1 mbar, the rate was 2:2 a=s. The results shown in Fig. 16 can be compared with the trend of the gas jet FWHM as a function of background pressure detailed in Fig. 9. For the Si detector on-axis, the alpha counts show a trend towards saturation at background pressures greater than 1 mbar, from which we postulate that the gas jet diameter must be less than the 15 mm diameter active area of the detector. The gas jet fluorescence measurements show a direct correspondence with the background pressure and above 1 mbar the FWHM is well within the size of the silicon detector. We note, however, that the presence of the detector will disturb the gas jet, and therefore, the interpretation of the above results remains a question. Although these measurements were not used in any quantitative analysis, they are useful in establishing some confidence in the ability to rely on the fluorescence variations in the photographs as a true representation of the gas jet density distribution. 7. Computational fluid dynamics simulations Theoretical models, as presented in Section 2, must rely on many simplifying assumptions to be solvable. Thus, such models can only provide an approximate guideline for the construction and operation of a shaped nozzle required, for example, for the IGISOL LIST method. In the experimental conditions we deal with a three-dimensional flow of compressible gas including turbulence and temperature variations. A link between the theory of supersonic gas flow and experimental requirements for the studies of gas jets can be established by the use of computational fluid dynamics (CFD) simulations. A key element in the application of CFD simulations to the optimization of the IGISOL LIST method is finding the appropriate parameters of the gas flow model. Such a calibration of the computational model can be done by experimental study of gas flow as initiated for the IGISOL system by Rasi et al. [17]. Further development of those investigations has been presented in the preceding sections. Comparison of experimental results to an outcome of CFD simulations can serve as a test of the computational model. Fig. 17 presents an example of the gas density simulated for helium flowing out from a converging-diverging nozzle for a pressure ratio of.75%. The figure can be qualitatively compared to the experimental results as shown in Figs. 7 and 11. Moreover, Fig. 17 shows that with CFD methods it is possible to get information for pressure ratios which are beyond the current pumping capacity of the gas jet measurement set-up. It is also possible to compute the pressure, velocity and temperature fields in a gas jet which are difficult to determine experimentally. As described in Section 6, an a-recoil source can be used to study the transport of particles by gas flow. In particular, due to the suitable lifetimes of the daughter products, one can estimate the travel time of particles from the gas chamber to a Si detector placed after the nozzle. It is possible to simulate a mutual interaction between a discrete phase (such as atoms of interest) and flowing gas, which is another way of comparing simulated Fig. 17. Simulated contours of gas density in a plane parallel to the flow direction. The colour scale covers the range kg/m 3. Inlet pressure is 2 mbar and the background pressure is set to 1.5 mbar. The nozzle on the right is the same as in Fig. 4, nozzle B. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.) and experimental results. Such atom transport simulations were feasible for the Warsaw IGISOL system [4] and provided evacuation times of the same order as experimental tests [41]. The computer model discussed here consisted of nozzle B as shown in Fig. 4 and an arbitrary cylindrical volume (diameter 22 mm, length 35 mm) after the nozzle. The nozzle geometry was imported from a CAD file used by the JYFL mechanical workshop to fabricate the actual nozzle for the gas jet measurement set-up shown in Fig. 6. The geometry of the whole model was created and meshed with the Gambit programme. Computer modelling of fluid flow in three dimensions was performed with the Fluent software package [42]. A properly defined computer model can be used to investigate elements of the ion guide system which directly interact with the flowing gas. In this manner, their shapes and sizes can be optimized with the CFD simulations. In the IGISOL LIST method the simulations are ongoing and are being used to find an appropriate shape of a nozzle which should result in a sufficiently compact jet such that an optimum overlap with the laser beams and efficient transport of ions into the SPIG structure can be attained. 8. Conclusion and outlook In summary we have studied the gas jet expanding from an ion guide as a function of different background pressure conditions and exit nozzle types. This work is motivated by the requirement for an optimal overlap efficiency between atoms and counter-propagating laser beams for the gas-cell-based LIST technique. Qualitative information has been obtained via visual observations made possible through excitation, by electrical discharge, of both helium and argon gas within a chamber. In order to represent changes of gas jet intensity, reduced noise contour maps were made from greyscale photographs. Quantitative measurements of the gas jet diameter were made at a fixed distance of 2cm from the ion guide by extracting intensity plots of the radial gas jet cross-section. We have shown that the FWHM of the gas jet is rather insensitive to the use of a simple exit hole or converging-diverging nozzle, and also to the different operating gas cell pressures, 25 or 56 mbar. On the other hand, the jet FWHM shows a striking dependence on the background pressure, rapidly decreasing from 4 mm at the lowest pressure ( :1 mbar) before leveling off after.5 mbar, wherein the FWHM is 6 mm in diameter. This is an important observation as the aperture of the repeller electrode of the JYFL SPIG is currently 6 mm, and therefore, background