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Low-pressure pseudospark switches for ICF pulsed power
Nuclear Instruments and Methods in Physics Research A 415 (1998) 327—331
Low-pressure pseudospark switches for ICF pulsed power K. Frank*, Ch. Bickes, U. Ernst, M. Iberler, J. Meier, U. Prucker, M. Schlaug, J. Schwab, J. Urban, D.H.H. Hoffmann Physics Department I, University of Erlangen-Nuremberg, Erwin-Rommel-Strasse 1, D-91058 Erlangen, Germany
Abstract Hollow-electrode pseudospark switches are gas-filled, low-pressure, high-current plasma switches which are based on cold cathode emission. They have the capability to satisfy at least a part of switching requirements for different applications in ICF drivers. The main purpose of the submitted paper is therefore to discuss the following realistic ways for the use of pseudospark switches. There are intense international activities aimed at investigating different approaches for the ignition of an ICF capsule. Most of these efforts utilize lasers of varying wavelengths to deliver the energy to initiate the ablation of the target, the compression and ignition of the fuel, and the propagation of the fusion burn. One alternative to this scheme is to provide the drive energy in form of a light ion beam produced by an efficient pulse power accelerator. A related method uses beams of heavy ion beams from high intensity versions of traditional high-energy accelerators. Dependent on the ICF driver for the power conditioning unit (PCU) arise totally different demands. These extremely different requirements mainly rely on the very specific character of the load. Flashlamps, pumping high power lasers represent a non-linear, low-impedance load. Relatively low switching voltage is necessary, but a high chargetransfer capability. Induction cells or magnetic compression units have a high impedance. Consequently high voltage (up to several 100 kV) is required to feed the energy in Marx modules and the following voltage adders produce megavolt voltages, which determines likewise the specific data of the used switch. ( 1998 Elsevier Science B.V. All rights reserved. Keywords: Pulsed power systems; Plasma switches; Gas discharge; Electrodes; Spectroscopy
1. Introduction Hollow-cathode pseudospark switches are gasfilled, low-pressure, high-current plasma switches which are based on cold cathode emission [1]. This makes the construction simpler than with traditionally used ones, but this has to be paid with
* Corresponding author. Tel.: #49 9131 85 7147; fax: #49 9131 85 8774.
electrode erosion mainly due to arc spot formation. Although successful experiments have been performed to eliminate this process, i.e. by multichannel operation, there still remains the challenge to find, besides refractory metals, an electrode material with superior performance. Parts of sintered or impregnated silicon carbide (SiC, SiSiC) or boron nitride (BN), pressed and shrunk into the bulk electrode material, yield a diffuse switch plasma development during arc formation [2].
0168-9002/98/$19.00 ( 1998 Elsevier Science B.V. All rights reserved. PII: S 0 1 6 8 - 9 0 0 2 ( 9 8 ) 0 0 4 0 3 - 3
IV. ACCELERATORS
328
K. Frank et al. /Nucl. Instr. and Meth. in Phys. Res. A 415 (1998) 327—331
At lower peak currents there is a limiting factor for application of pseudospark switches in pulsed electrical circuits with respect to suddenly appearing events of current interruption. This effect is well-known as current quenching. In contrary to thyratrons the pseudospark switch shows quenching at relatively low discharge currents (I ( $*4 5—10 kA) [3]. By adding a small amount of heavy noble gas (i.e. krypton) to the working gas D this 2 effect can be suppressed completely. By this and new semiconductor electrode the dynamic range of high voltage operation could be enlarged from not less than 100 V up to more than 30 kV with a twostage design. Based upon continuous efforts in fundamental research combined with the application of new technologies prototypes of “sealed-off” pseudospark switches are under construction for two specific applications (see type 1 and type 2 in Table 1). The most important data about the requirements are summarized in Table 1. It is exactly the 10th anniversary that the first experimental results about the development of high-current pseudospark switches were reported at the 6th International Pulsed Power Conference [4]. Over the last decade medium power pseudospark switches (20 kV/15 kA) achieved technological standard of commercial competitors. In a next strong effort “sealed-off” high power switches (20 kV/100 kA) are envisioned for pulsed power application, especially to replace ignitrons and spark-gaps, following the original goal.
Table 1 Operational data for the pseudospark switches Type 1 and Type 2 Operational data
Type 1
Type 2
Hold-off voltage º (kV) Peak current I (kA) 1 Pulse length ¹ (ls) Repetition rate f (s~1) Lifetime (no. of pulses)
10(º(35 30(I (100 1 5(¹(30 0.1(f(5 '106
0.1(º(20 0.1(I (20 1 1(¹(40 0.0001(f(10 '105
in the shape of waveform, jitter and delay, no irregularities like impedance drop and quenching. The charge injection trigger has the highest lifetime, but this advantage is compensated by its strong pressure dependence of operation. On the other hand, there is tremendous need to replace shortliving spark gaps in pulse generators driving magnetoforming units or systems for rock fragmentation. In this case very high currents and high charge transfer per shot stress the trigger unit extremely. The high plasma density during the main discharge formerly has caused severe erosion with the old surface discharge trigger system. Sometimes, a closure of the trigger gap occurred by metallic droplets released from the electrode. Summarized: both systems are not able to meet the demands from Table 1. Therefore, two novel trigger systems have been studied.
3. Novel trigger systems 2. Technological aspects on the way to “sealed-off” devices
3.1. Semiconductor surface discharge trigger
The commonly used trigger system for mechanism and high power pseudospark switches are the charge injection trigger, based upon a pulsed glow discharge, and the surface discharge trigger [1,5,6]. Both systems have shown very good reliability in the actual application. In order to compete with ignitrons in the trigger ability at low hold-off voltage however it is necessary to have an highly efficient trigger which is featured by an extremely high dynamic range of voltage operation from 1% to 99% of self breakdown voltage (see Table 1) between 100 V and 20 kV with unchanged performance
The trigger unit consists of a SiC-cylinder with a length of 28 mm and a diameter of 8 mm. At the right side of Fig. 1, it is extended by a graphite cylinder which contains the real metallic contact 2. Contact 1 is formed by a copper spring by means of which the surface discharge is initiated towards the contact 2, by applying a voltage pulse of 1—5 kV. The whole system is mounted on a ceramic plate which acts simultaneously as a protection against the plasma of the discharge. The unit is integrated into the hollow cathode at the rear side.
K. Frank et al. /Nucl. Instr. and Meth. in Phys. Res. A 415 (1998) 327—331
329
adaptation to the working range of gas pressure can be realized. 3.2. Pulsed dielectric trigger
Fig. 1. Sketch of trigger set-up.
The main difference to the old surface discharge trigger [7] is the long path of breakdown (38 mm). This provides an extended diffuse plasma corona which yields excellent delay and jitter values. In the old system the surface discharge ignited only over a distance of a tenth of mm, formed by the trigger gap. This configuration produced a localized high plasma density. In the new system the emitted pulsed current is large enough to trigger switches reliably down to 500 V hold-off voltage (see Fig. 2). Additionally the discharge characteristics of the surface breakdown can be manipulated by adjusting the impedance of the circuit, e.g. by an external resistance. By this an optimal
The system consists of a disc of high-dielectric (e+2000, H"15 mm, d"0.1—1.0 mm) material. Opposite to the discharge region the surface is metallized and carries the contact 1 (Fig. 3). !On the other side tiny finger contacts arranged on a circle of H 10 mm, form the contact 2 for the trigger pulse. Between the tips of these finger contacts and the ceramic there exist many tiny protrusions and cracks represented in detail in Fig. 4. If an high voltage pulse (º "1—4 kV, 53 q"0.25—1 ls) is applied to one of the contacts a very strong electric field (up to 108 V/cm) builds up the triple points of the finger contact. This causes efficient electron emission which can be used, depending on trigger voltage polarity, to trigger the switch. With a “sealed-off” device fitted with a dielectric trigger as described in Ref. [3] reliable triggering of the switch could be reproduced down to charging voltages of the switch as low as 80 V. Compared to an ignitron, used in the same application, this signifies a reduction of more than 200 V.
Fig. 2. Typical waveforms of high voltage trigger pulses and the correlated emitted current, measured by a Faraday cup.
IV. ACCELERATORS
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K. Frank et al. /Nucl. Instr. and Meth. in Phys. Res. A 415 (1998) 327—331
Fig. 3. Lay-out of a high dielectric trigger.
electrodes. The discharge is initially homogeneous and symmetrical around the central apertures with further expansion over the entire SiC surface at later stage of the discharge. The cross-section of the discharge plasma remains large even at high peak currents up to 120 kA, whereas metallic electrodes have the tendency to contract in a bright arc spot [8]
5. Conclusions
Fig. 4. Detail of finger contact.
4. Lifetime considerations for “sealed-off” systems High erosion rates are still the main drawback in high-current, high charge-transfer applications for pseudospark switches. As explained in Section 2 at high peak currents and with metallic electrodes the plasma contracts in unified arc spots at the cathode and anode with erosion rate one magnitude higher in comparison to a plasma stage maintained by a manifold of small cathode spots. In contrast to metal when graphite is used as cathode material, the erosion rate is small. Due to its porosity the permanent outgassing of the graphite lowers the hold-off voltage. As no practicable solution was available to avoid or to shorten the outbaking process of graphite electrode, this material was rejected. Furthermore, carbon dust led to an increased misfiring rate. Instead of graphite SiC (silicon carbide) has been chosen, primarily due to its negligible porosity. Up to now, there is no evidence in literature that this material had been formerly implemented for electrode material. Experiments with peak current up to 120 kA revealed that neither the current shape, nor voltage breakdown differ from those in the case of metal
There has been a considerable progress in understanding of physics and technology of high power pseudospark switches, which allows their implementation in technical applications. These switches have a very wide dynamic range of breakdown voltage operation: from 1% to 99%. Single-gap systems still have the disadvantage of small hold-off voltage, which is lower than 25 kV and which requires construction of two-stage systems. For high voltage application in this way a more reliable hold-off voltage as high as 30 kV would be achieved. Based upon better understanding of fundamental discharge processes it is possible to suppress the quenching phenomenon. This might be helpful in some applications for medical use such as kidney-stone disintegrator. Semiconductor electrode materials are the best choice for devices with considerably reduced electrode erosion and prevention from statistically distributed drop of impedance. The feasibility of a durable composite of semiconductor and metallic bulk electrode will facilitate tremendously the construction of “sealed-off” pseudospark switches for pulsedpower applications, especially to replace ignitron and spark gaps. Up to now the tested prototypes operate with hold-off voltages up to 20 kV, peak currents up to 150 kA and pulse length up to 20 ls. Tests with longer pulse length are planned in order to extend the commercial applicability of these devices.
Acknowledgements The authors would like to express gratitude to their colleagues Dr. Scheibe, U. Mandel (TU
K. Frank et al. /Nucl. Instr. and Meth. in Phys. Res. A 415 (1998) 327—331
Magdeburg), Dr. Herold (Hilo-Test Inc.) and Dr. Stingl, F. Wittig (CeramTech Inc.) for their assistance in this research. This project is funded by the German BMBF under contract Nos. 13N6803 and 13N6822.
References [1] K. Frank, O. Almen, P. Bickel, J. Christiansen, A. Go¨rtler, W. Hartmann, C. Kozlik, A. Linsenmeyer, H. Loscher, F. Peter, A. Schwandner, R. Stark, Proc. IEEE 80 (6) (1992) 958. [2] A. Go¨rtler, A. Schwandner, J. Christiansen, K. Frank, IEEE Trans. Plasma Sci. 21 (5) (1993) 1.
331
[3] L. Ducimetie`re, P. Faure, U. Janson, H. Riege, M. Schlaug, G.H. Schro¨der, E. Vossenberg, Pseudospark Switch Development For The LHC Extraction Kicker Pulse Generator, Conference Record of the 1996 22nd Int. Power Modulator Symp., Boca Raton, FL, 1996, pp. 149—152. [4] K. Frank, E. Boggasch, J. Christiansen, A. Go¨rtler, W. Hartmann, C. Kozlik, G. Kirkmann, C. Braun, V. Dominic, M.A. Gundersen, Proc. 6th Pulsed Power Conf., Arlington, 1987, p. 213. [5] D. Bloess et al., Nucl. Instr. and Meth. 205 (1,2) (1983) 173. [6] R. Tkotz, M. Schlaug, J. Christiansen, K. Frank, A. Go¨rtler, A. Schwandner, IEEE Trans. Plasma Sci. 24 (1) (1996) 53. [7] T. Mehr, H. Arenz, P. Bickel, J. Christiansen, K. Frank, A. Go¨rtler, F. Heine, D. Hoffmann, R. Kowalewicz, M. Schlaug, R. Tkotz, IEEE Trans. Plasma Sci. 23 (1995) 275. [8] A. Schwandner, A. Go¨rtler, J. Christiansen, K. Frank, D.H.H. Hoffmann, U. Prucker, Proc. 17th Int. Symp. on Discharges and Electrical Insulations in Vacuum, vol. II, 1996, 1014.
IV. ACCELERATORS
Low-pressure pseudospark switches for ICF pulsed power K. Frank*, Ch. Bickes, U. Ernst, M. Iberler, J. Meier, U. Prucker, M. Schlaug, J. Schwab, J. Urban, D.H.H. Hoffmann Physics Department I, University of Erlangen-Nuremberg, Erwin-Rommel-Strasse 1, D-91058 Erlangen, Germany
Abstract Hollow-electrode pseudospark switches are gas-filled, low-pressure, high-current plasma switches which are based on cold cathode emission. They have the capability to satisfy at least a part of switching requirements for different applications in ICF drivers. The main purpose of the submitted paper is therefore to discuss the following realistic ways for the use of pseudospark switches. There are intense international activities aimed at investigating different approaches for the ignition of an ICF capsule. Most of these efforts utilize lasers of varying wavelengths to deliver the energy to initiate the ablation of the target, the compression and ignition of the fuel, and the propagation of the fusion burn. One alternative to this scheme is to provide the drive energy in form of a light ion beam produced by an efficient pulse power accelerator. A related method uses beams of heavy ion beams from high intensity versions of traditional high-energy accelerators. Dependent on the ICF driver for the power conditioning unit (PCU) arise totally different demands. These extremely different requirements mainly rely on the very specific character of the load. Flashlamps, pumping high power lasers represent a non-linear, low-impedance load. Relatively low switching voltage is necessary, but a high chargetransfer capability. Induction cells or magnetic compression units have a high impedance. Consequently high voltage (up to several 100 kV) is required to feed the energy in Marx modules and the following voltage adders produce megavolt voltages, which determines likewise the specific data of the used switch. ( 1998 Elsevier Science B.V. All rights reserved. Keywords: Pulsed power systems; Plasma switches; Gas discharge; Electrodes; Spectroscopy
1. Introduction Hollow-cathode pseudospark switches are gasfilled, low-pressure, high-current plasma switches which are based on cold cathode emission [1]. This makes the construction simpler than with traditionally used ones, but this has to be paid with
* Corresponding author. Tel.: #49 9131 85 7147; fax: #49 9131 85 8774.
electrode erosion mainly due to arc spot formation. Although successful experiments have been performed to eliminate this process, i.e. by multichannel operation, there still remains the challenge to find, besides refractory metals, an electrode material with superior performance. Parts of sintered or impregnated silicon carbide (SiC, SiSiC) or boron nitride (BN), pressed and shrunk into the bulk electrode material, yield a diffuse switch plasma development during arc formation [2].
0168-9002/98/$19.00 ( 1998 Elsevier Science B.V. All rights reserved. PII: S 0 1 6 8 - 9 0 0 2 ( 9 8 ) 0 0 4 0 3 - 3
IV. ACCELERATORS
328
K. Frank et al. /Nucl. Instr. and Meth. in Phys. Res. A 415 (1998) 327—331
At lower peak currents there is a limiting factor for application of pseudospark switches in pulsed electrical circuits with respect to suddenly appearing events of current interruption. This effect is well-known as current quenching. In contrary to thyratrons the pseudospark switch shows quenching at relatively low discharge currents (I ( $*4 5—10 kA) [3]. By adding a small amount of heavy noble gas (i.e. krypton) to the working gas D this 2 effect can be suppressed completely. By this and new semiconductor electrode the dynamic range of high voltage operation could be enlarged from not less than 100 V up to more than 30 kV with a twostage design. Based upon continuous efforts in fundamental research combined with the application of new technologies prototypes of “sealed-off” pseudospark switches are under construction for two specific applications (see type 1 and type 2 in Table 1). The most important data about the requirements are summarized in Table 1. It is exactly the 10th anniversary that the first experimental results about the development of high-current pseudospark switches were reported at the 6th International Pulsed Power Conference [4]. Over the last decade medium power pseudospark switches (20 kV/15 kA) achieved technological standard of commercial competitors. In a next strong effort “sealed-off” high power switches (20 kV/100 kA) are envisioned for pulsed power application, especially to replace ignitrons and spark-gaps, following the original goal.
Table 1 Operational data for the pseudospark switches Type 1 and Type 2 Operational data
Type 1
Type 2
Hold-off voltage º (kV) Peak current I (kA) 1 Pulse length ¹ (ls) Repetition rate f (s~1) Lifetime (no. of pulses)
10(º(35 30(I (100 1 5(¹(30 0.1(f(5 '106
0.1(º(20 0.1(I (20 1 1(¹(40 0.0001(f(10 '105
in the shape of waveform, jitter and delay, no irregularities like impedance drop and quenching. The charge injection trigger has the highest lifetime, but this advantage is compensated by its strong pressure dependence of operation. On the other hand, there is tremendous need to replace shortliving spark gaps in pulse generators driving magnetoforming units or systems for rock fragmentation. In this case very high currents and high charge transfer per shot stress the trigger unit extremely. The high plasma density during the main discharge formerly has caused severe erosion with the old surface discharge trigger system. Sometimes, a closure of the trigger gap occurred by metallic droplets released from the electrode. Summarized: both systems are not able to meet the demands from Table 1. Therefore, two novel trigger systems have been studied.
3. Novel trigger systems 2. Technological aspects on the way to “sealed-off” devices
3.1. Semiconductor surface discharge trigger
The commonly used trigger system for mechanism and high power pseudospark switches are the charge injection trigger, based upon a pulsed glow discharge, and the surface discharge trigger [1,5,6]. Both systems have shown very good reliability in the actual application. In order to compete with ignitrons in the trigger ability at low hold-off voltage however it is necessary to have an highly efficient trigger which is featured by an extremely high dynamic range of voltage operation from 1% to 99% of self breakdown voltage (see Table 1) between 100 V and 20 kV with unchanged performance
The trigger unit consists of a SiC-cylinder with a length of 28 mm and a diameter of 8 mm. At the right side of Fig. 1, it is extended by a graphite cylinder which contains the real metallic contact 2. Contact 1 is formed by a copper spring by means of which the surface discharge is initiated towards the contact 2, by applying a voltage pulse of 1—5 kV. The whole system is mounted on a ceramic plate which acts simultaneously as a protection against the plasma of the discharge. The unit is integrated into the hollow cathode at the rear side.
K. Frank et al. /Nucl. Instr. and Meth. in Phys. Res. A 415 (1998) 327—331
329
adaptation to the working range of gas pressure can be realized. 3.2. Pulsed dielectric trigger
Fig. 1. Sketch of trigger set-up.
The main difference to the old surface discharge trigger [7] is the long path of breakdown (38 mm). This provides an extended diffuse plasma corona which yields excellent delay and jitter values. In the old system the surface discharge ignited only over a distance of a tenth of mm, formed by the trigger gap. This configuration produced a localized high plasma density. In the new system the emitted pulsed current is large enough to trigger switches reliably down to 500 V hold-off voltage (see Fig. 2). Additionally the discharge characteristics of the surface breakdown can be manipulated by adjusting the impedance of the circuit, e.g. by an external resistance. By this an optimal
The system consists of a disc of high-dielectric (e+2000, H"15 mm, d"0.1—1.0 mm) material. Opposite to the discharge region the surface is metallized and carries the contact 1 (Fig. 3). !On the other side tiny finger contacts arranged on a circle of H 10 mm, form the contact 2 for the trigger pulse. Between the tips of these finger contacts and the ceramic there exist many tiny protrusions and cracks represented in detail in Fig. 4. If an high voltage pulse (º "1—4 kV, 53 q"0.25—1 ls) is applied to one of the contacts a very strong electric field (up to 108 V/cm) builds up the triple points of the finger contact. This causes efficient electron emission which can be used, depending on trigger voltage polarity, to trigger the switch. With a “sealed-off” device fitted with a dielectric trigger as described in Ref. [3] reliable triggering of the switch could be reproduced down to charging voltages of the switch as low as 80 V. Compared to an ignitron, used in the same application, this signifies a reduction of more than 200 V.
Fig. 2. Typical waveforms of high voltage trigger pulses and the correlated emitted current, measured by a Faraday cup.
IV. ACCELERATORS
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K. Frank et al. /Nucl. Instr. and Meth. in Phys. Res. A 415 (1998) 327—331
Fig. 3. Lay-out of a high dielectric trigger.
electrodes. The discharge is initially homogeneous and symmetrical around the central apertures with further expansion over the entire SiC surface at later stage of the discharge. The cross-section of the discharge plasma remains large even at high peak currents up to 120 kA, whereas metallic electrodes have the tendency to contract in a bright arc spot [8]
5. Conclusions
Fig. 4. Detail of finger contact.
4. Lifetime considerations for “sealed-off” systems High erosion rates are still the main drawback in high-current, high charge-transfer applications for pseudospark switches. As explained in Section 2 at high peak currents and with metallic electrodes the plasma contracts in unified arc spots at the cathode and anode with erosion rate one magnitude higher in comparison to a plasma stage maintained by a manifold of small cathode spots. In contrast to metal when graphite is used as cathode material, the erosion rate is small. Due to its porosity the permanent outgassing of the graphite lowers the hold-off voltage. As no practicable solution was available to avoid or to shorten the outbaking process of graphite electrode, this material was rejected. Furthermore, carbon dust led to an increased misfiring rate. Instead of graphite SiC (silicon carbide) has been chosen, primarily due to its negligible porosity. Up to now, there is no evidence in literature that this material had been formerly implemented for electrode material. Experiments with peak current up to 120 kA revealed that neither the current shape, nor voltage breakdown differ from those in the case of metal
There has been a considerable progress in understanding of physics and technology of high power pseudospark switches, which allows their implementation in technical applications. These switches have a very wide dynamic range of breakdown voltage operation: from 1% to 99%. Single-gap systems still have the disadvantage of small hold-off voltage, which is lower than 25 kV and which requires construction of two-stage systems. For high voltage application in this way a more reliable hold-off voltage as high as 30 kV would be achieved. Based upon better understanding of fundamental discharge processes it is possible to suppress the quenching phenomenon. This might be helpful in some applications for medical use such as kidney-stone disintegrator. Semiconductor electrode materials are the best choice for devices with considerably reduced electrode erosion and prevention from statistically distributed drop of impedance. The feasibility of a durable composite of semiconductor and metallic bulk electrode will facilitate tremendously the construction of “sealed-off” pseudospark switches for pulsedpower applications, especially to replace ignitron and spark gaps. Up to now the tested prototypes operate with hold-off voltages up to 20 kV, peak currents up to 150 kA and pulse length up to 20 ls. Tests with longer pulse length are planned in order to extend the commercial applicability of these devices.
Acknowledgements The authors would like to express gratitude to their colleagues Dr. Scheibe, U. Mandel (TU
K. Frank et al. /Nucl. Instr. and Meth. in Phys. Res. A 415 (1998) 327—331
Magdeburg), Dr. Herold (Hilo-Test Inc.) and Dr. Stingl, F. Wittig (CeramTech Inc.) for their assistance in this research. This project is funded by the German BMBF under contract Nos. 13N6803 and 13N6822.
References [1] K. Frank, O. Almen, P. Bickel, J. Christiansen, A. Go¨rtler, W. Hartmann, C. Kozlik, A. Linsenmeyer, H. Loscher, F. Peter, A. Schwandner, R. Stark, Proc. IEEE 80 (6) (1992) 958. [2] A. Go¨rtler, A. Schwandner, J. Christiansen, K. Frank, IEEE Trans. Plasma Sci. 21 (5) (1993) 1.
331
[3] L. Ducimetie`re, P. Faure, U. Janson, H. Riege, M. Schlaug, G.H. Schro¨der, E. Vossenberg, Pseudospark Switch Development For The LHC Extraction Kicker Pulse Generator, Conference Record of the 1996 22nd Int. Power Modulator Symp., Boca Raton, FL, 1996, pp. 149—152. [4] K. Frank, E. Boggasch, J. Christiansen, A. Go¨rtler, W. Hartmann, C. Kozlik, G. Kirkmann, C. Braun, V. Dominic, M.A. Gundersen, Proc. 6th Pulsed Power Conf., Arlington, 1987, p. 213. [5] D. Bloess et al., Nucl. Instr. and Meth. 205 (1,2) (1983) 173. [6] R. Tkotz, M. Schlaug, J. Christiansen, K. Frank, A. Go¨rtler, A. Schwandner, IEEE Trans. Plasma Sci. 24 (1) (1996) 53. [7] T. Mehr, H. Arenz, P. Bickel, J. Christiansen, K. Frank, A. Go¨rtler, F. Heine, D. Hoffmann, R. Kowalewicz, M. Schlaug, R. Tkotz, IEEE Trans. Plasma Sci. 23 (1995) 275. [8] A. Schwandner, A. Go¨rtler, J. Christiansen, K. Frank, D.H.H. Hoffmann, U. Prucker, Proc. 17th Int. Symp. on Discharges and Electrical Insulations in Vacuum, vol. II, 1996, 1014.
IV. ACCELERATORS