TESLA style superconducting radio frequency (SRF) cavities

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1 IEEE TRANSACTIONS ON APPLIED SUPERCONDUCTIVITY, VOL. 25, NO. 3, JUNE Sudden Vacuum Loss in Long Liquid Helium Cooled Tubes Ram C. Dhuley, Student Member, IEEE and Steven W. Van Sciver Abstract Sudden vacuum loss in straight long tubes cooled by liquid helium is investigated. The scenario resembles an accident in a superconducting particle accelerator when the beam-cavity suddenly loses its vacuum to atmosphere. Following this accident, air will propagate down the vacuum channel and freeze on the cold walls. To warrant against catastrophes of this accident, it is vital to know the propagation speed of air in the vacuum space and the heat load on the helium bath. An experimental setup has been developed to determine these parameters. Experiments are conducted wherein a large nitrogen gas tank is rapidly vented to a high vacuum tube ( 10 4 Pa) immersed in liquid helium at 4.2 K. The measurements comprise of the tube pressure to determine the propagation speed, the tank pressure to determine the gas mass flow rate into the tube, and the tube temperature to estimate the heat load. Flash solidification of the gas on the cold tube, which is apparent in the measurements, limits the propagation speed to the order of 10 m/s. Based on the mass flow measurement a heat deposition rate of 60 kw/m 2 on the vacuum tube is estimated, while the heat transfer rate to the helium bath is predicted to exceed 20 kw/m 2. Index Terms Sudden vacuum loss, accelerator safety, transient helium heat transfer, cryodeposition, gas propagation. I. INTRODUCTION TESLA style superconducting radio frequency (SRF) cavities in linear accelerators (LINACs) are operated with high vacuum on their inside, while being immersed in a bath of liquid helium (LHe). A string of such cavities housed in a cryomodule forms a long vacuum channel cooled by liquid helium. Like other cryogenic systems, this configuration is prone to catastrophic failure resulting from sudden loss of vacuum. An accidental rupture at a cryomodule interconnect will result in a large in-rush of the surrounding air into the cold channel. The air will cryodeposit (solidify) on the channel walls and rapidly transfer heat to the helium bath. The potential severity of this accident drove several particle accelerator laboratories to conduct beam-line vacuum loss tests during the accelerator development stage [1], [2]. The general purpose of these tests is to verify the effectiveness of the pressure relief system foreseeing such an accident. Technical outcomes of these tests specifically apply to the system under consideration and are therefore limited. Due to the longitudinal geometry of the cavity string (length being much larger than cross section), the air entering from a Manuscript received August 8, 2014; accepted October 30, Date of publication November 5, 2014; date of current version January 15, This work is supported by the U.S. Department of Energy under Grant DE-FG02-96ER The authors are with the Mechanical Engineering and National High Magnetic Field Laboratory (NHMFL), Florida State University, Tallahassee, FL USA ( dhuley@magnet.fsu.edu; vnsciver@magnet.fsu.edu). Digital Object Identifier /TASC Fig. 1. Propagation of air in a LHe cooled channel resulting from a sudden vacuum loss. (a) Warm air enters an actively cooled vacuum region. (b) Cryodeposition causes local heating of the cavity wall, the process saturates and the gas moves forward. (c) The extent of heated zone increases as the cryodeposition front moves forward in the vacuum space. local rupture will propagate and cryodeposit along the vacuum channel. The incident heat flux will then depend on the extent to which this air has traveled. This case is unlike the situations in which a small experimental section [3] or a liquid helium container [4] suddenly loses its insulating vacuum. The heat flux in such situations can be estimated from the in-flow rate of air, its enthalpy change, and the surface area of the helium container [5]. The propagation of rapidly in-flowing air is extremely fast in a vacuum tube at room temperature [6] (several times the speed of sound in air). Takiya et al. [7] measured this speed to be as high as 1000 m/s. However when the tube is sufficiently cold to freeze the air, the propagation is significantly slow. A loss of vacuum test at DESY [2] found this speed to be 3 m/s in a niobium cavity string cooled by 2 K He II. Dalesandro et al. [8] observed this speed to be 4 7 m/s in a thin walled stainless steel vacuum tube immersed in a 2 K He II bath. This slowing can be understood as follows. Fig. 1 shows a vacuum tube immersed in liquid helium. The vacuum is quickly broken to set up an air flow in the tube from the left to the right. On encountering the cold surface the air solidifies on the walls and does not propagate forward. As more air solidifies at a certain location, the solid air layer grows and attains a thickness, which impedes further solidification at that location. This decreased solidification is due to large thermal resistance of the solid layer [9], which inhibits the heat extraction required for the gas to solid phase change. The solid layer thickness at which cryodeposition diminishes can be termed as the critical thickness. Dynamics of this cryodeposition process on a simpler IEEE. Personal use is permitted, but republication/redistribution requires IEEE permission. See for more information.

2 IEEE TRANSACTIONS ON APPLIED SUPERCONDUCTIVITY, VOL. 25, NO. 3, JUNE 2015 Fig. 3. Instrumentation- CAD drawing of the cut section of the copper tube immersed in LHe (center), a thermometer encapsulate mounted on the vacuum tube (left), and a pressure probe heat sunk to an aluminum rod (right). Fig. 2. Schematic of the experimental setup. geometry has been discussed in our previous work [10], [11]. A critical thickness is also shown to exist given certain conditions [12] [14]. Slow propagation of air in a helium cooled vacuum tube is a consequence of cryodeposition on the cold tube walls. The heat flux to the helium bath will depend on the mass of gas flowing in (the total energy input), and the extent of its propagation (the surface area for heat transfer). Given an occurrence of a sudden rupture, it is therefore vital to be able to predict the resulting gas flow rate and its propagation speed. An experimental setup has been developed to simulate a sudden vacuum loss, measure the resulting gas in-flow rate and determine its propagation speed in the cold tube. In this paper we describe the propagation measurements for a test case, and estimate the heat load on the cold system. II. EXPERIMENTAL SETUP A. Vacuum Loss Simulation Setup The experimental setup is schematically shown in Fig. 2. The principal component of the propagation measurement apparatus is a 3 mm thick, 1.5 (38 mm) OD OFHC copper tube of length 1.5 m. This copper tube is suspended in a conventional liquid helium cryostat from a top plate with a thin walled stainless steel (SS) extension tube. The copper tube, actively cooled by liquid helium, forms the main cryodeposition site. The SS section of the vacuum tube extends from the cryostat to a large gas tank. A fast opening bellows type solenoid valve (SV) isolates the vacuum tube from the gas tank. A venturi tube between the vacuum tube and the SV generates a choked flow after the SV opens. In the choked regime the flow rate can be determined from the tank depressurization rate. An electrical trigger signal opens the SV to simulate a sudden vacuum loss. For the present case the experiment was conducted using normal liquid helium at 4.2 K as the coolant, and nitrogen as the working gas. Nitrogen as the substitute for air simplifies the heat transfer analysis by defining the energy input to the system in terms of the sensible and latent heats of a single gas (unlike air, which is a mixture of several gases). B. Vacuum Tube Thermometry Lake Shore Cernox surface mount thermometers measured the temperature rise of the vacuum tube due to cryodeposition. As shown in Fig. 3, the thermometers are placed on the tube s outer surface, which is in contact with liquid helium. Special thermometer encapsulates were designed to minimize the interference of the surrounding liquid helium on the tube temperature measurement. In these encapsulates, a low thermal conductivity epoxy (Stycast 2850 FT) covers the entire body of the thermometer package except the specific surface that senses temperature. The capsules are then installed in spot faces milled on the tube s outer surface. A thin indium disc placed in the spot face enhances thermal contact between the thermometer and the tube surface. Apiezon N grease seals any radial gap between the thermometer capsule and the spot face. The grease solidifies when cold, and resists the penetration of liquid helium into the spot face. The encapsulate construction and installation are depicted in Fig. 3. The tube carries four thermometers equally spaced over 1.5 m. The measurement of transient temperature of the tube benefits from the high thermal diffusivity of copper in liquid helium. The characteristic diffusion time for radial heat transfer across the wall of a 3 mm thick copper tube at 4.2 K is 25 μs. The onset of cryodeposition on the tube s inner surface can therefore be detected with no significant time lag by the thermometers embedded on the outer surface. Additionally, the high thermal conductivity of copper limits the formation of a large temperature gradient across the tube wall. A single thermometer then provides an excellent measure of the tube temperature averaged over its thickness. This temperature is used in the heat transfer calculations presented in Section III-B.

3 DHULEY AND VAN SCIVER: SUDDEN VACUUM LOSS IN LONG LIQUID HELIUM COOLED TUBES C. Vacuum Tube Pressure Measurement Kulite pressure probes (XCQ series) measured the pressure rise in the vacuum tube. The probe layout is shown in Fig. 3. These probes are chosen due to their miniature size (diameter 2.5 mm, length 10 mm). The natural frequency (typically > 100 khz) resolves the rapid pressure changes expected after a sudden vacuum loss. An aluminum rod (insert), which runs along the tube centerline, supports four probes spaced by 0.5 m, over a length of 1.5 m. This insert absorbs the self-heat ( 85 mw) generated in the probes, and prevents their overheating in vacuum. The insert also acts as a thermal buffer to inhibit the radiation cooling of the probes enclosed in a 4.2 K tube. A thermometer (Lake Shore PT-100) and a heater (Minco Polyimide Thermofoil) placed on the insert, regulate its temperature. This arrangement limits the cooling of the insert and probes to 10 K below room temperature. The probes thus operate in the factory compensated temperature range with less than 0.2% thermal zero and sensitivity shifts. D. Data Acquisition and Processing During the experiment, the pressure probes were powered by TENMA voltage supply and the output data were sampled at 20 khz per probe on a National Instruments USB DAQ board. The timing resolution of 0.05 ms was chosen by conducting a separate room temperature vacuum break experiment, and adequately resolving the resulting propagation speed ( 650 m/s). To determine the pressure rise times in the helium cooled tube, the high frequency noise in the pressure data was smoothened using Savitzky-Golay smoothening filter. An appropriate filter design for each data set reduced the noise below 10 Pa. Rise time was then defined as the time when the pressure rose above the noise bounds of the smoothened signal. The time distortion due to smoothening was < 1 ms for all pressure data and is negligible compared to the propagation timescales. The tube temperature data were sampled on Data Translation DT9824 DAQ boards at 4.8 khz (the limit on this board). A similar procedure was adopted to determine the temperature rise times. III. EXPERIMENTAL RESULTS A. Propagation Timescales Sudden vacuum loss was simulated by opening the SV and venting the nitrogen tank (100 kpa, 295 K, 85 liters) to the vacuum tube (4.2 K, 10 4 Pa). Fig. 4 shows the resulting fall in the tank pressure (top), the rise in the tube pressure (center), and the transient heating of the tube (bottom). The tank pressure data is used to calculate the rate of mass flow into the tube. As shown further in Section III-B, this mass provides an estimate of the total energy input to the system. The arrival of gas is detected by P1 at the entrance of the helium cooled region merely 5 ms after the vacuum break. Flash cryodeposition of this gas at the entrance results in a simultaneous temperature rise at T1. The arrival times recorded by the sensors are plotted with their locations in Fig. 5. The measured arrival time data are interpolated to estimate the Fig. 4. Tank pressure (top), the vacuum tube pressure (center) and the vacuum tube temperature (bottom) after opening the SV at t =0. Fig. 5. Temperature T and pressure P rise times in the cold section of the vacuum tube. Markers denote the measurements while solid lines are the interpolated curves. x =0corresponds to the start of the cold section. arrival times at locations between the sensor stations. These are shown as solid lines in Fig. 5. Following observations from the data are noted. First, the wall temperature rises before the tube pressure at any location. This observation can be understood from the cryodeposition dynamics at a certain location. The vapor pressure of nitrogen near 4.2 K is Pa [15]. Initial cryodeposition on a clean 4.2 K tube surface is therefore extremely fast. This flash cryodeposition locally heats the tube, but sustains a state of vacuum even after the arrival of gas at that location. With the continued build-up of cryodeposit at this location, the rate of cryodeposition falls (see discussion on Fig. 1), the local pressure begins to rise, and the gas is eventually detected by the probe. As the thermometers respond before the pressure probes, we infer that the temperature rise times estimate the propagation speed better than the pressure rise times. Second, the gas propagation decreases along the tube as observed in the tube temperature data of Fig. 4. Based on the temperature rise times in Fig. 5, we obtain average speeds of

4 IEEE TRANSACTIONS ON APPLIED SUPERCONDUCTIVITY, VOL. 25, NO. 3, JUNE 2015 Fig. 6. Comparison of the heat deposited on the tube with the heat stored in bulk copper. 10 m/s, 4 m/s, and 2.5 m/s respectively for regions I, II, and III between the thermometer stations. The underlying cause of this deceleration is to be studied further. The flash cryodeposition diminishes around 430 ms when P4 starts to read pressure. Hereafter the gas encounters the rigid tube end and reflects back. In the absence of flash cryodeposition the entire vacuum tube pressurizes uniformly as the gas is further drawn from the supply tank. B. Heat Transfer Estimates The heat load on the vacuum tube for the duration of flash cryodeposition ( 430 ms) is estimated from the mass of nitrogen that solidified on the tube, and the associated enthalpy change. Nitrogen loses 520 kj/kg on changing state from 100 kpa, 295 K gas to 15 K solid [16]. The mass of nitrogen that entered the tube is calculated from the tank pressure data using the isentropic discharge method [17]. A separate test to ascertain the accuracy of this method, yielded less than 4% variation between the calculated and the measured flow rates. The mass of nitrogen that did not solidify is calculated using the tube pressure of 7 kpa (maximum in the tube) and the corresponding saturation temperature of 60 K (minimum temperature to exhibit the gas phase). The use of maximum pressure and minimum possible temperature yields an overestimate of the mass in the gas phase. The value obtained is less than 7% of the total mass that has entered the tube. The fraction being small is neglected, and the incident heat deposition is then calculated using the mass that enters the tube. The result is shown in Fig. 6. Noting the extent of cryodeposition (1.5 m in 430 ms), we obtain an average heat deposition rate of 60 kw/m 2 on the vacuum tube. Despite high incident heat load, the quick initiation of film boiling restricts the flow of heat from the tube to the helium bath. Based on Steward s transient heat transfer data for a vertically oriented heater surface in liquid helium [18], the temperature T peak in Fig. 4 marks the transition to film boiling. Although it is difficult to quantify the heat transfer to the helium bath in this transitional regime, a qualitative estimate can be gleaned by comparing the heat retained in the tube and the incident heat. The temperature needed to determine the heat retained in the tube is estimated by spatial interpolation of the measured temperature data. The heat retained is compared to the incident heat in Fig. 6. The values closely follow, which indicates that a large fraction of the incident heat accumulates in the tube material. An error exists since the calculated tube heat content exceeds the incident heat. This error, though not quantifiable, is largely attributed to the inaccuracy of the interpolated temperature data, which are generated from the spatially limited measurements. The combined effect of flash freezing (high incident heat load on the tube) and transition to film boiling (slow heat removal by helium) causes the tube to heat rapidly. After the initial phase of flash cryodeposition, the rate of heating is slowed. With slower cryodeposition, the entire tube gradually stabilizes near K and the heat transfer to helium enters the steady film boiling regime. With similar steady film boiling temperatures, heat transfer rates in slight excess of 20 kw/m 2 have been reported [18]. This value also fits in the kw/m 2 range obtained from the CEBAF loss of cavity vacuum experiment [1]. The Breen-Westwater correlation for large diameter tubes [19] with this steady film boiling condition predicts a heat transfer rate of 14.5 kw/m 2. IV. CONCLUSION A brief duration of flash cryodeposition is evident from the temperature and pressure data at any location in the tube. This cryodeposition restricts the flow of gas beyond that location and causes an overall propagation delay. Based on the measurements, we obtain a propagation speed of 10 m/s at the start of the actively cooled section. The speed continuously decreased over the passage, reaching as low as 2.5 m/s near the far end. This deceleration is favorable since it will limit the contamination of a long SRF cavity string. In general, the speed obtained in this study is of the order of 10 m/s, as observed in previous experiments with a niobium cavity string [2] and a stainless steel tube [8]. The transient heat transfer properties of the tube material thus have a minor influence on the propagation speed. The extent of cryodeposition zone and hence the incident heat flux on the tube depend on the gas propagation speed. Based on the rate of mass in-flow and the duration of flash cryodeposition over the tube length, we obtain a heat deposition rate of 60 kw/m 2 on the tube. Though this rate is high, the early inception of film boiling on the tube s outer surface restricted the heat transfer to helium. A large fraction of the incident heat accumulated in the bulk tube material. As cryodeposition subsided, the entire tube attained a temperature around 40 K. Based on some prior findings in literature, we anticipate a steady film boiling heat transfer rate exceeding 20 kw/m 2 under these conditions. ACKNOWLEDGMENT Thanks to the members of NHMFL-FSU Cryolab for technical assistance. REFERENCES [1] M. Wiseman et al., Loss of cavity vacuum experiment at CEBAF, in Advances in Cryogenic Engineering. New York, NY, USA: Springer- Verlag, 1994, pp

5 DHULEY AND VAN SCIVER: SUDDEN VACUUM LOSS IN LONG LIQUID HELIUM COOLED TUBES [2] T. Boekman et al., Experimental tests of fault conditions during the cryogenic operation of a XFEL prototype cryomodule, in Proc. ICEC22/ICMC20, 2009, pp [3] S. M. Harrison, Loss of vacuum experiments on a superfluid helium vessel, IEEE Trans. Appl. Supercond., vol. 12, no. 1, pp , Mar [4] W. Lehmann and G. Zahn, Safety aspects for LHe cryostats and LHe containers, in Proc. Int. Cryogenic Eng. Conf., 1978, vol. 7, pp [5] C. Heidt, S. Grohmann, and M. Süer, Modeling the pressure increase in liquid helium cryostats after failure of the insulating vacuum, in Adv. Cryogenic Eng. Trans. CEC, 2014, vol. 1573, pp [6] A. H. Shapiro, The Dynamics and Thermodynamics of Compressible Fluid Flow, 1st ed. New York, NY, USA: Ronald, 1953, pp [7] T. Takiya, F. Higashino, Y. Terada, and A. Komura, Pressure wave propagation by gas expansion in a high vacuum tube, J. Vac. Sci. Technol. A, Vac. Surf. Films, vol. 17, no. 4, pp , Jul [8] A. A. Dalesandro, R. C. Dhuley, J. C. Theilacker, and S. W. Van Sciver, Results from sudden catastrophic loss of vacuum on scaled superconducting radio frequency cryomodule, in Adv. Cryogenic Eng. Trans. CEC, 2014, vol. 1573, pp [9] K. W. Rogers, Experimental investigations on solid nitrogen formed by cryopumping, Celestial Res. Corp., Pasadena, CA, USA, NASA Tech. Rep. CR-553, [10] R. C. Dhuley, E. S. Bosque, and S. W. Van Sciver, Cryodeposition of nitrogen gas on a surface cooled by helium II, in Adv. Cryogenic Eng. Trans. CEC, 2014, vol. 1573, pp [11] E. S. Bosque, R. C. Dhuley, and S. W. Van Sciver, Transient heat transfer in helium II due to sudden vacuum break, in Adv. Cryogenic Eng. Trans. CEC, 2014, vol. 1573, pp [12] E. S. Bosque, Transient heat transfer in helium II due to vacuum break, Ph.D. dissertation, Florida State Univ., Tallahassee, FL, USA, [13] E. S. Bosque, Rapid cryodeposition of nitrogen gas onto a He II-cooled surface following a vacuum break, Part 1: Mass transport, Cryogenics, submitted for publication. [14] E. S. Bosque, Rapid cryodeposition of nitrogen gas onto a He II-cooled surface following a vacuum break, Part 2: Heat transport, Cryogenics, submitted for publication. [15] K. M. Welch, Capture Pumping Technology: An Introduction, 1sted. Elmsford, NY, USA: Pergamon, 2011, p. 12. [16] R. B. Scott, Cryogenic Engineering, 3rd ed. Boulder, CO, USA: Met- Chem Research Inc., 1988, p [17] J. E. A. John and T. G. Keith, Gas Dynamics, 3rd ed. Upper Saddle River, NJ, USA: Prentice-Hall, 2006, pp [18] W. G. Steward, Transient helium heat transfer phase I Static coolant, Int. J. Heat Mass Transf., vol. 21, no. 7, pp , Jul [19] S. W. Van Sciver, Helium Cryogenics, 2nd ed. New York, NY, USA: Springer-Verlag, 2012, pp