OPERATING INSTRUCTIONS FOR SUPERCONDUCTING MAGNET CRYOSTATS INTRODUCTION The Janis Research Company's Superconducting Magnet/Cryostat System is one of the most versatile tools available to the scientist today. The system will give many years of trouble free rewarding service, if the proper operating instructions as contained in this manual, are followed. Many options are available to increase the versatility of the basic system. Since all these options are not always incorporated into the typical unit, this manual treats the operation of the basic system. Optional items will be described in their own (enclosed) manuals. UNPACKING AND SET-UP Before removing the dewar (and enclosed magnet) from the shipping crate, it is advisable to have prepared a stand to support it. As a temporary expedient, the front side may be removed from the shipping crate, and the dewar left in the inner frame. In this manner, the dewar can be visually inspected for any damage incurred during shipment. Many dewars are equipped with a mounting flange, and for those so equipped, all support should be from this flange. For dewars not equipped with this flange, support the dewar upright on the outer tail (bottom) flange. CAUTION: AT ALL TIMES WHEN HANDLING OR SUPPORTING THE DEWAR, DO NOT EXERT ANY LATERAL FORCES ON ANY DEWAR TAILS, AS MISALIGNMENT OF THE TAILS AND POSSIBLE THERMAL SHORTS MAY RESULT. To remove the dewar from the shipping crate, first remove the tie down screws holding the mounting flange to the crate. Do not remove the large plastic capped screws, as these are the leveling screws for the mount. Most dewars are supplied with lifting lugs, and these can be used to lift the dewar (and magnet) vertically out of the crate. If this is not possible, the remainder of the shipping crate must first be dismantled. support@janis.com 1 www.janis.com
PREPARATION FOR COOL-DOWN Most dewars are shipped with the vacuum space evacuated. This is a result of the final testing at the factory, and it helps ensure a clean vacuum space. As a precaution against a deterioration of the vacuum, which arises sometimes during transit or a prolonged storage period, the dewar should be re-evacuated prior to use. This is best done with a good pumping station (e.g., a cold - trapped rotary/diffusion pumping station) capable of bringing the ultimate pressure down to approximately 10-5 Torr. Any inserts with independent vacuum jackets must also be evacuated at this time. An engineering drawing showing the details of your system is enclosed for your reference. Some units include shipping spacers, which must be removed prior to cooling. The spacers usually support the helium reservoir against the nitrogen reservoir (or radiation shield), which in turn is supported by additional spacers against the outer jacket. If your cryostat has these supports, an overhead crane will be needed to lift the nitrogen and helium reservoirs out of the outer shell. This will usually involve opening an O-ring seal, which can be easily re-assembled after removing the spacers. It will also generally require removal of a radiation shield, which is simply bolted to the bottom of the nitrogen reservoir. Once all shipping spacers are removed, the cryostat must be re-assembled and evacuated (and if possible leak checked). Every vacuum jacket is protected against cold leaks with a pressure relief valve, which will vent any pressure that exceeds 2 to 4 psig. This pressure relief is usually located at the evacuation valve. Provision for installation of a thermocouple vacuum gauge is usually located at the same evacuation valve. The evacuation valves supplied for the dewar and insert vacuums are mostly of the bellows sealed type. After evacuating the jacket, the valve should be firmly closed, but care should be exercised to avoid damaging the seat with too much pressure. When evacuation of the (dewar or insert) jacket is initiated, always be sure that the pressure on the pump side of the evacuation valve is lower than the jacket pressure. This is done to avoid drawing oil vapor from the pump into the vacuum jacket. Thus, one should not pump the vacuum jacket while liquid helium is in the dewar, since the liquid helium will usually cryopump to a lower pressure (10-6 Torr less) than the pumping station in use. The helium reservoir containing the magnet is provided with at least one helium vent port or pumping arm. This may be also used to evacuate the helium reservoir before and during cool-down in order to remove any air, moisture or nitrogen gas (see below). CAUTION: NEVER PUMP ON THE HELIUM RESERVOIR UNLESS THE DEWAR IS UNDER VACUUM. THIS COULD RESULT IN MAJOR DAMAGE (COLLAPSE) TO THE DEWAR AND THE ENCLOSED MAGNET. The helium reservoir vent or pumping arm will usually end with a rubber stopper or a spring loaded, quench relief flange. This acts as a helium vent/relief in case of a magnet quench. support@janis.com 2 www.janis.com
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COOLDOWN PROCEDURE 1. General Before introducing any liquid cryogens into the dewar, check carefully for water in both the nitrogen and helium reservoirs. This is particularly important for dewars with an open nitrogen reservoir, due to condensation on the walls during warm up from a previous run. If not removed, this water will freeze upon cooling, and could cause severe damage to the dewar, magnet and any cryostat insert. If possible, evacuate the helium reservoir (with the dewar jacket under vacuum - see last caution) in order to remove all the air, and then back fill with helium gas. Similar or slightly different procedures will also be necessary for any cryostat inserts present in the system. Such procedures will be described in the enclosed description for the insert (e.g., "SuperVaritemp" System Operation). In either case, one should at least purge the helium space thoroughly with helium gas, and isolate it with one-way valves (e.g. pressure relief) to prevent air from entering back into this space. 2. Liquid Nitrogen Transfer (a) Nitrogen Reservoir The nitrogen reservoir should now be filled with liquid nitrogen. If an automatic device is supplied, check the instructions for this device. Otherwise, maintain a steady flow of liquid into the reservoir, but not so fast that a large spattering occurs. It is always advisable to maintain the liquid nitrogen level as high as possible, since the efficiency of some dewars will decrease rapidly when the nitrogen level drops far below the top of the reservoir. (b) Helium Reservoir It is very import to cool the helium reservoir, the magnet and any cryostat insert to as close to liquid nitrogen temperature as possible. This must be done to conserve the amount of liquid helium necessary for final cooldown to 4.2 K. This can be done in one of two ways. The first method involves direct transfer of liquid nitrogen into the helium reservoir. This is the faster method, and could take from one to several hours to cool the magnet/cryostat assembly to 77 K. The temperature may be monitored by a thermometer inside the reservoir, or judged by the amount (or lack) of boiling, of the liquid nitrogen. The next step is to remove all the liquid nitrogen from the helium reservoir. This can be done by inserting a tube, which reaches the bottom of the helium reservoir, and blowing the liquid nitrogen out through this tube, by over-pressurizing the reservoir (with helium or nitrogen gas). If an initial helium fill mechanism is available with your cryostat, it should be used for this process. All vent ports or pressure relief valves (on the vapor cooled magnet leads) should be sealed to allow pressurization (5-10 psig). Liquid nitrogen will start flowing out of this tube once the helium reservoir is pressurized. This process should continue for support@janis.com 4 www.janis.com
about five minutes after it appears that no liquid nitrogen is flowing out of this tube. This is to ensure that all the liquid nitrogen is removed from the helium reservoir. Note that solid nitrogen has a very large heat capacity (an order of magnitude greater than copper), so even an inch or two left at the bottom of the reservoir will require large amounts of liquid helium to cool them to 4.2 K. The helium reservoir must now be evacuated and back filled with gaseous helium. The pressure inside the helium reservoir must drop to about one torr. Should the pressure level off at about 90 torr, this may indicate the presence of liquid (or frozen) nitrogen in the helium reservoir. The pressurizing and pumping procedure should be repeated to ensure that all the nitrogen has been removed, and the helium reservoir filled with helium gas. The second method of cooling the helium reservoir (and its contents) to nitrogen temperature is to fill the nitrogen reservoir with liquid nitrogen, while maintaining an over pressure of helium gas in the helium reservoir. A combination of radiational and conductive cooling will eventually cool the helium reservoir close to nitrogen temperatures. This process will take at least overnight, and possibly up to several days (in bigger units) for the helium reservoir/magnet/insert to cool down. If an over pressure of helium gas is not used, one should seal the reservoir (after purging it with helium gas) with a one way valve to prevent air from entering when the temperature (and the pressure) of the helium reservoir drops. For either of the two methods above, any cryostat insert in the helium reservoir should be kept either under continuous pumping (SuperVaritemp), or be sealed with only helium gas in the sample space (exchange gas inserts). A separate instruction sheet is usually enclosed to describe the cooldown and operation of any insert included. 3. Liquid Helium Transfer In many cases, a liquid helium transfer line is supplied with the system, with a built-in "initial fill" adapter in the helium reservoir. The purpose of such an adapter is to bring the liquid helium entry point below the magnet during the initial helium fill. The transfer should start at a steady slow rate in order to make full use of the cooling power of the cold helium vapor as it rises and escapes through the helium vent tube(s). This rate should be maintained for about 15-30 minutes, and the temperature inside the reservoir monitored carefully (if this option is included). When the temperature inside the reservoir drops to approximately 10 K, the liquid helium transfer rate should be increased, and the helium level monitored through any supplied level probe. The amount of liquid helium needed will be a function of the size of the reservoir, the coil, as well as the efficiency of the transfer. For the smaller reservoirs, about five liquid helium liters, plus one liter per five pounds of magnet is required to cool the reservoir and magnet from 77 to 4.2 K. This, along with the overall capacity of the helium reservoir, should offer a rough figure for the amount of liquid helium required for initial cool-down. A simple method, which tells you when to increase the helium transfer rate, is to monitor the resistance of the coil (with the power supply leads disconnected) with an ohmmeter. Once the resistance drops to zero (the coil is superconducting), this indicates that the magnet has cooled down sufficiently and thus liquid helium should easily collect. When the liquid helium level meter indicates that the helium reservoir is full, the transfer should be terminated, and the transfer line removed. If an initial helium fill tube has been used in the transfer, it support@janis.com 5 www.janis.com
should also be removed to reduce the heat load into the liquid helium. All fill and vent ports should now be sealed, with the helium venting through the pressure relief valves supplied on the helium reservoir It is important to maintain the liquid helium level above the coil at all times. This is absolutely necessary when a current is passing through the superconducting coil, in order to prevent a magnet quench and possible damage to the coil. With this in mind, any subsequent liquid helium transfers should be made above the level of the remaining liquid in the helium reservoir. This can be done without the initial fill line (if supplied), or simply by keeping the tip of the transfer line above the remaining liquid. support@janis.com 6 www.janis.com
SUPERCONDUCTING MAGNET OPERATION A. General Precautions During the operation of any superconducting magnet system, certain hazards to the operator and the equipment exist, and should be noted. The following precautions should therefore be taken before any attempt is made to charge the magnet. Remove (or tie down) any objects (tools, screws, etc.) that could be magnetized and attracted to the magnet when the field is turned on. Also remove any personal articles such as watches or credit cards that could be affected by intense magnetic fields. This is particularly important in large coils and units with a room temperature bore. Make absolutely sure that the liquid helium level is above the superconducting coil before the field is turned on, and throughout the whole time that current is passing through the coil. Failure to do so may result in a magnet quench (sudden loss of field associated with the coil going "normal") that could damage the coil. Also, any quench relief (usually located at the helium reservoir pumping arm) should be checked for proper operation, in order to protect the dewar and operator in the unlikely event of a magnet quench. Exercise extreme care when handling the power circuit for the magnet. In particular, never disconnect the power supply from the circuit while it is providing a current to the magnet. A lethal voltage may develop at the terminal being disconnected. This is due to the large change in flux and associated large emf that can develop when a sudden interruption in the current (and magnetic field) occurs. The proper procedure for handling the magnet power circuit will be detailed below. For systems that are supplied with a bi-polar magnet power supply, please refer to the accompanying manual and familiarize yourself with the instrument prior to attaching it to the magnet circuit. Normally the voltage taps and high current leads are connected with matching polarity, so that charging the magnet with a positive current will generate a positive voltage across the magnet. If this does not occur, it is an indication of a wrong polarity and the leads can be reversed. Make all the necessary electrical connections according to the enclosed pin wiring diagrams. These could include the persistent switch heater, magnet voltage taps, magnet current, etc., in addition to the main magnet leads and any other sweep or secondary coils supplied. Particular importance should be given to the high current magnet circuit, where any loose contacts could result in dangerous overheating during the current flow. Such a situation can occur with detachable leads if they have been detached and air has been accidentally allowed in the reservoir and has condensed at the joint prior to re-attaching this joint. support@janis.com 7 www.janis.com
Make certain that the liquid helium level is above the superconducting coil, and preferably up to its highest recommended level. B. Charging the Magnet Most systems are supplied with a magnet power supply, which contains a sweep generator to charge the magnet at various rates, and special protection (usually diodes) to protect against voltage mismatch with the magnet. They also contain appropriate meters to monitor the current passing through the circuit and the voltage output of the power supply and the magnet voltage. In addition, they have a switch to allow ramping the current (up or down) in the magnet, or pausing at some specific current during the sweep. Some units have special circuits (such as energy absorbers) to allow relatively quick discharges for magnets with large inductance, as well as other features such as quench detectors, fast ramp switches or computer interfaces. The following procedure assumes that a standard magnet power supply with a sweep generator is available. The enclosed manual for the power supply will describe any additional features provided. Check the enclosed magnet specifications and note the rated field, the rated current at that field and the recommended charging voltage. Also check whether a persistent current switch is supplied at the coil. Never exceed the rated current indicated in these specifications. In some systems one current may be rated at 4.2 K, and a higher one at lower temperatures, so care should be taken to reach the lower temperature before increasing the current to the higher value. Make all the necessary electrical connections according to the enclosed pin wiring diagrams. These could include the persistent switch heater, magnet voltage taps, magnet current, etc., in addition to the main magnet leads and any other sweep or secondary coils supplied. Particular importance should be given to the high current magnet circuit, where any loose contacts could result in dangerous overheating during the current flow. Make certain that the liquid helium level is above the superconducting coil, and preferably up to its highest recommended level. Read the supplied manual for the superconducting magnet power supply and turn the power supply on, then let it warm up (between 15 & 30 minutes). Set the voltage limit to its maximum values (typically 5-10 volts). Set the current limit to a value equal to or less than the rated magnet current. This should be checked by shorting the high current cables (from the power supply), before attaching them to the magnet, ramping the current, and checking the final value. The power supply has meters to monitor the magnet current and voltage, plus a sweep generator to help ramp the current up, down or pause the current at some intermediate value. support@janis.com 8 www.janis.com
Make all the necessary electrical connections according to the pin wiring diagram supplied. This will include voltage taps, magnet high current leads, persistent switch heater, etc. The power supply should be turned off when attaching these wires. If the magnet has a persistent current switch, the persistent switch heater power source should be turned on, in order to heat up the switch and turn it "normal". Wait about 15 seconds before charging the magnet. The persistent switch heater current will already have been stored in the programmer, as listed in the magnet specifications. Calculate the fastest allowable charging rate (di/dt) from the recommended magnet charging voltage (V) and its inductance L, using the relation V = L.(dI/dt). If V is in volts (usually 1 to 3 volts for most laboratory magnets), and L is in Henries (usually 10 to 20 Henries), then the charging rate is obtained in Amperes/second. This rate will give the shortest allowable time to bring the magnet to its rated current and field. If this rate is exceeded, it could result in generating too much heat in the coil, which can force it into its normal (resistive state), causing the magnet to quench and dissipate all its stored energy in the helium reservoir. The magnet could also quench if it is charged to a current higher than its rated current. Set the charging rate, and start charging the magnet by turning the sweep generator ramp switch to its "up" position. The current in the magnet starts increasing and the voltage across the magnet should slowly stabilize as it reaches the value LdI/dt Once the current limit is reached, the voltage across the magnet should drop to zero (current is constant), and the power supply output voltage should also drop close to zero (since the lead resistance is quite small). During this charging process, intermediate values of the current (and field) may be achieved by using the Pause position on the ramping switch. For magnets with a persistent current switch, the switch heater power supply may now be turned off to bring the switch to its superconducting mode. The current in the coil will now flow in a closed (superconducting) loop, and the current passing through the magnet leads may now be reduced to zero in order to eliminate Joule heating in those leads. This can be done by ramping (fast ramping is allowed) the current from the power supply down to zero. Do not change the setting on the current limit and do not disconnect the power supply from the circuit. If the persistent switch needs to be turned back to its "normal" state (e.g., for changing the field), the current should be ramped back up to the same value it had when the magnet was placed in the persistent mode. This ensures that this same current is again passing through the leads. Only at this point should the persistent switch heater power supply be switched on to change the persistent switch back to its "normal" mode. This interrupts the closed superconducting loop in the coil, and returns the power supply and magnet leads into the same circuit as the coil. To discharge the coil, the current should be ramped down at a rate no faster than the maximum charging rate. Never use the fast ramping switch on the power supply while discharging the magnet. As the current decreases, the voltage across the magnet reverses the sign, and remains at a non-zero value until the magnet is fully discharged (or the discharge stopped at some intermediate current value). support@janis.com 9 www.janis.com
CAUTION: DO NOT DISCONNECT THE POWER SUPPLY FROM THE CIRCUIT AS LONG AS ANY CURRENT IS FLOWING. A LETHAL VOLTAGE CAN OTHERWISE DEVELOP AT THE TERMINAL BEING DISCONNECTED. Once the current drops to zero, turn the persistent switch heater off, then turn off the power supply first and the programmer off last. The system can then be allowed to warm up gradually to room temperature. Once the coil is "normal", it is then quite safe to disconnect the power supply or cables from the circuit. Any safety pressure relief valves should be kept in position to allow venting of the cryogen gases as the dewar warms up, and prevent any air or moisture from condensing inside the dewar. TROUBLESHOOTING All power supplies and other electronic components supplied as part of your superconducting magnet/cryostat system have their accompanying manuals. The operator should always refer to these manuals for proper operating procedures, and in case of any difficulties with these parts. Should any difficulty arise with the dewar or superconducting coil supplied, it is recommended that you contact Janis Research before any repair work is undertaken. In addition to the description of the difficulty, it is always helpful to obtain the model and serial number of the system in question. This will enable our scientists to uniquely identify this system, and help in resolving any difficulty that may arise. support@janis.com 10 www.janis.com