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A high stability accelerator for ion beam diagnostics in a tokamak
Nuclear Ins~ents Noah-dolled
and Methods in Physics Research B56/57
1033
(1991) 1033-1035
A high stability accelerator for ion beam diagnostics in a tokamak J.B. Schroeder, S.J. Lunstrum, J.R. Adney, S.H. Phillips and R.D. Rathmell National Electrostatics
Corp., Middleton,
WI 53562-0310,
USA
A 2 MY single ended accelerator has been developed for probing a Tokamak plasma with a heavy ion beam. This b&b current Pelletron is designed to deliver up to 200 PA of 2 MeV Tlf for plasma diagnostics. Stringent energy stability requirements have led to the development of an improved accelerator voltage control system incorporating a capacitive tank liner. The special design
features and performance of this system will be discussed.
1. Introduction
In 1985 National Electrostatics Corporation (NEC) delivered the first Heavy Ion Beam Probe to the Fusion Research Center at the University of Texas in Austin. This accelerator was purchased by Rensselaer Polytechnic Institute for inst~ation on the Texas Ex~~rnent~ Tokamak (TEXT) to serve as a non-invasive probe of the plasma potentials within TEXT during its 500 ms confinement pulse [l]. Thallium was the ion of choice due to its mass (205 amu) and the availability of a simple, low energy spread ion source. The voltage multiplier driven accelerator could produce up to 50 JLA of Tl+ at 500 keV with a maximum total beam energy spread of 50 eV, resulting in the ability to resolve plasma potential fluctuations of f 2.5 V [2]. Five years of nearly trouble-free operation later, the Heavy Ion Beam Probe is still gathering useful data, but TEXT is undergoing a major upgrade. The peak magnetic fields at the core of the upgraded TEXT will be too strong to allow a 500 keV Tic beam to pass through and still reach the analyzer. NEC is now building Heavy
Ion Beam Probe II to match the upgraded TEXT confinement field [3]. Beam Probe II is to provide up to 200 PA of T1+ at 2 MeV to allow penetration of all parts of the plasma throughout the confinement pulse. NEC was led to develop the special energy stabilization system described here to satisfy the requirement for beam energy spread of 100 eV or better.
2. System configuration Fig. 1 shows the general system layout. The Model 6SH-4 Pelletron is a 2 MV single-ended electrostatic accelerator equipped with four Pelletron charging chains. It uses the standard all-metal and ceramic accelerating tube. The 36 in. diameter insulating co1um.n which supports the high voltage terminal consists of two cast acrylic plates and column hoops whose elliptical shape was computer designed to minimize electric fields. Voltage grading is done with resistors mounted on the accelerating tube. Pure SF, gas is used for insulation in
N& Pelletron Model 6SH-4 ElectroSatic Analyzers 0
8
5
1 I s Meters
Capacitive Liner Driver
TOKAMAK
Fig. 1. Heavy Ion Beam Probe II system configuration. 0168-583X/91/$03.50
0 1991 - Elsevier Science Publishers B.V. (North-Holland)
XIV. ACCELERATOR
TECHNOLOGY
1034
.I.B. Schroeder et al. / A high stability accelerator
the pressure vessel which is 12.5 ft long and 5 ft in diameter. A cylindrical stainless steel electrode or “liner” is mounted inside the tank opposite the base of the high voltage terminal. This liner is used in the energy stabilization system. The theft beam is produced by an ion source in the high voltage terminal. As the beam emerges from the accelerating tube it is focused by an in-tank, electrostatic, quadrupole triplet. A + 3.5 * deflector diverts the beam to either of two NEC built, cylindrical electrostatic analyzers. Both analyzers have a bending radius of 1.5 m and operate with up to 80 kV (+ 40 kV) across the gap. Great care was taken in their design and construction to maintain the 30 mm plate to plate spacing to within + 76 urn. Such precision is required to insure that all ions travel through the analyzer with the same bending radius. The 44O and 68” bending angles of the analyzers create two alternative incident beam paths which maximize the cross sectional area of the TEXT plasma which can be probed. A pair of scanners rapidly scan the beam through the plasma during the confinement pulse. Finally, a parallel-plate energy analyzer with a position sensitive detector (provided by Rensselaer Polytechnic Institute) detects the beam after it emerges from the tokamak.
3. Energy control system NEC’s standard terminal potential stabilizer (TPS) system uses a GVM signal (or the slit current difference for an energy analyzed beam) as the feedback error signal to stabilize the terminal voltage by controlling the current drain through a corona probe facing the high voltage terminal. The GVM signal is only capable of showing relatively slow voltage variation (5 1 Hz) due to the dc filtering of the ac signal generated as the grounded rotating vane alternately covers and uncovers the stator plates connected to the amplifier. The amplifier is designed to eliminate the dependence of the output voltage on rotor frequency. Similarly the slit current amplifiers have a relatively low bandwidth for low slit currents. To allow for faster error correction it has been standard for many years to use GPO plates to feedback faster error signals to the TPS. An adjustable crossover frequency determines the frequency above which the CPO error signal takes over from the GVM or slit error signals for feedback. This system routinely provides terminal voltage stability of +O.Ol% for Pelletrons. The Heavy Ion Beam Probe II requires at least a factor of 2 better stability. Increasing gain in the standard system does not improve stability because the bandwidth of the control loop as limited by the RC time constant of the column results in a 90” phase shift of the terminal voltage response with respect to probe control voltage at - 1
Hz. The flight time of the ions from the probe to the terminal is a negligible contribution. It has been shown [4] that one can achieve faster error correction by using a capacitive liner to wrrect for terminal voltage variation. To achieve this a cylindrical ring is installed close to the tank wall facing the terminal. The liner is driven by a bipolar high voltage power supply (Trek Model 609A-3) capable of producing & 10 kV. This liner can change the terminal voltage by up to 25% of the liner voltage. In the new system, the corona probe current is still used for low frequency error correction in very much the same way as before. In addition to that, the CPO signal (above 1 Hz) is used to drive the liner power supply. With this system, terminal voltage error correction can be achieved from dc to greater than 100 Hz and therefore higher control gain and better stability is possible.
4. Experimental system While the Model 6SH-4 Pelletron was under construction the new voltage stabilization system was studied and refined. An in-house 1 MV single-ended Pelletron (Model 3SH) was employed for this purpose. It was fitted with an aluminum tank liner, GVM, GPO, and corona probe similar to the 6SH-4 design. Early attempts to operate the voltage stabilizer were hampered by two problems. First, the liner driver power supply was damaged by accelerator sparks on several occasions. Layers of spark protection were built into the power supply to eventually make it rugged enough to live through the worst sparks. The second problem was caused by the liner voltage affecting the GVM signal. This was a result of the GVM being located within a few inches of the liner due to the limited number of ports on the 3SH tank. This problem was overcome by creating a concentric GVM-CPO design which was mounted on the end of the tank. In this location the high voltage terminal shell completely shields these devices from the liner. Measurements were taken to confirm this fact. The experimental system was set up so that the accelerator voltage could be stabilized in either of two modes. The first mode was NEC’s standard control configuration which uses the CPO and GVM (or slit) signal feedback to control the wrona probe. The second mode uses both corona probe and liner. By alternating between the two modes the effect of the liner could be evaluated. In general the circuits could be tuned to produce the least terminal voltage ripple by observing the CPO signal on an oscilloscope while making adjustments. This gives a good measure of the range of voltage fluctuations as well as an approximate value of
J. B. Schroeder et al. / A high stabi&
the average ripple. However, a more elaborate method was used to quantify the ripple. The terminal voltage variation plot (fig. 2) was obtained by connecting the calibrated CPO preamp output to a bipolar A/D (12 bit) converter in a PC-AT compatible computer. The CPO was calibrated so that a 1 V output is equivalent to a 50 V variation in the terminal potential. When the terminal voltage is stable, one reads 0 V on the A/D. As the terminal voltage changes from this stable point, a non-zero voltage (- 5 V to + 5 V) is fed to the A/D. The A/D is read at about 3000 times/s for 15 min. Periodically, data collection is suspended after 10000 events to update the CRT which displays a plot of counts vs channel. The data is averaged over 5 channels out of 4096 before plotting to reduce clutter on the graph. The plot shown in fig. 2 was from data taken with 1 MV terminal voltage. A series of similar plots were made while varying the terminal potential from 0.45 MV to 1.1 MV.
accelerator
1035
250
1
Probe and liner control
I
0
0.2
0.4
0.6
0.8
1.0
1.2
Accelerator terminal voltage (Megavolts) Fig. 3. Voltage deviation as a function of terminal voltage stabilized with and without the liner.
5. Results The results of these measurements are shown in fig. 3 where terminal voltage variation is plotted as a function of terminal voltage. A linear regression was used to draw lines through the data points. The upper line shows the performance of the standard system which uses only the corona probe to control voltage. The lower line shows the significant improvement in voltage stability that results when the liner is added. At 1 MV the voltage deviation is 190 V FWHM without the liner. Voltage control with the liner is remarkably good, only 31 V FWHM.
6. Conclusion The results of these preliminary tests give good reason to be optimistic about the stability performance of
the Beam Probe II. An interesting feature of the liner system which can be seen in fig. 3 is that voltage deviation does not increase in direct proportion to terminal voltage. Rather, there seems to be a noise floor of about 23 V which is augmented by a voltage dependent component of about 8 V/MV. Extrapolating this line to 2 MV would give a voltage variation of 39 V FWHM, but it remains to be seen whether the contributions of noise and greater RC constant on the 6SH-4 will allow us to do as well. Further research must be done to determine the apparent source of noise in the system. Reduction of this noise could result in even better voltage stability.
References
0 -300
-200
-100
0
Deviation,
100
200
300
Volts
Fig. 2. Terminal voltage distribution during a 15 min run at 1 MV.
[l] P.M. Schoch, J.C. Forster, W.C. Jennings and R.L. Hickok, Rev. Sci. Instr. 57 (1986) 1825. [2] P.M. Schoch, A. Carnevah, K.A. Conner, T.P. Crowley, J.C. Forster, R.L. Hickok, J.F. Lewis, J.G. Schatz, Jr. and G.A. Hallock, Rev. Sci. Instr. 59 (1988) 1646. (31 R.L. Hickok and P.M. Schoch, Rev. Sci. Instr. 59 (1988) 1685. [4] P. Amdt and K. Ziegler, Proc. Symp. of North Eastern Accelerator Personnel (1986) p. 103.
XIV. ACCELERATOR
TECHNOLOGY
and Methods in Physics Research B56/57
1033
(1991) 1033-1035
A high stability accelerator for ion beam diagnostics in a tokamak J.B. Schroeder, S.J. Lunstrum, J.R. Adney, S.H. Phillips and R.D. Rathmell National Electrostatics
Corp., Middleton,
WI 53562-0310,
USA
A 2 MY single ended accelerator has been developed for probing a Tokamak plasma with a heavy ion beam. This b&b current Pelletron is designed to deliver up to 200 PA of 2 MeV Tlf for plasma diagnostics. Stringent energy stability requirements have led to the development of an improved accelerator voltage control system incorporating a capacitive tank liner. The special design
features and performance of this system will be discussed.
1. Introduction
In 1985 National Electrostatics Corporation (NEC) delivered the first Heavy Ion Beam Probe to the Fusion Research Center at the University of Texas in Austin. This accelerator was purchased by Rensselaer Polytechnic Institute for inst~ation on the Texas Ex~~rnent~ Tokamak (TEXT) to serve as a non-invasive probe of the plasma potentials within TEXT during its 500 ms confinement pulse [l]. Thallium was the ion of choice due to its mass (205 amu) and the availability of a simple, low energy spread ion source. The voltage multiplier driven accelerator could produce up to 50 JLA of Tl+ at 500 keV with a maximum total beam energy spread of 50 eV, resulting in the ability to resolve plasma potential fluctuations of f 2.5 V [2]. Five years of nearly trouble-free operation later, the Heavy Ion Beam Probe is still gathering useful data, but TEXT is undergoing a major upgrade. The peak magnetic fields at the core of the upgraded TEXT will be too strong to allow a 500 keV Tic beam to pass through and still reach the analyzer. NEC is now building Heavy
Ion Beam Probe II to match the upgraded TEXT confinement field [3]. Beam Probe II is to provide up to 200 PA of T1+ at 2 MeV to allow penetration of all parts of the plasma throughout the confinement pulse. NEC was led to develop the special energy stabilization system described here to satisfy the requirement for beam energy spread of 100 eV or better.
2. System configuration Fig. 1 shows the general system layout. The Model 6SH-4 Pelletron is a 2 MV single-ended electrostatic accelerator equipped with four Pelletron charging chains. It uses the standard all-metal and ceramic accelerating tube. The 36 in. diameter insulating co1um.n which supports the high voltage terminal consists of two cast acrylic plates and column hoops whose elliptical shape was computer designed to minimize electric fields. Voltage grading is done with resistors mounted on the accelerating tube. Pure SF, gas is used for insulation in
N& Pelletron Model 6SH-4 ElectroSatic Analyzers 0
8
5
1 I s Meters
Capacitive Liner Driver
TOKAMAK
Fig. 1. Heavy Ion Beam Probe II system configuration. 0168-583X/91/$03.50
0 1991 - Elsevier Science Publishers B.V. (North-Holland)
XIV. ACCELERATOR
TECHNOLOGY
1034
.I.B. Schroeder et al. / A high stability accelerator
the pressure vessel which is 12.5 ft long and 5 ft in diameter. A cylindrical stainless steel electrode or “liner” is mounted inside the tank opposite the base of the high voltage terminal. This liner is used in the energy stabilization system. The theft beam is produced by an ion source in the high voltage terminal. As the beam emerges from the accelerating tube it is focused by an in-tank, electrostatic, quadrupole triplet. A + 3.5 * deflector diverts the beam to either of two NEC built, cylindrical electrostatic analyzers. Both analyzers have a bending radius of 1.5 m and operate with up to 80 kV (+ 40 kV) across the gap. Great care was taken in their design and construction to maintain the 30 mm plate to plate spacing to within + 76 urn. Such precision is required to insure that all ions travel through the analyzer with the same bending radius. The 44O and 68” bending angles of the analyzers create two alternative incident beam paths which maximize the cross sectional area of the TEXT plasma which can be probed. A pair of scanners rapidly scan the beam through the plasma during the confinement pulse. Finally, a parallel-plate energy analyzer with a position sensitive detector (provided by Rensselaer Polytechnic Institute) detects the beam after it emerges from the tokamak.
3. Energy control system NEC’s standard terminal potential stabilizer (TPS) system uses a GVM signal (or the slit current difference for an energy analyzed beam) as the feedback error signal to stabilize the terminal voltage by controlling the current drain through a corona probe facing the high voltage terminal. The GVM signal is only capable of showing relatively slow voltage variation (5 1 Hz) due to the dc filtering of the ac signal generated as the grounded rotating vane alternately covers and uncovers the stator plates connected to the amplifier. The amplifier is designed to eliminate the dependence of the output voltage on rotor frequency. Similarly the slit current amplifiers have a relatively low bandwidth for low slit currents. To allow for faster error correction it has been standard for many years to use GPO plates to feedback faster error signals to the TPS. An adjustable crossover frequency determines the frequency above which the CPO error signal takes over from the GVM or slit error signals for feedback. This system routinely provides terminal voltage stability of +O.Ol% for Pelletrons. The Heavy Ion Beam Probe II requires at least a factor of 2 better stability. Increasing gain in the standard system does not improve stability because the bandwidth of the control loop as limited by the RC time constant of the column results in a 90” phase shift of the terminal voltage response with respect to probe control voltage at - 1
Hz. The flight time of the ions from the probe to the terminal is a negligible contribution. It has been shown [4] that one can achieve faster error correction by using a capacitive liner to wrrect for terminal voltage variation. To achieve this a cylindrical ring is installed close to the tank wall facing the terminal. The liner is driven by a bipolar high voltage power supply (Trek Model 609A-3) capable of producing & 10 kV. This liner can change the terminal voltage by up to 25% of the liner voltage. In the new system, the corona probe current is still used for low frequency error correction in very much the same way as before. In addition to that, the CPO signal (above 1 Hz) is used to drive the liner power supply. With this system, terminal voltage error correction can be achieved from dc to greater than 100 Hz and therefore higher control gain and better stability is possible.
4. Experimental system While the Model 6SH-4 Pelletron was under construction the new voltage stabilization system was studied and refined. An in-house 1 MV single-ended Pelletron (Model 3SH) was employed for this purpose. It was fitted with an aluminum tank liner, GVM, GPO, and corona probe similar to the 6SH-4 design. Early attempts to operate the voltage stabilizer were hampered by two problems. First, the liner driver power supply was damaged by accelerator sparks on several occasions. Layers of spark protection were built into the power supply to eventually make it rugged enough to live through the worst sparks. The second problem was caused by the liner voltage affecting the GVM signal. This was a result of the GVM being located within a few inches of the liner due to the limited number of ports on the 3SH tank. This problem was overcome by creating a concentric GVM-CPO design which was mounted on the end of the tank. In this location the high voltage terminal shell completely shields these devices from the liner. Measurements were taken to confirm this fact. The experimental system was set up so that the accelerator voltage could be stabilized in either of two modes. The first mode was NEC’s standard control configuration which uses the CPO and GVM (or slit) signal feedback to control the wrona probe. The second mode uses both corona probe and liner. By alternating between the two modes the effect of the liner could be evaluated. In general the circuits could be tuned to produce the least terminal voltage ripple by observing the CPO signal on an oscilloscope while making adjustments. This gives a good measure of the range of voltage fluctuations as well as an approximate value of
J. B. Schroeder et al. / A high stabi&
the average ripple. However, a more elaborate method was used to quantify the ripple. The terminal voltage variation plot (fig. 2) was obtained by connecting the calibrated CPO preamp output to a bipolar A/D (12 bit) converter in a PC-AT compatible computer. The CPO was calibrated so that a 1 V output is equivalent to a 50 V variation in the terminal potential. When the terminal voltage is stable, one reads 0 V on the A/D. As the terminal voltage changes from this stable point, a non-zero voltage (- 5 V to + 5 V) is fed to the A/D. The A/D is read at about 3000 times/s for 15 min. Periodically, data collection is suspended after 10000 events to update the CRT which displays a plot of counts vs channel. The data is averaged over 5 channels out of 4096 before plotting to reduce clutter on the graph. The plot shown in fig. 2 was from data taken with 1 MV terminal voltage. A series of similar plots were made while varying the terminal potential from 0.45 MV to 1.1 MV.
accelerator
1035
250
1
Probe and liner control
I
0
0.2
0.4
0.6
0.8
1.0
1.2
Accelerator terminal voltage (Megavolts) Fig. 3. Voltage deviation as a function of terminal voltage stabilized with and without the liner.
5. Results The results of these measurements are shown in fig. 3 where terminal voltage variation is plotted as a function of terminal voltage. A linear regression was used to draw lines through the data points. The upper line shows the performance of the standard system which uses only the corona probe to control voltage. The lower line shows the significant improvement in voltage stability that results when the liner is added. At 1 MV the voltage deviation is 190 V FWHM without the liner. Voltage control with the liner is remarkably good, only 31 V FWHM.
6. Conclusion The results of these preliminary tests give good reason to be optimistic about the stability performance of
the Beam Probe II. An interesting feature of the liner system which can be seen in fig. 3 is that voltage deviation does not increase in direct proportion to terminal voltage. Rather, there seems to be a noise floor of about 23 V which is augmented by a voltage dependent component of about 8 V/MV. Extrapolating this line to 2 MV would give a voltage variation of 39 V FWHM, but it remains to be seen whether the contributions of noise and greater RC constant on the 6SH-4 will allow us to do as well. Further research must be done to determine the apparent source of noise in the system. Reduction of this noise could result in even better voltage stability.
References
0 -300
-200
-100
0
Deviation,
100
200
300
Volts
Fig. 2. Terminal voltage distribution during a 15 min run at 1 MV.
[l] P.M. Schoch, J.C. Forster, W.C. Jennings and R.L. Hickok, Rev. Sci. Instr. 57 (1986) 1825. [2] P.M. Schoch, A. Carnevah, K.A. Conner, T.P. Crowley, J.C. Forster, R.L. Hickok, J.F. Lewis, J.G. Schatz, Jr. and G.A. Hallock, Rev. Sci. Instr. 59 (1988) 1646. (31 R.L. Hickok and P.M. Schoch, Rev. Sci. Instr. 59 (1988) 1685. [4] P. Amdt and K. Ziegler, Proc. Symp. of North Eastern Accelerator Personnel (1986) p. 103.
XIV. ACCELERATOR
TECHNOLOGY