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The development of a high current H− injector for the proton storage ring at LAMPF
Nuclear Instruments and Methods in Physics Research BlO/ll North-Holland. Amsterdam
891
(1985) 891495
THE DEVELOPMENT OF A HIGH CURRENT H - INJECTOR FOR THE PROTON STORAGE RING AT LAMPF * R.L. YORK, Ralph R. STEVENS, Jr.. R.A. DeHAVEN, and R. KANDARIAN Los Alamos National Laboratop.
J.R. MCCONNELL,
E.P. CHAMBERLIN
Los Alamos, New Mexico 87545, USA
A new high-current H- Injector has been installed at LAMPF for the proton storage ring. The injector is equipped with a muiticusp surface-production H- ion source that was developed at LAMPF. The ion source is capable of long-term operation at 20 mA of H- current at 10% duty factor and with normalized beam emittance of 0.08 cm mrad (95% beam fraction). Details of the development program, the injector design, and initial operating experience are discussed. Included in the discussion is a comparison of intensity and emittance measurements of the same H- beam at 100 keV and 750 keV.
1. Introduction The construction of the proton storage ring at LAMPF necessitated the development of a new high-intensity H- ion source. To satisfy requirements of the proton storage ring and those of LAMPF, the new Hsource must produce a 20 mA peak intensity H- beam with sufficient quality to match the acceptance of the accelerator. The new source must also be capable of long-term stable operation at up to 10% duty factor. An acceierator version of the multicusp, surface-pr~uction negative-ion source has been developed at LAMPF to meet these requirements. Because of the installation of this new type of source, the more stringent chopping and bunching requirements entailed in operating the storage ring, and increasing the peak intensity H- beam by a factor of 40, a complete rebuilding of the Hinjector was necessary. The new injector is now operational and some initial studies of the H- beam from the high-voltage Cockcroft-Walton dome have been accomplished.
centrated on increasing the brightness of a 20 mA beam. Early experiments proved that the emittance of the extracted H- beam was almost completely determined by the geometrical admittance of the ion source. This means that the ion beam produced on the surface of the converter electrode has sufficiently large emittance to fill the phase-space region determined by the aperture stops of the source. Thus, the emittance of the extracted H- ion beam is determined by the diameter of the converter (3.81 cm), the size of the exit aperture in the repeller electrode (1.0 cm) and the flight path from the
CUSPED-FIELD
MAGNETS CONVERTER
2. Development of the new H - ion source The multicusp, surface-production H- ion source was originally developed for neutral beam injectors at LBL by Ehlers and Leung 111.However, since their goal was a low-brightness high-current beam (1.0 A) and our desire was to produce a high brightness beam of relatively moderate intensity, the development program at LAMPF followed a different course than that initiated at Berkeley. The development effort at LAMPF con* Supported by the United States Energy Research and Development Administration Contract
W-7405-ENC-36.
0168-583X/85/$03.30 Q Elsevier Science Pubhshers B.V. (North-Holland Physics Publishing Division)
FILAMENTS
CESIUM
CELL
ELECTRODE
Fig. 1. The multicusp, surface-production
H- ion source.
VIII. ACCELERATOR TECHNOLOGY
R. L. York et cd /
892
z z
$0
E z
Derdopmenr
of a high current H _ injector
0.10 t
u
0
OBSERVED
EMITTANCES, 1
I
100 CONVERTER
I
I
200
300
POTENTIAL
(VOLTS)
Fig. 2. Calculated geometrical admittance and measured normalized emittance vs converter voltage.
converter to the exit aperture (12.62 cm) as shown in fig. 1. The agreement between the geometrical admittance prediction for the emittance as a function of converter voltage and some measured emittance values for 20 mA beams is shown in fig. 2. In this type of geometry the curvature of the converter surface is also an important parameter. This parameter has not yet been optimized. However, initial studies have shown that a converter surface which is a spherical cap with a radius equal to the distance from the converter surface to the exit aperture yields - 25% more H- current than a flat converter surface. Since higher brightness was the objective of the LAMPF development program all ion source prototypes were built with geometrical admittances that restricted the beam to normalize emittance of less than 0.12 cm . mrad. The testing of the various prototype H- ion sources was accomplished using the high-voltage test stand in the injector complex at LAMPF [2]. This test stand provides the capability of performing intensity and emittance measurements for both unanalyzed and mass-analyzed 200 keV beams. The optimized design of the multicusp surface-production H- ion source is shown in fig. 1. The source employs a cylindrical, stainless steel housing with ten rows of magnets around the outer cylindrical surface. The end plates contain four rows of magnets to complete the full-line cusp geometry with eight of the ten rows. The source housing is 20 cm in diameter and 23 cm long. Two tungsten fiiarnents (0.15 cm diameter Xl7 cm long) are mounted in the end plates to provide primary electrons to produce the arc discharge. The filaments are positioned so that most of the emission surface is in the magnetic field-free region. The filament holders are covered with boron-nitride cylinders with molybdenum caps to prevent sputtering erosion of the holders. The cesium transfer tube is also made of molybdenum and is isolated from the source housing to limit its participation in the arc discharge. The converter surface is molybdenum. The converter
-150
0I
I
4
a FLIGHT
12 PATH
1%
I 20
(cm)
Fig. 3. (a) The dipole, cusped-field magnet arrangement at the exit of the source. (b) Plots of the magnetic field experienced by the beam exiting the source.
shaft is covered with quartz to prevent unnecessary current drain on the converter power supply. Water cooling is provided to the filament holders, endplates, repeller, converter electrode, and magnets. The source housing is cooled only indirectly through contact with the individual magnet holders and typically operates at 40°C. The H- ion beam is extracted through a break in the cusp-field confinement geometry. The magnets are positioned symmetrically around the source housing, extending the length of the cylinder except along the beam axis. Here, a section of a line-cusp magnet has been removed and replaced by two similar magnets positioned above and below the extraction aperture in a symmetric manner as shown in fig. 3a. This magnet arrangement essentially retains the plasma-confinement geometry while providing an almost magnetic-field-free path for beam extraction. All of the magnets around the cylinder and the endplates are s~a~um-cobalt magnets except those in the area of the beam axis which are Alnico-8 magnets. Plots of the magnetic fields experienced by the beam traveling from the converter through the extraction aperture is shown in fig. 3b. The two plots represent the magnetic field at the center and at the bottom edge of the extraction aperture. The field at the top edge of the aperture is simply the mirror image of the curve shown. The small dipole-cusped field in the extraction region rejects almost all of the secondary
electrons formed on the converter while only slightly shearing the extracted H- ion beam ih the horizontal plane. However, since the beam is much larger than the extraction aperture, this shearing action does not affect the X-Y symmetry of the extracted beam and only introduces a few percent emittance growth in the horizontal plane. In the early stages of development, the arc discharge was operated continuously and pulsed beam generated by pulsing the converter voltage. However, because maintaining an optimum cesium coverage on the converter was very difficult and to extend filament lifetime, operation of the source discharge was switched to pulse mode. Experimental data show that operating the arc in pulsed mode allows the converter to become recoated with cesium while the arc is off. In operating the source in pulsed mode some difficulty was encountered in obtaining a stable arc discharge. A stable arc could be established by increasing the pressure inside the source to greater than 2.5 X 10m3 Torr or by increasing the arc voltage to greater than 200 V. However, both of these values were detrimental to the operation of the source. This problem was solved by putting a high voltage spike on the caning of the arc voltage pulse. This high voltage spike permits a rapid (SO ps) and consistent turn on of the discharge and thus permits the source to run with lower gas flow to minimize stripping of Hions inside the source. Operating at a lower discharge voltage during the arc pulse minimizes sputtering of the filaments and thus increases their lifetimes. Cesium is transferred continuously into the source using a temperature controller to regulate the cesium-oven temperature. Although other converter materials such as niobium and titanium have been tested, molybdenum has been proven to be the best based on H- yield and resistance to sputtering [3]. We have increased the brightness of the H- beam by changing the size of the converter. When a 5.08 cm diameter converter was used in the
source, maximum beam intensity was obtained at 40 A of ate current. However, when a 3.81 cm diameter was used in the source the beam intensity peaked at 60 A of arc current. This reduction in the admittance of the source and increase in arc current improved the brightness of the beam by a factor of 2.25. The normal performance of the source is now a 20 mA H- beam with a normalized e~ttance of 0.08 cm - mrad, as shown in fig. 4.
3. Lifetime tests Several lifetime tests have been carried out to evaluate the long-term performance of this ion source for accelerator service. The lifetime tests were performed with the source operating at 10% duty factor and producing a beam with a peak-intensity of 20 mA. To accomplish this, the source was operated at 60 A of arc current with the 0.15 cm diameter tungsten filaments heated to a temperature of - 2780 K. The source lifetime under these conditions is approximately 200 h and is limited by failure of the filaments. Burnout occurs at the center of the filaments shortly after a 6% reduction in filament diameter occurs. If thermal evaporation of filament material were the only factor in determining loss of filament material, a lifetime of 336 h would be expected. Therefore the observed lifetime implies that sputtering by plasma ions is responsible for one-third the material loss of these filaments. Although this source lifetime should be adequate for source operation because the source will initially operate at less than 10% duty factor and at less than maximum current, other filament materials such as La$ are being considered to extend the source lifetime. During the lifetime test the beam-current and emittance values were monitored to determine the H- beam stabilities. The values from one of these tests are shown in fig. 4. In general, the beam intensity and emittance were found to be quite stable. The only tuning necessary was adjustment of the cesium reservoir temperature with a general trend toward high temperatures as a function of time.
4. H - injector and transport
TIME
(HOURS)
_
Fig. 4. Normalized emittance and beam current vs time.
The H- cusp-field ion source was designed to minimize the ratio of eIectron current to extracted Hcurrent. The biasing of the repefler electrode slightly positive (+ 2 V) at the exit of the source reduces the extracted electron current to 25% of the H- ion current. However, even this relatively small current could be a potent source of X-rays at an energy of 750 keV. Thus, in order to limit the X-rays at the ground level Htransport line and to match the beam optics to the VIII. ACCELERATOR TECHNOL~Y
894
R. L. York et al. /
Development
existing injector high-voltage column, an appropriate beam transport system was constructed in the high-voltage equipment dome. Initially the H- beam is accelerated to an energy of 100 keV for transport in the dome and then accelerated to ground level through a 650 kV high voltage column. The ion source, its associated power supplies, control system and the beam transport system are housed in a 3.4 m X 4.6 m X 3.4 m Cockcroft-Walton high voltage equipment dome. Alternating current power is supplied to the dome with a 75 kVA 208/3# isolation transformer. The transformer is in two stages each capable of 450 kV dc isolation and with a capacitance of 500 pF. When in operation, the ion source and transport line require 35 to 37 kVA. The dome transport system is detailed in fig. 5. The transport line includes two solenoid focusing lenses, several X-Y steering magnets and 4.5” bending magnet to reject electrons extracted from the source [4]. The transport line also contains an emittance station and two wire scanners to assist in the tuning of the beam. The deflector plates in the transport line can be used to control the duty factor of the beam and to turn off the beam when necessary. This procedure avoids turning off the source, which can cause an over accumulation of cesium inside the source and adversely affect the tune of the source.
of o
htgh current
H
injector
Fig. 5. Beam transport line inside the high voltage equipment
dome.
The flexibility of this system has been a great asset in the initial operation of the new injector. Its resistance to high-voltage arc-down transients has been excellent. More operating experience with this new control system will be necessary to evaluate its potential but so far its performance appears quite promising. However, since this injector must be a production system, the experimental CAMAC system is backed up with a proven operating system which is patterned after the existing LAMPF H’-injector control system. This injector control system encodes and transmits a serial word from the injector control room to the high voltage dome. Thus this system accomplishes the functions of a hard wired local control system and data transmission using only four fiber optic links through serial bit transmission.
5. Injector control system 6. Injector operations The design of the control system for the new Hinjector was especially challenging because of the two high-voltage potentials in the system. In order to maximize system flexibility and build a system which consisted of commercially available components a CAMAC-based system was chosen. The system consists of three crates linked by serial fiber optics. The main crate in the injector control room houses the LSI 11/23 computer, video ter~nal/cursor/inter~pt driver, and shaft encoder modules for controlling the remote stepping motors. The two remote crates in the high voltage dome contain sufficient modules to perform the four types of control/readout. These are stepping motor drivers for power supply programming, analog/digital converters for data conversion, relay drivers for opening valves and relay contact sensors for state determination. All three crates contain modules to implement the serial communication link. The control program for the system was deliberately written to be independent of hardware arrangement. The hardware configuration is input to the program through a channel table. This table acts as a software patch-panel which mirrors the external world; thus the program requires no modification to reflect changes in wiring or crate configuration.
The initial operation of the new injector was hampered by problems in the 100 kV equipment racks. The first problem involved supplying ac power to these racks through three isolation transformers. The leakage inductance of these transformers acts as a high-impedance power source. This impedance caused severe spiking on the ac line voltage when the high-power SCR power supplies were operated. Addition of capacitance across the ac Iines reduced the spiking to a level that allowed the power supplies to operate. Since the line spikes generated by the SCR supplies caused the supplies to drop regulation, addition of line filters to the ac system merely accentuated the problem. Subsequent problems were caused by transient damaging electronic equipment in the 100 kV-rack during arc downs. Crude measurements of these transients indicated peak currents of several thousand amperes and submicrosecond duration. These high currents induce large voltages across the small inductance of the ground strip between the source and the high voltage racks, resulting in failures of the source power supplies. Attempts to add protection to the individual supplies were successful in several cases. Attempts were also made to limit the current involved in the faults by
R. L. York et ul. /
Dedopment
of o high current
895
H _ injector
the source’s previously determined capability of 20 mA, and refine the beam transport optics.
+17.70
0
7. Conclusion -17.70 0
-1.26 a 0 F
VERTICAL
+1.26
POSITION
(cm1
: > Fig. 6. Isometric
plot of a 680 keV
emittance measurement
adding resistance to the spark gaps on the column, to the primary shield of the isolation transformer, and in series with the high voltage power supply. Since much of the stored energy is in the isolation transformer there is no convenient way to add resistance to limit the fault current when the fault is inside the accelerating column. Despite these high-voltage problems, we have accomplished emittance measurements of the H- beam at 80 keV in the high-voltage dome and at ground level at an energy of 680 keV. An isometric plot of an emittance scan taken at 680 keV for a 10.5 mA H- beam is shown in fig. 6. The normalized emittance is 0.12 cm . mrad at 90% beam fraction. This emittance is larger than expected due to some spherical aberrations on the beam which we believe are produced in the optics of the beam transport. If allowances are made for these aberrations the agreement between the 80 keV and 680 keV data are quite reasonable. In the near future we hope to increase the beam energy to 750 keV, increase the intensity to
The multicusp surface-production H- ion source has been shown to be capable of producing beams of the required quality, intensity and reliability for the operation of the proton storage ring at LAMPF. Although some development of the high-voltage injector system is still required to increase its reliability, the new Hinjector has passed its initial operational tests. We thank all the people of the accelerator operations and accelerator support groups at LAMPF for their hard work in building the new H- injector. This work was supported by the US Department of Energy under Contract W-7405-ENG-36.
References [l] K.W. EhIers and K.N. Leung, Rev. Sci. Instr. 51 (1980) 721. [2] R.R. Stevens, Jr.. J.R. McConnell, E.P. Chamherlin, R.W. Hamm and R.L. York, Proc. Linear Accelerator Conf. Brookhaven, BNL 51134 (1979) p. 405. (31 R.L. York and R.R. Stevens, Jr., 3rd Symp. on The Production and Neutralization of Negative Ions and Beams, Brookhaven National Laboratory (Nov., 1983) p. 410. [4] R.R. Stevens. Jr., R.L. York, J.R. McConnell and R. Kandarian, Los Alamos National Laboratory Report, LAUR-84-1234 (June, 1984) Proc. Linear Accelerator Conf. Seeheim/Darmstadt, West Germany (May 1984).
VIII.
ACCELERATOR
TECHNOLOGY
891
(1985) 891495
THE DEVELOPMENT OF A HIGH CURRENT H - INJECTOR FOR THE PROTON STORAGE RING AT LAMPF * R.L. YORK, Ralph R. STEVENS, Jr.. R.A. DeHAVEN, and R. KANDARIAN Los Alamos National Laboratop.
J.R. MCCONNELL,
E.P. CHAMBERLIN
Los Alamos, New Mexico 87545, USA
A new high-current H- Injector has been installed at LAMPF for the proton storage ring. The injector is equipped with a muiticusp surface-production H- ion source that was developed at LAMPF. The ion source is capable of long-term operation at 20 mA of H- current at 10% duty factor and with normalized beam emittance of 0.08 cm mrad (95% beam fraction). Details of the development program, the injector design, and initial operating experience are discussed. Included in the discussion is a comparison of intensity and emittance measurements of the same H- beam at 100 keV and 750 keV.
1. Introduction The construction of the proton storage ring at LAMPF necessitated the development of a new high-intensity H- ion source. To satisfy requirements of the proton storage ring and those of LAMPF, the new Hsource must produce a 20 mA peak intensity H- beam with sufficient quality to match the acceptance of the accelerator. The new source must also be capable of long-term stable operation at up to 10% duty factor. An acceierator version of the multicusp, surface-pr~uction negative-ion source has been developed at LAMPF to meet these requirements. Because of the installation of this new type of source, the more stringent chopping and bunching requirements entailed in operating the storage ring, and increasing the peak intensity H- beam by a factor of 40, a complete rebuilding of the Hinjector was necessary. The new injector is now operational and some initial studies of the H- beam from the high-voltage Cockcroft-Walton dome have been accomplished.
centrated on increasing the brightness of a 20 mA beam. Early experiments proved that the emittance of the extracted H- beam was almost completely determined by the geometrical admittance of the ion source. This means that the ion beam produced on the surface of the converter electrode has sufficiently large emittance to fill the phase-space region determined by the aperture stops of the source. Thus, the emittance of the extracted H- ion beam is determined by the diameter of the converter (3.81 cm), the size of the exit aperture in the repeller electrode (1.0 cm) and the flight path from the
CUSPED-FIELD
MAGNETS CONVERTER
2. Development of the new H - ion source The multicusp, surface-production H- ion source was originally developed for neutral beam injectors at LBL by Ehlers and Leung 111.However, since their goal was a low-brightness high-current beam (1.0 A) and our desire was to produce a high brightness beam of relatively moderate intensity, the development program at LAMPF followed a different course than that initiated at Berkeley. The development effort at LAMPF con* Supported by the United States Energy Research and Development Administration Contract
W-7405-ENC-36.
0168-583X/85/$03.30 Q Elsevier Science Pubhshers B.V. (North-Holland Physics Publishing Division)
FILAMENTS
CESIUM
CELL
ELECTRODE
Fig. 1. The multicusp, surface-production
H- ion source.
VIII. ACCELERATOR TECHNOLOGY
R. L. York et cd /
892
z z
$0
E z
Derdopmenr
of a high current H _ injector
0.10 t
u
0
OBSERVED
EMITTANCES, 1
I
100 CONVERTER
I
I
200
300
POTENTIAL
(VOLTS)
Fig. 2. Calculated geometrical admittance and measured normalized emittance vs converter voltage.
converter to the exit aperture (12.62 cm) as shown in fig. 1. The agreement between the geometrical admittance prediction for the emittance as a function of converter voltage and some measured emittance values for 20 mA beams is shown in fig. 2. In this type of geometry the curvature of the converter surface is also an important parameter. This parameter has not yet been optimized. However, initial studies have shown that a converter surface which is a spherical cap with a radius equal to the distance from the converter surface to the exit aperture yields - 25% more H- current than a flat converter surface. Since higher brightness was the objective of the LAMPF development program all ion source prototypes were built with geometrical admittances that restricted the beam to normalize emittance of less than 0.12 cm . mrad. The testing of the various prototype H- ion sources was accomplished using the high-voltage test stand in the injector complex at LAMPF [2]. This test stand provides the capability of performing intensity and emittance measurements for both unanalyzed and mass-analyzed 200 keV beams. The optimized design of the multicusp surface-production H- ion source is shown in fig. 1. The source employs a cylindrical, stainless steel housing with ten rows of magnets around the outer cylindrical surface. The end plates contain four rows of magnets to complete the full-line cusp geometry with eight of the ten rows. The source housing is 20 cm in diameter and 23 cm long. Two tungsten fiiarnents (0.15 cm diameter Xl7 cm long) are mounted in the end plates to provide primary electrons to produce the arc discharge. The filaments are positioned so that most of the emission surface is in the magnetic field-free region. The filament holders are covered with boron-nitride cylinders with molybdenum caps to prevent sputtering erosion of the holders. The cesium transfer tube is also made of molybdenum and is isolated from the source housing to limit its participation in the arc discharge. The converter surface is molybdenum. The converter
-150
0I
I
4
a FLIGHT
12 PATH
1%
I 20
(cm)
Fig. 3. (a) The dipole, cusped-field magnet arrangement at the exit of the source. (b) Plots of the magnetic field experienced by the beam exiting the source.
shaft is covered with quartz to prevent unnecessary current drain on the converter power supply. Water cooling is provided to the filament holders, endplates, repeller, converter electrode, and magnets. The source housing is cooled only indirectly through contact with the individual magnet holders and typically operates at 40°C. The H- ion beam is extracted through a break in the cusp-field confinement geometry. The magnets are positioned symmetrically around the source housing, extending the length of the cylinder except along the beam axis. Here, a section of a line-cusp magnet has been removed and replaced by two similar magnets positioned above and below the extraction aperture in a symmetric manner as shown in fig. 3a. This magnet arrangement essentially retains the plasma-confinement geometry while providing an almost magnetic-field-free path for beam extraction. All of the magnets around the cylinder and the endplates are s~a~um-cobalt magnets except those in the area of the beam axis which are Alnico-8 magnets. Plots of the magnetic fields experienced by the beam traveling from the converter through the extraction aperture is shown in fig. 3b. The two plots represent the magnetic field at the center and at the bottom edge of the extraction aperture. The field at the top edge of the aperture is simply the mirror image of the curve shown. The small dipole-cusped field in the extraction region rejects almost all of the secondary
electrons formed on the converter while only slightly shearing the extracted H- ion beam ih the horizontal plane. However, since the beam is much larger than the extraction aperture, this shearing action does not affect the X-Y symmetry of the extracted beam and only introduces a few percent emittance growth in the horizontal plane. In the early stages of development, the arc discharge was operated continuously and pulsed beam generated by pulsing the converter voltage. However, because maintaining an optimum cesium coverage on the converter was very difficult and to extend filament lifetime, operation of the source discharge was switched to pulse mode. Experimental data show that operating the arc in pulsed mode allows the converter to become recoated with cesium while the arc is off. In operating the source in pulsed mode some difficulty was encountered in obtaining a stable arc discharge. A stable arc could be established by increasing the pressure inside the source to greater than 2.5 X 10m3 Torr or by increasing the arc voltage to greater than 200 V. However, both of these values were detrimental to the operation of the source. This problem was solved by putting a high voltage spike on the caning of the arc voltage pulse. This high voltage spike permits a rapid (SO ps) and consistent turn on of the discharge and thus permits the source to run with lower gas flow to minimize stripping of Hions inside the source. Operating at a lower discharge voltage during the arc pulse minimizes sputtering of the filaments and thus increases their lifetimes. Cesium is transferred continuously into the source using a temperature controller to regulate the cesium-oven temperature. Although other converter materials such as niobium and titanium have been tested, molybdenum has been proven to be the best based on H- yield and resistance to sputtering [3]. We have increased the brightness of the H- beam by changing the size of the converter. When a 5.08 cm diameter converter was used in the
source, maximum beam intensity was obtained at 40 A of ate current. However, when a 3.81 cm diameter was used in the source the beam intensity peaked at 60 A of arc current. This reduction in the admittance of the source and increase in arc current improved the brightness of the beam by a factor of 2.25. The normal performance of the source is now a 20 mA H- beam with a normalized e~ttance of 0.08 cm - mrad, as shown in fig. 4.
3. Lifetime tests Several lifetime tests have been carried out to evaluate the long-term performance of this ion source for accelerator service. The lifetime tests were performed with the source operating at 10% duty factor and producing a beam with a peak-intensity of 20 mA. To accomplish this, the source was operated at 60 A of arc current with the 0.15 cm diameter tungsten filaments heated to a temperature of - 2780 K. The source lifetime under these conditions is approximately 200 h and is limited by failure of the filaments. Burnout occurs at the center of the filaments shortly after a 6% reduction in filament diameter occurs. If thermal evaporation of filament material were the only factor in determining loss of filament material, a lifetime of 336 h would be expected. Therefore the observed lifetime implies that sputtering by plasma ions is responsible for one-third the material loss of these filaments. Although this source lifetime should be adequate for source operation because the source will initially operate at less than 10% duty factor and at less than maximum current, other filament materials such as La$ are being considered to extend the source lifetime. During the lifetime test the beam-current and emittance values were monitored to determine the H- beam stabilities. The values from one of these tests are shown in fig. 4. In general, the beam intensity and emittance were found to be quite stable. The only tuning necessary was adjustment of the cesium reservoir temperature with a general trend toward high temperatures as a function of time.
4. H - injector and transport
TIME
(HOURS)
_
Fig. 4. Normalized emittance and beam current vs time.
The H- cusp-field ion source was designed to minimize the ratio of eIectron current to extracted Hcurrent. The biasing of the repefler electrode slightly positive (+ 2 V) at the exit of the source reduces the extracted electron current to 25% of the H- ion current. However, even this relatively small current could be a potent source of X-rays at an energy of 750 keV. Thus, in order to limit the X-rays at the ground level Htransport line and to match the beam optics to the VIII. ACCELERATOR TECHNOL~Y
894
R. L. York et al. /
Development
existing injector high-voltage column, an appropriate beam transport system was constructed in the high-voltage equipment dome. Initially the H- beam is accelerated to an energy of 100 keV for transport in the dome and then accelerated to ground level through a 650 kV high voltage column. The ion source, its associated power supplies, control system and the beam transport system are housed in a 3.4 m X 4.6 m X 3.4 m Cockcroft-Walton high voltage equipment dome. Alternating current power is supplied to the dome with a 75 kVA 208/3# isolation transformer. The transformer is in two stages each capable of 450 kV dc isolation and with a capacitance of 500 pF. When in operation, the ion source and transport line require 35 to 37 kVA. The dome transport system is detailed in fig. 5. The transport line includes two solenoid focusing lenses, several X-Y steering magnets and 4.5” bending magnet to reject electrons extracted from the source [4]. The transport line also contains an emittance station and two wire scanners to assist in the tuning of the beam. The deflector plates in the transport line can be used to control the duty factor of the beam and to turn off the beam when necessary. This procedure avoids turning off the source, which can cause an over accumulation of cesium inside the source and adversely affect the tune of the source.
of o
htgh current
H
injector
Fig. 5. Beam transport line inside the high voltage equipment
dome.
The flexibility of this system has been a great asset in the initial operation of the new injector. Its resistance to high-voltage arc-down transients has been excellent. More operating experience with this new control system will be necessary to evaluate its potential but so far its performance appears quite promising. However, since this injector must be a production system, the experimental CAMAC system is backed up with a proven operating system which is patterned after the existing LAMPF H’-injector control system. This injector control system encodes and transmits a serial word from the injector control room to the high voltage dome. Thus this system accomplishes the functions of a hard wired local control system and data transmission using only four fiber optic links through serial bit transmission.
5. Injector control system 6. Injector operations The design of the control system for the new Hinjector was especially challenging because of the two high-voltage potentials in the system. In order to maximize system flexibility and build a system which consisted of commercially available components a CAMAC-based system was chosen. The system consists of three crates linked by serial fiber optics. The main crate in the injector control room houses the LSI 11/23 computer, video ter~nal/cursor/inter~pt driver, and shaft encoder modules for controlling the remote stepping motors. The two remote crates in the high voltage dome contain sufficient modules to perform the four types of control/readout. These are stepping motor drivers for power supply programming, analog/digital converters for data conversion, relay drivers for opening valves and relay contact sensors for state determination. All three crates contain modules to implement the serial communication link. The control program for the system was deliberately written to be independent of hardware arrangement. The hardware configuration is input to the program through a channel table. This table acts as a software patch-panel which mirrors the external world; thus the program requires no modification to reflect changes in wiring or crate configuration.
The initial operation of the new injector was hampered by problems in the 100 kV equipment racks. The first problem involved supplying ac power to these racks through three isolation transformers. The leakage inductance of these transformers acts as a high-impedance power source. This impedance caused severe spiking on the ac line voltage when the high-power SCR power supplies were operated. Addition of capacitance across the ac Iines reduced the spiking to a level that allowed the power supplies to operate. Since the line spikes generated by the SCR supplies caused the supplies to drop regulation, addition of line filters to the ac system merely accentuated the problem. Subsequent problems were caused by transient damaging electronic equipment in the 100 kV-rack during arc downs. Crude measurements of these transients indicated peak currents of several thousand amperes and submicrosecond duration. These high currents induce large voltages across the small inductance of the ground strip between the source and the high voltage racks, resulting in failures of the source power supplies. Attempts to add protection to the individual supplies were successful in several cases. Attempts were also made to limit the current involved in the faults by
R. L. York et ul. /
Dedopment
of o high current
895
H _ injector
the source’s previously determined capability of 20 mA, and refine the beam transport optics.
+17.70
0
7. Conclusion -17.70 0
-1.26 a 0 F
VERTICAL
+1.26
POSITION
(cm1
: > Fig. 6. Isometric
plot of a 680 keV
emittance measurement
adding resistance to the spark gaps on the column, to the primary shield of the isolation transformer, and in series with the high voltage power supply. Since much of the stored energy is in the isolation transformer there is no convenient way to add resistance to limit the fault current when the fault is inside the accelerating column. Despite these high-voltage problems, we have accomplished emittance measurements of the H- beam at 80 keV in the high-voltage dome and at ground level at an energy of 680 keV. An isometric plot of an emittance scan taken at 680 keV for a 10.5 mA H- beam is shown in fig. 6. The normalized emittance is 0.12 cm . mrad at 90% beam fraction. This emittance is larger than expected due to some spherical aberrations on the beam which we believe are produced in the optics of the beam transport. If allowances are made for these aberrations the agreement between the 80 keV and 680 keV data are quite reasonable. In the near future we hope to increase the beam energy to 750 keV, increase the intensity to
The multicusp surface-production H- ion source has been shown to be capable of producing beams of the required quality, intensity and reliability for the operation of the proton storage ring at LAMPF. Although some development of the high-voltage injector system is still required to increase its reliability, the new Hinjector has passed its initial operational tests. We thank all the people of the accelerator operations and accelerator support groups at LAMPF for their hard work in building the new H- injector. This work was supported by the US Department of Energy under Contract W-7405-ENG-36.
References [l] K.W. EhIers and K.N. Leung, Rev. Sci. Instr. 51 (1980) 721. [2] R.R. Stevens, Jr.. J.R. McConnell, E.P. Chamherlin, R.W. Hamm and R.L. York, Proc. Linear Accelerator Conf. Brookhaven, BNL 51134 (1979) p. 405. (31 R.L. York and R.R. Stevens, Jr., 3rd Symp. on The Production and Neutralization of Negative Ions and Beams, Brookhaven National Laboratory (Nov., 1983) p. 410. [4] R.R. Stevens. Jr., R.L. York, J.R. McConnell and R. Kandarian, Los Alamos National Laboratory Report, LAUR-84-1234 (June, 1984) Proc. Linear Accelerator Conf. Seeheim/Darmstadt, West Germany (May 1984).
VIII.
ACCELERATOR
TECHNOLOGY