An electrostatic separator with built-in high-voltage generators

NUCLEAR I N S T R U M E N T S

AND METHODS

148 (19781

203-207 : ©

NORTH-HOLLAND

P U B L I S H I N G CO.

AN ELECTROSTATIC SEPARATOR WITH BUILT-IN HIGH-VOLTAGE GENERATORS AKIRA YAMAMOTO, AKIHIRO MAKI and ASAO KUSUMEGI

National Laboratory /br High Energy Physics, Oho-machi, Tsukuba-gun, Ibaraki, 300-32, Japan Received 9 December 1976 and in revised form 23 May 1977 An electrostatic separator with high-voltage generators mounted directly onto the separator chamber is described together with the present performance of the separator of this type. A maximum voltage of 900 kV was obtained with the 3 m separator and of 800 kV with the 9 m separator for an interelectrode spacing of 10 c m

1. Introduction In the field of high-energy physics, electrostatic separators have been used extensively for the separation of secondary particles with momenta below approximately 6 G e V / c 1,2). A conventional way to supply high-voltages across the electrodes of electrostatic separators is to use special cables between the high-voltage generators and the electrodes through lead-in bushings3). This method of using high-voltage cables results in considerable technical problems, such as insulation of cables, complex end treatments with stress cones and oil treatments when connecting or disconnecting the cables. The high-voltage applied to the electrode of the separator is several hundred kV and therefore, in general, the size of the cable becomes large, 5-10 cm diameter. The heavy weight and the bulky stress cones at the terminals for both shielded and unshielded cables make the handling of these cables tedious and cumbersome. If unshielded cables were used to reduce the capacity of highvoltage cables, sparks along the cable surface cause operational problems, namely, severe electrical noises and damages to the equipment. An attempt had been made at Argonne to eliminate high-voltage cables by charging the electrode to the required potential with an energetic electron beam4). This technique has proved to be impractical, however. We have solved these problems entirely by mounting the high-voltage generators directly onto the vacuum chamber. As a result, no high-voltage appears outside the vacuum chamber. An additional benefit that has been realized later is that the stored energy is much less for this type of separator than for conventional ones with high-voltage cables that inevitably add more capacitance to the system. Therefore, the spark damage to the

electrodes and insulators is minimized. We obtained a maximum voltage of 900 kV with a 3 m separator and of 800kV with a 9 m separator across a gap spacing of 10 cm. We have operated these separators for several months and we anticipate further improvement mainly by additional conditioning.

2. Configuration of the KEK electrostatic separator 2.1. HIGH-VOLTAGE POWER SUPPLY

A compact high-voltage power supply has been developed and manufactured for the KEK elec-

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trostatic separatorS). The power supply consists of a pair of Cockcroft-Walton type high-voltage generators of positive and negative polarities and a control console containing oscillators, power amplifiers and regulators. Each of the high-voltage generators is a Cock croft-Walton type voltage multiplier of 30 stages capable of delivering a maximum voltage of 600kV with a current of up to l m A . . T h e high-voltage generator is contained in a rectangular steel vessel of outside dimensions 3 0 0 x 7 0 0 x 1 2 7 0 m m 3 with a lead-out bushing of 893 mm length and 100ram in diameter. The Cockcroft-Walton circuit and filter circuit are molded together and assembled in such a way as not to form an abrupt change of electric field inside the vessel. All the components were vacuumimpregnated with insulating oil, immersed in it and then the assembly was sealed in a vacuum. A series resistor chain is contained inside the leadout bushing. The total weight of the generator is 250 kg. The schematic diagram of the voltage generator is shown in fig. I. The control console contains an independent pair of high-frequency oscillators and regulators

SEPARATOR

205

for positive and negative polarities. The stability of the voltage is regulated to better than 1x 10 4 at 600 kV with a load current of l mA. The ripple voltage is less than 60 V..The block diagram of the high-voltage power supply is shown in fig. 2. 2.2. STRUCTUREOF KEK 9 m SEPARATOR The schematic view of the KEK 9 m separator is shown in fig. 3. The basic unit of the cylindrical chamber, 3 m long and 1.4 m inner diameter, is designed to form 6 m or 9 m separator by connecting two or three units in tandem. For normal operation a stainless-steel electrode, 400 mm wide and 2870 mm long, is used as the anode at the bottom and an anodised aluminium plate of the same dimension is used as the cathode at the top in the basic unit. These electrodes are placed on or hang by a pair of porcelain support insulators with recessed corrugation of 345 mm length. A pair of stress shields is used on each side of the support insulators. The gap spacing between these electrodes can be adjusted from 20 to 200 mm from the outside of the chamber. Each high-voltage generator is mounted on the top of the chamber through a porcelain bushing,

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206

A. Y A M A M O T O et al.

800 mm long and 250 mm diameter, with recessed corrugation. The high-voltage terminal of the leadin bushing is covered with a stress shield cap. 2.3. PUMPINGSYSTEMAND GAS SUPPLY The vacuum chamber is evacuated by a turbomolecular pump of 1000 I/s backed up by an oilrotary pump of 500 l/rain, and two ion pumps of 1000 I/s as a main system for the 9 m separator. An auxiliary system of a Roots pump of 19 000 l/rain backed up by an oil-rotary pump of 65001/min is prepared for the initial pumping from atmospheric pressure. The ultimate pressure is 3× 10-6 torr. The gate valves of the ion pumps are closed while gas is fed to the chamber. The schematic diagram of the pumping system for the 9 m separator is shown in fig. 4.

3. Present operation and performance A series of tests was performed on a model separator and the 3 m separator to obtain as high a maximum voltage across the electrodes as possible% These tests were: (1) test of the components, particularly, of lead-in bushings and support insulators, (2) the effect of conditioning, (3) the effect of the type of gas, (4) the effect of the electrode material, (5) the dependence on polarity, (6) the effect of deconditioning and (7) the effect of the dimensions of components and others. The results

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of these tests are described elsewhereT). Most of these data are similar to those obtained at other laboratoriesS), and some are a little different. Present performances of both the 3 m separator and the 9 m separator are shown in figs. 5 and 6, respectively. The data were obtained-with the conditions as described in sect. 2, the gap spacing was fixed at 100 mm and a gas mixture of neon and helium was usedg). The maximum attainable voltage for the 3 m separator was 900 kV. The sparking rate was rather high at this voltage, a few sparks per 10 rain. The rate went down to 4-5 sparks per hour at 850 kV and to 3-5 sparks per day at 800 kV. The deconditioning rate was 1 × 10 -5 tort per day at 700 kV. With the 9 m separator, the maximum attainable voltage was 800 kV across the electrodes. The sparking rate was - 5 sparks per hour at 800 kV. It went down to ~ 1 spark per hour at 750 kV and 2 sparks per day at 700 kV. The deconditioning rate was approximately the same as that of the 3 m separator.

4. Conclusion We have presented the results of the operation and performance of the 3 m and 9 m KEK electrostatic separators together with their design details. We believe that the separators with the built-in high-voltage generators are superior to those with conventional high-voltage cables. Many

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E L E C T R O S T A T I C SEPARATOR

cable manufacturers have developed more flexible and thinner cables than those used previously. However, the problem of the stored energy attributed to the cable capacitance is less in the present separator than in those with high-voltage cables. On the other hand, the present structure has a small disadvantage in that it requires more shielding blocks around the chamber compared to the usual separator design, because of the additional volume of the high-voltage generators. A more compact electrostatic separator with highvoltage generators has been developed at KEK. Another problem is the effect of radiation on the electrical properties of the high-voltage generator. The effect will be examined in the active beam lines in the near future. The authors are indebted to the other members of the beam line group of the laboratory for their continuing interest and useful discussions, and to Messrs. M. Taino and H. Ishii for their collaboration. The authors are grateful to Prof. K. Miyake of Kyoto University and Mr. K. Gomi of INS, Univ. of Tokyo, for initiating the work with the model separator and for useful discussions. Thanks are also due to the continuing effort of Nichicon Capacitor Ltd. for manufacturing the

207

high-voltage power supplies. One of the authors (A.K.) wishes to thank Drs. R. C. Sah and W. Kilpatrick of LBL, Dr. J. Murray of SLAC, Drs. R. Klein and R. Kustom of ANL, Dr. H. Foeische of BNL and Drs. D. Gray and A. Eastwood of RHEL for useful discussions. The other author (A.Y.) wishes to thank Drs. L. Danloy and R. Tinguely of CERN for illuminating discussions. Finally, the authors would like to thank Dr. D. I. Lowenstein of BNL for carefully reading the manuscript. References ~) P. Eberhard, M. L. Good and H. K. Ticho, Rev. Sci. Instr. 31 (1960) 1054. 2) K. H. Davies, Rutherford High Energy Laboratory, Report RHEL/R 222 (1971). 3) C. Germain and R. Tinguely, Nucl. Instr. and Meth. 20 (1963) 21. 4) D. J. DeGeeter and A. Yokosawa, Proc. Ist Int. Syrup. on Insulation o f h(gh-voltage in vacuum, M.I.T. (1964) p. 441. 5) Nichicon Capacitor, Kyoto, Japan. 6) A. Yamamoto, A. Maki, Y. Maniwa and A. Kusumegi+ Jap. J. Appl. Phys. 16 (1977)343. 7) A. Yamamoto, A. Maki and A. Kusumegi, .KEK Preprint76-4 (1976). 8) F. Rohrbach, High-voltage technolo,~ (ed L. L. AIston; Owford Univ. Press, London+ 1968) p. 350. 9) L. Danloy and P. Simon, Proc. 5th Int. Syrup. on Discharge and electrical insulation in vacuum, Poznan, Poland (1972) p. 367.