A superconducting solenoid with a conductive bore tube is under construction for use with the time projection chamber (TPC) detector at PEP. It will be a uniform induction of 1.5 T over a 6.3 m/sup 3/ volume. Its stored energy will be 11 MJ while maintaining a radiation thickness of 0.3 radiation lengths for the coil package. The coil will operate at a current density of 7 x 10/sup 8/ Am/sup -2/ and it will be cooled by force flow two phase helium in a tube. The final design details are given here

Burns, W.A.

Eberhard, P.H.

Garfinkel, B.

Gibson, G.H.

Green, M.A.

Miller, P.B.

Ross, R.R.

Smits, R.G.

Taylor, J.D.

Van Slyke, H.W.

UNT Digital Library

A MAGNET SYSTEM FOR THE TIME PROJECTION CHAMBER AT PEP H. A. Green, P. H. Eberhard, J. D. Taylor, H, A. Burns, B. GarfIntel, G. B* Gibson, P. B. Killer, R. R. Ross, R. G. Smlts, and H. V. Van Slyke* ABSTRACT A superconducting aolrnoid with * conductive bore tube is under construction for use with the time projec­tion chamber (TPC) detector at ?EP. It will be a uniform induction of 1.5 T over a 6.3 m^ volune. Its stored energy will be 11 MJ vhlle maintaining a radlc-tion titickness of 0.3 radiation lengths for the coil package. The coll vlll operate at a current density of 7 x 10 8 Am"2 and it will be cooled by force flow two phase helium is a tube. The final design details are given here. I. INTRODUCTION High Energy Physics has historically played a major role in the development of superconducting magnet technology. The new colliding beam facility PEP which is under construction at the Stanford Linear Accelerator Center requires new technology in ••any of the experiments which are being built. The PEP-4 experiment, which is a collaboration of scientists from LEL, UCLA, CCR, Johns Hopkins and Yale, has an entirely new type of detector which is surrounded by a lar*>e thin superconducting solenoid.1 The choice of a superconducting solenoid is dictated by both economics and physics. The PEP-4 experiment, which also is called the TPC experiment, is built in layers like an onion (see Pig. 1). The time projection chamber (TPC), which is the heart of the experiment, is in a magnetic field which bend:, charged particles in order to measure theit momentum. The next layer is the super­conducting solenoid which generates this field. Other detectors such as the argon calorimeters and muon detectors detect and analyze neutral particles and particles which are very penetrating. These detectors are outside the solenoid. The TPC solenoid provides an induction of 1.5 T in a room temperature volume 2.04 m in diameter and 3.25 m long. This magnetic field should have good uniformity In order that the TPC function correctly. The magnet is thin from a radiation standpoint so that Y r&ys and electrons can be detected outside the coll. The TPC solenoid Is almost totally surrounded by experimental apparatus. The Inside bore of the magnet is filled with the TPC, its electronics and two end cap argon gas calorimeters which are attached to the iron poles. The outside surface of the cryostat Is surrounded by argon calorimeters and drift chambers. Service from the endB Is restricted became nearly 25000 wire pairs from the TPC which must be brought out past the end of the magnet. In reality services for the solenoid can only be supplied from the outside corner on either side of the supports at 3, 7, and 11 o'clock at th.; north end of the magnet. II. BASIC PRINCIPLES OF THE TPC MAGNET DESIGN The high energy physics use of the magnet has led to a rather unconventional magnet design. The TPC magnet is unusual in the following ways: 1} The coll is thin; the total radiation thickness of the magnet is 0.65 radiation lengths (including an 11 atm pressure vessel for the TPC). 2) The current density in the coil superconductor is an order of magnitude higher Lavrence Berkeley Laboratory, University of California, Berkeley, CA 94720 Superconducting Coil Pb-Sclnl. Shomr Cvrlolty i Iron IWi itport H I t-fmt m «o taomt of «fc I ^ . ^ ^ ^ M ^ T J r T S T i ^ F l 8 * l * A c r 0 ° « c t l o n o f t h e ra experiment « m E ! o n i m b c M t i t t o n . t t h k H i p f a y » l ^ and i t s companion PEP-9 experiment. ISy™"*- ***»* °» l»»ll««t « I M M ay b - | • • • * • * , or NprmaK tut Hi m M ". UNLIMITED 1 r than conventional superconducting magnet of the sane size and stored energy. The high coil current density Is dictated by the lev radiation thickness requlreaent. 3) The Magnet is cooled by a forced two phase tubular cooling system. This feature eliminates a cryogen vessel and all of its necks for relief valves and other services. The IPC nagnet has a number of other advantages over conventional superconducting nagnets. These Include lev cold xass, enhanced cryogenic safety, in short cooldown tima and lover capital cost. The TPC magnet design principles were developed over the last three years, and a number of test coils have been built. 2» 3» 6 High current density operation of the coil Is possible because there are shorted secondary circuits which carry the magnet current during a magnet quench. The shorted secondary circuits affect the quench process in the following way: 1) lhe shorted secondary permits the current to shift- from the coil to itself as the quench proceeds. This reduces the hotBpot temperature in the superconductor and it permits the shorted secondary to absorb a substantial amount of the magnet stored energy. 2) The Bhorted secondary circuit permits fast coil current decay while limiting the transient voltages In the magnet. 3) The shorted secondary causes the entire coil to become normal long before it wculd through ordinary quench propagation. This phenomenon is called "quench back". 4) The shorted secondary circuit pennitB effective methods of "dynamic quench protection" to be used, which would not be used on conventional superconducting COIIB.3 Since the use of shorted secondarys penults sefe high current density operation of the coil, the TPC magnet can be made adiabatically Btable rather than cryogenically stable. As a result, tubular two phase cooling can be used Instead of a helium bath vessel. High current density operation of the coll reduces the cold mass of the coll. The combination of tubular cooling and low coll mass will greatly simplify the cryogenic system for the magnet while enhancing its cryogenic sifety. III. A DESCRIPTION OF THE TPC MAGNET SYSTEM The TPC magnet consists of five primary subsystems. They are: 1) The coll package, 2) The cryostat vacuum vessel and coil package support system, 3) The helium refrigeration distribution system, 4) The power supply and quench protection system, and 5) The Iron poles and return yoke. This report deals with the first four subsystems, but the iron pole and return yoke are importan. if the field uniformity requirements for the TPC are to be met. Table 1 presents the parameters of the magnet and a number of itB subsystem parameters. The coll package consists of three layers of material which are wound on an annealed 1100 aluminum bore tube which has an RRR (residual resistance ratio) of 25. (Figure 2 shows a cross section of the coll package.) A layer of ultra pure aluminum (RRR>1500) insulated turn to turn by spacers made from fish line is wound in the bore tube. The ultra pure aluminum and bora tube are the aborted secondary circuits, the key to quench protection. During a quench, the ultra pure aluminum shifts current from the coil quickly causing quench back while the bora tube absorbs most of the nagnet atored energy. Two layera 1.0 x 3.7 ma copper based superconductor are wound over the ultra pure aluminum. (Tabic 1 shows the basic superconductor parameters.) The coil layer to layer and layer to ground Insulation Is epoxy impregnated glass tape. The final layer Is the forced flc*» cooling tube. Three oval tubas are wound, but only one carries two phase helium. The layer of cooling tube helps to carry some of the magnetic forces The magnet cryostat vacuum vessel consists of inner and outer aluminum cylinders. See Fig. 3 for a view of the magnet coil package in its vacuum vessel. The inner cylinder Is also an ASHE Coded 11 at* pressure vessel for the TPC gas. The helium is contained in the cooling tuba which has a pressure rating in excess of 100 atcu The magnet contains 'only about 65 liters of liquid helium, therefore, the ratio of vacuum volume to liquid volume (about 40) leads to a very safe cryogenic system. The magnet coil package is supported by compression rods at each end in alx places.3 (See Fig. 4.) The number of supports is dictated by deflection of the thin coil package. All services entering the magnet enter through the outer corner of the North vail of the vacuum vessel. The gas cooled electrical leads, which are at the 11 o'clock position, are completely burled within the vacuum enclosure. Liquid helium and liquid nitrogen enter and exit at the ~> o'clock position. Vacuum services are at the 3 o'clock position. Refrigeration will be supplied to the TPC magnet by a 200W refrigerator to be builr by CTi. A control dewer separated the refrigerator from the load. The control dewar system contiols both cool-down and normal opevatlon of the magnet. The control dewar itself contains 200 liters of liquid hell'im, two heat exchangers, a helium pump, and various valves. /V Squoshed 3/4" 0.0. Turj« soet r""̂  Jntutalod if, L JIi . J | L ,•' J/V tvAFV.V~>[wK*^ *Smm ClOJl Tope / \ , llOOJliuminum Fish line Cord Space Uninsulated Ultra Pure - 3mm Diameter Aluminum Wire 3mm Diameter 0 10 20 Fig, 2. A cross section of the TPC magnet coll package. Fig. 3. The end of the TPC coil package within the vacuum vessel {at a radial support rod). Outer Vacuum Vessfll Support Ring Fig. 4. 0 0.2 0.4 0.6 0.8 1 I U _ l L_ A cut away view of the north end of the TPC magnet within the vacuum vessel. The helium pimp provides backup circulation of liquid in the event of refrigerator failure. The control dewar Is connected to the coll cryostat with semi-flf.xlble shielded transfer lines which carry both liquid nitrogen and liquid helium to and from the c-.il. The magnet will be powered by a 3000 A six phase power supply which Is SCR regulated on the primary side of the transformer. The power supply can provide up to 10 V which Is sufficient to charge the magnet in as little as 40 nlnutes. The quench protection system for the magnet consists of 4 distinct elements. They are: 1) The 9.5 mm thick 1100 aluminum bore tube; 2) The layer of ultra pure aluminum 3) An SCR circuit breaker with a varlstor resistor across the leads, and 4) A capacitor bank which discharges into a center tap between the two layers of the coll. The first two quench protection elements are passive. The latter two elements are dynamic quench protection systems5 which require the quench to be detected quickly. Both of the dynamic quench protection methods have been tested on a 2 HJ thin solenoid. This work la reported In the proceedings of this conference.? ACKNOWLEDGMENTS This work has bean performed under the auspices of the basic research division of the United States Department of Energy, The author* gratefully acknowledge the efforts of the following people In their role as contributors to the TPC msgnst project: A. P. Baton*, 1. X. Brown, C. L. Covey and X. E. L«c. "A Proposal for a PEP Facility Baaed on the Time Projection Chamber", PEP-4, December 1976. H. A. Green, "Large Superconducting Detector Magnets with Ultra Thin Colla for Use In High Fnargy Accelerators and Storage Rings", Proceedings of the 6th International Conference on Magnet Technology. August 1977, LBL-6717. M. A. Green, et al.,"A Large High Current Density Superconducting Solenoid for the Time Projection Chamber Experiment11! to be published in the Proceedings of the 7th International Cryogenic Engineering Conference, London, England, July 1978, LBL-7933. J. D. Taylor* et al., "Quench Protection Studies on a 2 HJ Magnet", this proceedings, September 197B, LBL-7S76. TABLE I The Basic Parameters of the TPC Magnet And it& Major Subsystems Magnet Package Parameters Vacuum Vessel Warm Inside Diameter Vacuum Vessel Warm Outside Diametur Vacuum Vessel Length Dlstaace Between Poles Package Mass Coil Package Parameters Coil Diameter Coll Length Coil Package Thickness Number of Turns in Coil Number of Turns of Pure Aluminum Coil Inductance with Iron Coll Package Cold Mass Superconductor Parameters 2.040 tn 2.440 m 3.840 m - 3.26 m 4400 kg 2.168 m 3.300 m -33 mm 1740 550 4.26 H 1440 kg Type of Conductor Nb-Tl Bare Matrix Dimension 0.9 x 3.6 ran Insulated Matrix Dimension 1.0 x 3.7 mm Insulation Type Formvar Copper to S/C Ratio 1.8 to 1 Number of Filaments 2200 Filament Diameter -26ym Twist Pitch -40mm Critical Current @ 4.2K & 2. .01 3400 A Magnet Design Electrical Parameters Design Central Induction Induction Uniformity in TPC Coll Current Magnet Stored Energy Superconductor Matrix Current Density EJ 2 Product*f5 Magnet Cooling System Parameters Static Refrigeration Load lead Gas Flow Cooling Circuit Length Cooling Tube Area Cooling Circuit Design Mass Plow Magnet Operating Temperature Two Phase Cooling Pressure Drop Cooling Tube Volume Estimated Magnet Cool Down Tine 1.5 T ±0.015 T 2270 A 11.0 x 106j 7.00 x,10 eAm - 2 5.4 x 10 2 4 JAV-* 30 H -0.25 ga - 1 360 a 177 m2 12 RB-1 ~4.a°K "0.4 atu 64 liters -30 hours 

Magnet system for the time projection chamber at PEP

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Magnet system for the time projection chamber at PEP

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