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CW accelerators suitable for spallation neutron sources
102
Nuclear Instruments and Methods in Physics Research A249 (1986) 102-115 North-Holland, Amsterdam
CW ACCELERATORS SUITABLE FOR SPALLATION NEUTRON SOURCES Bruce G. CHIDLEY Atomic Energy of Canada Limned, Research Company, Chalk River Nuclear Laboratories, Chalk River, Ontario, Canada KOJ IJO
An accelerator-driven steady state neutron source will require a high current, high energy, 100% duty factor accelerator. No suitable accelerator has yet been built, but the technology to do so exists and examples of all of the likely components have been tested. This paper reviews some of the component test programs with comments on the problems specific to this application. 1. Introduction Spallation neutron sources exist at several laboratories today, but they use pulsed accelerators . To build a steady state neutron facility to produce thermal fluxes five to ten times greater than existing reactors will require a high energy, high current, 100% duty factor accelerator with capabilities different from present accelerators. There is some flexibility in the accelerator specifications but a suitable choice would be an output of 100 mA of protons at 1 GeV. The output energy requirement presents no problem; it is the combination with high average current that makes the design difficult. Space charge effects limit the maximum current in an accelerator . In general the space charge limit increases with energy so the design of the low energy part of the accelerator is the most difficult. The space charge limit can be increased by lowering the resonant frequency of an accelerating structure, but as this increases the size and cost of the structure, a compromise is needed to determine a suitable frequency. A typical accelerator might have a low energy stage operating at low frequency followed by a high energy stage operating at a multiple of this frequency. This paper will discuss the continuous wave (cw), i .e . 100% duty factor, accelerators that have been designed or built for spallation neutron sources, and their associated problem areas. 2. Cw spallation neutron projects 2.1 . MTA The concept of using an accelerator to produce neutrons via spallation has a long history. The first project of this type was the MTA program [1,2] which lasted from 1949 to 1954 and produced the highest current cw proton linac to date . The MTA project was initiated at a time when
uranium was in short supply in the US and no domestic ore bodies were known. The aim was the production of 239 pu using high energy deuterons on a depleted uranium target . The ultimate goal was to produce about 500 kg of plutonium per year using a 500 MeV, 320 mA, 50 MHz deuteron accelerator . The first accelerator built was the Mark 1 (fig . 1), a cw Alvarez linac operating at 12 MHz, which was over 20 m in diameter and 20 m long. Because it could not sustain high enough fields to accelerate deuterons, it was operated only with protons, which could be accelerated from 66 keV to 12 MeV at 50 mA cw indefinitely, 100 mA cw for a few minutes, or 225 mA at a 20% duty cycle. The next accelerator, the A-54, was built when high power, higher frequency rf tubes were developed, and was a more modest 4 m in diameter, 7 m long, operating at 48 .6 MHz. It accelerated 220 mA cw of protons from 100 keV to 500 keV. The A-54 was not operated with deuterons, due not to a field gradient limitation, but because a 200 kV injector was not available. The third accelerator, the A-48, consisted of two quarter wave stem preaccelerators followed by two 20 m Alvarez tanks. The quarter wave stem resonators operated at 24.3 MHz so the Alvarez had only alternate beam buckets filled . It accelerated 100 mA cw protons to 3 .7 MeV or 30 mA cw deuterons to 7.5 MeV. Though progress had been satisfactory, the program was discontinued when uranium deposits were found in the US . The physical size of the resonator and vacuum vessel was impressive and the rf power requirements were high, but it had been demonstrated that the problems were not insurmountable . A legacy of the project was that for years afterwards Alvarez linacs - including two at Chalk River - were built from NITA copper clad steel . 2.2 . ING At Chalk River there has been an interest in cw accelerators suitable for a steady-state spallation neu-
103
B. G. Chulley / Cw accelerators for spallation neutron sources
T.-T 1,1 T- q , T-1 1-1 ~11-
Fig. 1 . The MTA Mark 1 accelerator. Table 1 Heat associated with neutron production Reaction
n/s per MW 2.4 X 10 15 5 X10 15 3 X1015 3 X 10'6 6 X 10 16 2.1 X 10 17
T(d, n) 9 Be(d,n) W(e, n) Fission (thermal) (fast) Spallation
tron source for many years . In July 1963 a study committee was organized to investigate a high neutron flux research facility to provide 1000 times the intensity of the neutron beams then available at the NRU reactor. The most promising candidate system was a high current proton linear accelerator with an energy of several
hundred MeV and a spallation target . The choice of the spallation reaction over fission, T(d, n), 9 Be(d, n) or W(e, n) was dictated by the amount of heat associated with neutron production (table 1) . Ultimately, target cooling determines the maximum source strength . This study led to the intense neutron generator (ING) proposal [3,4] for a 65 mA cw 1 GeV proton accelerator and a liquid lead-bismuth eutectic target (see fig. 2). The ING study was ended in 1968, but development work on high current cw accelerators continued as part of a spallation breeder study [5]. At the time the ING study concluded the design was as follows : The injector was a 750 kV Cockcroft-Walton followed by a 110 m Alvarez section (fig. 3) with an output energy of 106 MeV operating at 268.3 MHz and a 1430 m coupled cavity section with an output of 1 GeV operating at 805 MHz. Fig. 4 shows an artist's concept of the ING neutron production facility.
5045
358'
ALVAREZ ION
SOURCE
PREACC ELERATOR
0.75 MEV
BUNCNER
9
LINAC
CAVITIES
WALL LOSSES : 3 .2 MW BEAM POWER : 6 .9MW
266- 13 MHz
Fig. 2. The ING linac.
RF
POWER
1057 MEV
WAVEGUIOE LINAC
320
TANKS
WALL LOSSES " 22 .2MW BEAMPOWER :580MW
005 MHz RF POWER
10 00 MEV TO TARGET FACILITY
104
B.G. Cludley / Cw accelerators for spallation neutron sources
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B. G. Chidley / Cw accelerators for spallation neutron sources
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B.G. Chidley / Cw accelerators for spallation neutron sources
Work had begun on the following programs to study problems associated with various parts of the project . (1) An ion source test stand. (2) A 750 kV Cockcroft-Walton and do column [6] (fig. 5) . (3) A 1 .5 m Alvarez linac (750 keV to 3 MeV) [7] . (4) A 3 cell portion of Alvarez tank number 3 (fig. 6) . (5) The electron test accelerator (ETA) [8] - a 4 MeV electron linac similar to part of the coupled cavity section, except that it accelerated electrons rather than protons (fig . 7) . (6) A liquid lead-bismuth loop . Following the ING study, work on some of the programs continued under the spallation breeder project . This project was undertaken to assess the economic feasibility of a spallation breeder to produce fissile fuel . Although spallation breeding would not have been competitive at then current uranium prices, predicted future shortages might have led to price increases that would have made breeding attractive or even essential .
HIGH CURRENT TEST FACILITY Fig. 5 . High current test facility.
2.3. ZEBRA
The economics of a spallation breeder suggest an accelerator giving 1 GeV protons at 300 mA cw, and it was estimated that facilities of this size would be needed by the year 2000. This would entail a considerable development program and indeed several years of work would be required to reach a position from which intelligent decisions on an accelerator breeder could be made. A long term development program was envisioned at Chalk River, consisting of the following stages [9] (see fig. 8) . (1) ZEBRA (zero energy breeder accelerator) . A 300 mA cw 10 MeV linac which would be the full current of the breeder accelerator but 1% of the output energy . It would consist of a 108 MHz RFQ from 75 keV to 2 MeV followed by a 216 MHz Alvarez linac to 10 MeV . (2) EMTF (electronuclear materials test facility) . An intermediate energy, intermediate current facility
B.G. Chulley / Cw accelerators for spallation neutron sources
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B.G. Chidley / Cw accelerators for spallation neutron sources
109
B.G. Chulley / Cw accelerators for spallation neutron sources
STAGE 1 :
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STAGE 2 :
EMTF
100 m
500 m
TARGET
7 RF
-
e0
MW
RF
-
205 IAN
7
ILANKET
650 IIITe 110 rwe
500 m
Fig . 8 . Spallation breeder project . ZEBRA
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Fig . 9 . ZEBRA and pre-ZEBRA activities.
B. G. Chidley / Cw accelerators for spallation neutron sources
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B. G. Chidley / Cw accelerators for spallation neutron sources
(70 mA 200 MeV which would include a lead-bismuth target assembly. It would be used for materials and fundamental research as well as for accelerator development. (3) PILOT. A pilot or demonstration facility built as an extension of EMTF first to 70 mA 1 GeV and later to 300 mA 1 GeV . Preliminary or pre-ZEBRA work would include RFQ1 - a 270 MHz 75 mA cw radio frequency quadrupole accelerator (RFQ) - and 2BLAT - a 2 /3a 270 MHz Alvarez linac . Both would be used to study and verify construction concepts and design codes . Figs. 9 and 10 show the scope of the design work preliminary test hardware, and computer simulations which were in progress under the ZEBRA project when it ended in 1984. ZEBRA terminated for the same reasons as the MTA project - additional uranium deposits had been found, the growth in uranium consumption had decreased and it was believed that there would not be a shortage of uranium in the near future . 3. Experience at CRNL The obvious problems associated with cw operation of a linac are heating and high radiation fields . Radiation fields near an accelerator arise from X-rays and proton induced reactions from beam spill. Chalk River experiments were limited to low energy protons and provided little experience in this area. Heating effects are very significant and go beyond simply requiring additional cooling. 3.1. Ion source A breeder accelerator requires a reliable high-current, low emittance proton source. The ion source development achieved this, and also exploited results in spin-off applications such as high-current heavy-ion beams for sputtering, material treatment and ion implantation. Satisfactory high current do sources have been developed using a multi-aperture duoPIGatron ion source and an accel-decel column [11] (fig. 11). A 7 aperture 45 kV ion source has given output currents in excess of 490 mA (mixed charge states) during continuous spark free runs of several hours duration. Two improvements in performance are desirable. The proton fraction should be increased (it is 40-45% to date) and more durable cathodes need to be developed (present oxide cathodes degrade after about 250 h of full current operation). 3.2. Dc column The high current 750 kV do column of the ING project would stand full voltage indefinitely with no
beam current, but the maximum time between breakdown was current dependent and decreased rapidly as beam current increased [12] (fig. 12). The spark free time was 200 min at 6 mA, 90 min at 30 mA, 30 min at 40 mA. It was estimated that a beam spill of 10-20 pA was sufficient to initiate a breakdown . Efforts were directed towards improving column stability or tolerance to spilled beam by altering electrode geometry to shield the column insulators from the beam and reduce the field gradient at the insulator surface, and towards reducing the beam spill by altering electrode geometry near the beam axis and optimizing ion source parameters. The behaviour of this column was fairly typical but some columns at other laboratories have had better current capabilities. For example, the 600 kV DCX-2 injector column at ORNL [13] operated reliably up to 100 mA so a comparison may yield clues on how to build a better column . As noted earlier Alvarez linacs can and have been built to accept high currents at lower input energy, but at the expense of going to a low frequency, very large structure. Fortunately the radio frequency quadrupole was developed which can accept a lower input energy than the Alvarez linac and the 750 kV do column can be eliminated. 3.3. RFQ Although the ING accelerator design did not include an RFQ, a modern version would. For efficient operation, an RFQ should be run at electric field gradients near the sparking limit. There was some concern that a cw RFQ would not be able to operate reliably at high fields, even though this had been demonstrated for pulsed ones. A short unmodulated RFQ was built at Chalk River in 1983 which could be operated cw at suitable levels [18] (and even higher fields in the pulsed mode). RFQI is under construction and is scheduled for operation in 1987. 3.4. Alvarez section Two Alvarez tanks were built and operated cw [15,16]. The first one was a 3 cell portion of ING tank number 3 (50 MeV input) and was built to test design and construction techniques and study cw excitation but was not intended to accelerate a beam. The second tank duplicated the input end of ING tank number 1 and was intended to accelerate beam up to the space charge limit of approximately 50 mA. Because of limitations in the injector, it was operated only to 5 mA. Development of the second Alvarez tank and its operations taught many useful lessons . The tank has a theoretical Q of 56 000, a measured Q (cold) of 42 800, and an operating Q (hot) of 39 800,
B. G. Chulley / Cw accelerators for spallation neutron sources
FILAMENT SUPPLY 0-10V, 0-100A
ARC SUPPLY 0-40A 0-160V
ELECTRON FEED
PLASMA GENERATOR
PIG REGION
ACCEL SUPPLY 0-60kV 0-1A
EXTRAC COIUh
Fig. 11 . DuoPIGatron ion source and accel-decel column
and requires an input power of 166 kW to reach design fields . The cooling system is capable of removing this heat and regulating the tank temperature under steady state operation, but the temperature gradients established are power dependent. If a brief interruption occurs in drive power or a spark occurs, temperature changes will throw the tank off resonance and a programmed power run-up must be executed. This means that a loss of tank stability is particularly troublesome during rf conditioning . Troubles with the rf drive loop assembly are now felt
to have been due to vacuum leaks opening when operating temperatures were reached. Severe heating occurred at the junction between the drift tube stems and the cavity wall . For a perfectly tuned Alvarez linac the currents flowing along a drift tube stem perpendicular to the outer wall are fairly small and by themselves would be handled easily by conventional rf gaskets or fingerstock . However, the wall currents are considerably larger and cause overheating in a stem by flowing from the wall, over the rf contact to one side of the stem, around the stem, and
B.G. Chidley / Cw accelerators for spallation neutron sources
over the rf contact back to the wall. It should be noted that wall currents will take this route rather than take the lower resistance path by remaining on the wall and detouring around the stem, because the detour is possible only if the field in the cavity is modified locally. If the stem to wall contact is not adequate, heating or sparking will occur at the junction. Several attempts were made to solve the problem without a major redesign by installing collars with fingerstock - none of which worked . Eventually collars were soft soldered to the stems and the tank wall . While this solution allowed high power operation, it did not provide the mechanical flexibility required for accurate alignment of the drift tubes and would not be applicable to a similar problem which would arise with tuners or post couplers (which were not present in this tank). We have since developed a suitable junction and tested it in a resonant load facility [17] (fig . 13). 3.5. Coupled cavity accelerating section
The electron test accelerator (ETA) was built to study performance of biperiodic coupled cavity accelerating structures suitable for the acceleration of protons with energies greater than 200 MeV. It is a 4 MeV 25 mA electron linac using the LAMPF side coupled structure operating at 805 MHz and consists of one graded /3 and one constant ß tank . It operates cw and experiments have been run where greater than 80% of the rf power was transferred to an electron beam [181 . The ETA program had led to a program studying possible industrial applications of electron linacs . The accelerating structures differed from those at LAMPF in that they had additional cooling channels, but, as occurred in the Alvarez tank, power dependent
temperature distributions arose, resulting in frequency shifts - in this case different for each module . As with the Alvarez linac a programmed power run-up had to be executed . The rf drive is overcoupled at zero beam current and becomes undercoupled as beam current is increased. This results in reflected power and a standing wave in the drive line . Although this was tolerated in the ETA tests, a mechanism would be required to adjust coupling with beam current in an accelerator for a spallation source. For some runs the second tank (constant-/3) was operated at reduced power to increase the ratio of drive power delivered to the beam to power dissipated in the structure. Under these conditions greater than 80% of the power went to the beam, which did not create a control problem. This ratio is typical of what would be required in a spallation breeder. The beam can excite higher order modes in the linac structure, but this was not a problem for ETA [191.
DRIFT TUBE MOUNTING
zW pt 5 O U 2 W m
100 MAXIMUM
200 TIME
300 BETNEEN
Fig. 12 . Dc column stability.
400
BREAKDOIIINS
500 (MIN
1
Fig. 13 . Drift tube mounting assembly.
B. G. Chtdley / Cw accelerators for spallation neutron sources
BEAM
SET PHASE
Fig. 14. ETA control system.
Modifications of the ETA coupled cavity structure have been built whith improved cooling that are capable of cw operation at considerably higher gradients. The gradient as used in ETA is appropriate to minimize the total cost of linac structure plus rf power, but the improved cooling may ease the control problem by reducing the power dependent frequency shift. 3.6. Rf and control systems High power rf systems can be built at almost any desired frequency, although the market for such systems is small. A special development may be required for a suitable system . While this is no problem for a large project where the development cost can be spread over many units, for a small test study where the use of existing hardware is mandatory, the choice will be restricted . The control system requirements differ from low duty factor accelerators principally because the particle beam contains high power and can do a great deal of damage if it gets out of control. For this reason tolerances may be tighter and response time must be very fast . The ETA control system is shown in fig. 14 .
4. Conclusions All the components of an accelerator suitable for a cw spallation neutron source are designable using present technology . A cost estimate has not been given but using as guides the very rough estimates of $100000/m for structure costs and $1000/kW of rf power a cost of between $200M and $300M is obtained. References C.M . Van Atta, Proc . Information Meeting on Accelerator Breeding, Brookhaven National Laboratory, CONF770107 (1977) p. 9. [2] Design and Construction of the Livermore Linear Accelerator MTA Project, Livermore Research Laboratory, LRL-60 (1954) . The AECL Study for an Intense Neutron Generator, eds., G.A. Bartholomew and P.R . Tunnicliffe, Atormc Energy of Canada Limited, Report AECL-2600 (1966) . [41 ING Status Report July 1967, ed., T.G . Church, Atomic Energy of Canada Limited, Report AECL-2750 (1967) . [5] P.R . Tunnicliffe, Proc . Information Meeting on Accelerator Breeding, Brookhaven National Laboratory, CONF770107 (1977) p. 69 .
B.G. Chidley / Cw accelerators for spallation neutron sources [61 J.H . Ormrod, M.D. Snedden and J. Ungrin, Atomic Energy of Canada Limited, Report AECL-4224 (1972) . [7] B .G . Chidley, S.B . Hodge, J.C . Brown, J.H . Ormrod and J. Ungrm, Proc . 1972 Linear Accelerator Conf., Los Alamos Scientific Laboratory, LASL-5115 (1972) p. 218 . [8] J.S . Fraser, S.H. Kidner, J. McKeown and G.E . McMichael, ibid ., p. 226. [91 G.A . Bartholomew, Proc . 5th Meeting of the Int. Col-
[10] [11] [121 [131
laboration on Advanced Neutron Sources, All-Conf-45, Jülich (1981) p. 89. S.O . Schriber, Proc . 1984 Linear Accelerator Conf ., GSI81-11 (1984) p. 501. M.R . Shubaly and M.S . de Jong, IEEE Trans. Nucl . Sci. NS-30 (1983) 1399 . J. Ungrin, Atomic Energy of Canada Limited, Report AECL-6584 (1979) . G.C . Kelly and O.B . Morgan, Minutes of the 1964 Linear Accelerator Conf., UC-28, TID-4500 (1964) p. 456.
[14] R.M . Hutcheon, S.O. Schriber, J.C . Brown, D.W . Clements, H.F . Campbell, G.E . McMichael and M.S . de Jong, Proc. 1984 Linear Accelerator Conf ., GSI-84-11 (1984) p. 74. [15] B.G . Chidley, S.B . Hodge and J.D. Hepburn, Atomic Energy of Canada Limited, private communication (1978) . [16] J. Ungrin, B.G . Chidley, J.C . Brown, G.E . McMichael, D.W . Clements and H.F. Campbell, Atomic Energy of Canada Limited, Report AECL-8952 (1985) . [171 J. Ungrin, private communication (1985). [18] G.E . McMichael, J. McKeown and J.S. Fraser, Proc . 1979 Linear Accelerator Conf., Brookhaven National Laboratory, BNL-51134 (1979) p. 180. [19] J.-P. Labrie, K.C .D. Chan, J. McKeown, H. Eutcneuer and A.M . Vetter, Proc . 1984 Linear Accelerator Conf., GSI-84-11 (1984) p. 168.
Nuclear Instruments and Methods in Physics Research A249 (1986) 102-115 North-Holland, Amsterdam
CW ACCELERATORS SUITABLE FOR SPALLATION NEUTRON SOURCES Bruce G. CHIDLEY Atomic Energy of Canada Limned, Research Company, Chalk River Nuclear Laboratories, Chalk River, Ontario, Canada KOJ IJO
An accelerator-driven steady state neutron source will require a high current, high energy, 100% duty factor accelerator. No suitable accelerator has yet been built, but the technology to do so exists and examples of all of the likely components have been tested. This paper reviews some of the component test programs with comments on the problems specific to this application. 1. Introduction Spallation neutron sources exist at several laboratories today, but they use pulsed accelerators . To build a steady state neutron facility to produce thermal fluxes five to ten times greater than existing reactors will require a high energy, high current, 100% duty factor accelerator with capabilities different from present accelerators. There is some flexibility in the accelerator specifications but a suitable choice would be an output of 100 mA of protons at 1 GeV. The output energy requirement presents no problem; it is the combination with high average current that makes the design difficult. Space charge effects limit the maximum current in an accelerator . In general the space charge limit increases with energy so the design of the low energy part of the accelerator is the most difficult. The space charge limit can be increased by lowering the resonant frequency of an accelerating structure, but as this increases the size and cost of the structure, a compromise is needed to determine a suitable frequency. A typical accelerator might have a low energy stage operating at low frequency followed by a high energy stage operating at a multiple of this frequency. This paper will discuss the continuous wave (cw), i .e . 100% duty factor, accelerators that have been designed or built for spallation neutron sources, and their associated problem areas. 2. Cw spallation neutron projects 2.1 . MTA The concept of using an accelerator to produce neutrons via spallation has a long history. The first project of this type was the MTA program [1,2] which lasted from 1949 to 1954 and produced the highest current cw proton linac to date . The MTA project was initiated at a time when
uranium was in short supply in the US and no domestic ore bodies were known. The aim was the production of 239 pu using high energy deuterons on a depleted uranium target . The ultimate goal was to produce about 500 kg of plutonium per year using a 500 MeV, 320 mA, 50 MHz deuteron accelerator . The first accelerator built was the Mark 1 (fig . 1), a cw Alvarez linac operating at 12 MHz, which was over 20 m in diameter and 20 m long. Because it could not sustain high enough fields to accelerate deuterons, it was operated only with protons, which could be accelerated from 66 keV to 12 MeV at 50 mA cw indefinitely, 100 mA cw for a few minutes, or 225 mA at a 20% duty cycle. The next accelerator, the A-54, was built when high power, higher frequency rf tubes were developed, and was a more modest 4 m in diameter, 7 m long, operating at 48 .6 MHz. It accelerated 220 mA cw of protons from 100 keV to 500 keV. The A-54 was not operated with deuterons, due not to a field gradient limitation, but because a 200 kV injector was not available. The third accelerator, the A-48, consisted of two quarter wave stem preaccelerators followed by two 20 m Alvarez tanks. The quarter wave stem resonators operated at 24.3 MHz so the Alvarez had only alternate beam buckets filled . It accelerated 100 mA cw protons to 3 .7 MeV or 30 mA cw deuterons to 7.5 MeV. Though progress had been satisfactory, the program was discontinued when uranium deposits were found in the US . The physical size of the resonator and vacuum vessel was impressive and the rf power requirements were high, but it had been demonstrated that the problems were not insurmountable . A legacy of the project was that for years afterwards Alvarez linacs - including two at Chalk River - were built from NITA copper clad steel . 2.2 . ING At Chalk River there has been an interest in cw accelerators suitable for a steady-state spallation neu-
103
B. G. Chulley / Cw accelerators for spallation neutron sources
T.-T 1,1 T- q , T-1 1-1 ~11-
Fig. 1 . The MTA Mark 1 accelerator. Table 1 Heat associated with neutron production Reaction
n/s per MW 2.4 X 10 15 5 X10 15 3 X1015 3 X 10'6 6 X 10 16 2.1 X 10 17
T(d, n) 9 Be(d,n) W(e, n) Fission (thermal) (fast) Spallation
tron source for many years . In July 1963 a study committee was organized to investigate a high neutron flux research facility to provide 1000 times the intensity of the neutron beams then available at the NRU reactor. The most promising candidate system was a high current proton linear accelerator with an energy of several
hundred MeV and a spallation target . The choice of the spallation reaction over fission, T(d, n), 9 Be(d, n) or W(e, n) was dictated by the amount of heat associated with neutron production (table 1) . Ultimately, target cooling determines the maximum source strength . This study led to the intense neutron generator (ING) proposal [3,4] for a 65 mA cw 1 GeV proton accelerator and a liquid lead-bismuth eutectic target (see fig. 2). The ING study was ended in 1968, but development work on high current cw accelerators continued as part of a spallation breeder study [5]. At the time the ING study concluded the design was as follows : The injector was a 750 kV Cockcroft-Walton followed by a 110 m Alvarez section (fig. 3) with an output energy of 106 MeV operating at 268.3 MHz and a 1430 m coupled cavity section with an output of 1 GeV operating at 805 MHz. Fig. 4 shows an artist's concept of the ING neutron production facility.
5045
358'
ALVAREZ ION
SOURCE
PREACC ELERATOR
0.75 MEV
BUNCNER
9
LINAC
CAVITIES
WALL LOSSES : 3 .2 MW BEAM POWER : 6 .9MW
266- 13 MHz
Fig. 2. The ING linac.
RF
POWER
1057 MEV
WAVEGUIOE LINAC
320
TANKS
WALL LOSSES " 22 .2MW BEAMPOWER :580MW
005 MHz RF POWER
10 00 MEV TO TARGET FACILITY
104
B.G. Cludley / Cw accelerators for spallation neutron sources
a 0 .Û N N R Q z M
B. G. Chidley / Cw accelerators for spallation neutron sources
r
va z O' ~ a. oc oc W
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a ]C
a W m W tn H H OC
a
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0
W W N
CC
O H a OC W W
U U
a a W z
105
106
B.G. Chidley / Cw accelerators for spallation neutron sources
Work had begun on the following programs to study problems associated with various parts of the project . (1) An ion source test stand. (2) A 750 kV Cockcroft-Walton and do column [6] (fig. 5) . (3) A 1 .5 m Alvarez linac (750 keV to 3 MeV) [7] . (4) A 3 cell portion of Alvarez tank number 3 (fig. 6) . (5) The electron test accelerator (ETA) [8] - a 4 MeV electron linac similar to part of the coupled cavity section, except that it accelerated electrons rather than protons (fig . 7) . (6) A liquid lead-bismuth loop . Following the ING study, work on some of the programs continued under the spallation breeder project . This project was undertaken to assess the economic feasibility of a spallation breeder to produce fissile fuel . Although spallation breeding would not have been competitive at then current uranium prices, predicted future shortages might have led to price increases that would have made breeding attractive or even essential .
HIGH CURRENT TEST FACILITY Fig. 5 . High current test facility.
2.3. ZEBRA
The economics of a spallation breeder suggest an accelerator giving 1 GeV protons at 300 mA cw, and it was estimated that facilities of this size would be needed by the year 2000. This would entail a considerable development program and indeed several years of work would be required to reach a position from which intelligent decisions on an accelerator breeder could be made. A long term development program was envisioned at Chalk River, consisting of the following stages [9] (see fig. 8) . (1) ZEBRA (zero energy breeder accelerator) . A 300 mA cw 10 MeV linac which would be the full current of the breeder accelerator but 1% of the output energy . It would consist of a 108 MHz RFQ from 75 keV to 2 MeV followed by a 216 MHz Alvarez linac to 10 MeV . (2) EMTF (electronuclear materials test facility) . An intermediate energy, intermediate current facility
B.G. Chulley / Cw accelerators for spallation neutron sources
107
ro
N N d cC
G W W ' ' W W N
L
W
K
c 7. W
W W H F 0
zU W
W O Zh W f Z
= OL ~ V
W
KN d ~ dW
~of
Q z
108
B.G. Chidley / Cw accelerators for spallation neutron sources
109
B.G. Chulley / Cw accelerators for spallation neutron sources
STAGE 1 :
ZEBRA
STAGE 2 :
EMTF
100 m
500 m
TARGET
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B. G. Chidley / Cw accelerators for spallation neutron sources
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B. G. Chidley / Cw accelerators for spallation neutron sources
(70 mA 200 MeV which would include a lead-bismuth target assembly. It would be used for materials and fundamental research as well as for accelerator development. (3) PILOT. A pilot or demonstration facility built as an extension of EMTF first to 70 mA 1 GeV and later to 300 mA 1 GeV . Preliminary or pre-ZEBRA work would include RFQ1 - a 270 MHz 75 mA cw radio frequency quadrupole accelerator (RFQ) - and 2BLAT - a 2 /3a 270 MHz Alvarez linac . Both would be used to study and verify construction concepts and design codes . Figs. 9 and 10 show the scope of the design work preliminary test hardware, and computer simulations which were in progress under the ZEBRA project when it ended in 1984. ZEBRA terminated for the same reasons as the MTA project - additional uranium deposits had been found, the growth in uranium consumption had decreased and it was believed that there would not be a shortage of uranium in the near future . 3. Experience at CRNL The obvious problems associated with cw operation of a linac are heating and high radiation fields . Radiation fields near an accelerator arise from X-rays and proton induced reactions from beam spill. Chalk River experiments were limited to low energy protons and provided little experience in this area. Heating effects are very significant and go beyond simply requiring additional cooling. 3.1. Ion source A breeder accelerator requires a reliable high-current, low emittance proton source. The ion source development achieved this, and also exploited results in spin-off applications such as high-current heavy-ion beams for sputtering, material treatment and ion implantation. Satisfactory high current do sources have been developed using a multi-aperture duoPIGatron ion source and an accel-decel column [11] (fig. 11). A 7 aperture 45 kV ion source has given output currents in excess of 490 mA (mixed charge states) during continuous spark free runs of several hours duration. Two improvements in performance are desirable. The proton fraction should be increased (it is 40-45% to date) and more durable cathodes need to be developed (present oxide cathodes degrade after about 250 h of full current operation). 3.2. Dc column The high current 750 kV do column of the ING project would stand full voltage indefinitely with no
beam current, but the maximum time between breakdown was current dependent and decreased rapidly as beam current increased [12] (fig. 12). The spark free time was 200 min at 6 mA, 90 min at 30 mA, 30 min at 40 mA. It was estimated that a beam spill of 10-20 pA was sufficient to initiate a breakdown . Efforts were directed towards improving column stability or tolerance to spilled beam by altering electrode geometry to shield the column insulators from the beam and reduce the field gradient at the insulator surface, and towards reducing the beam spill by altering electrode geometry near the beam axis and optimizing ion source parameters. The behaviour of this column was fairly typical but some columns at other laboratories have had better current capabilities. For example, the 600 kV DCX-2 injector column at ORNL [13] operated reliably up to 100 mA so a comparison may yield clues on how to build a better column . As noted earlier Alvarez linacs can and have been built to accept high currents at lower input energy, but at the expense of going to a low frequency, very large structure. Fortunately the radio frequency quadrupole was developed which can accept a lower input energy than the Alvarez linac and the 750 kV do column can be eliminated. 3.3. RFQ Although the ING accelerator design did not include an RFQ, a modern version would. For efficient operation, an RFQ should be run at electric field gradients near the sparking limit. There was some concern that a cw RFQ would not be able to operate reliably at high fields, even though this had been demonstrated for pulsed ones. A short unmodulated RFQ was built at Chalk River in 1983 which could be operated cw at suitable levels [18] (and even higher fields in the pulsed mode). RFQI is under construction and is scheduled for operation in 1987. 3.4. Alvarez section Two Alvarez tanks were built and operated cw [15,16]. The first one was a 3 cell portion of ING tank number 3 (50 MeV input) and was built to test design and construction techniques and study cw excitation but was not intended to accelerate a beam. The second tank duplicated the input end of ING tank number 1 and was intended to accelerate beam up to the space charge limit of approximately 50 mA. Because of limitations in the injector, it was operated only to 5 mA. Development of the second Alvarez tank and its operations taught many useful lessons . The tank has a theoretical Q of 56 000, a measured Q (cold) of 42 800, and an operating Q (hot) of 39 800,
B. G. Chulley / Cw accelerators for spallation neutron sources
FILAMENT SUPPLY 0-10V, 0-100A
ARC SUPPLY 0-40A 0-160V
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Fig. 11 . DuoPIGatron ion source and accel-decel column
and requires an input power of 166 kW to reach design fields . The cooling system is capable of removing this heat and regulating the tank temperature under steady state operation, but the temperature gradients established are power dependent. If a brief interruption occurs in drive power or a spark occurs, temperature changes will throw the tank off resonance and a programmed power run-up must be executed. This means that a loss of tank stability is particularly troublesome during rf conditioning . Troubles with the rf drive loop assembly are now felt
to have been due to vacuum leaks opening when operating temperatures were reached. Severe heating occurred at the junction between the drift tube stems and the cavity wall . For a perfectly tuned Alvarez linac the currents flowing along a drift tube stem perpendicular to the outer wall are fairly small and by themselves would be handled easily by conventional rf gaskets or fingerstock . However, the wall currents are considerably larger and cause overheating in a stem by flowing from the wall, over the rf contact to one side of the stem, around the stem, and
B.G. Chidley / Cw accelerators for spallation neutron sources
over the rf contact back to the wall. It should be noted that wall currents will take this route rather than take the lower resistance path by remaining on the wall and detouring around the stem, because the detour is possible only if the field in the cavity is modified locally. If the stem to wall contact is not adequate, heating or sparking will occur at the junction. Several attempts were made to solve the problem without a major redesign by installing collars with fingerstock - none of which worked . Eventually collars were soft soldered to the stems and the tank wall . While this solution allowed high power operation, it did not provide the mechanical flexibility required for accurate alignment of the drift tubes and would not be applicable to a similar problem which would arise with tuners or post couplers (which were not present in this tank). We have since developed a suitable junction and tested it in a resonant load facility [17] (fig . 13). 3.5. Coupled cavity accelerating section
The electron test accelerator (ETA) was built to study performance of biperiodic coupled cavity accelerating structures suitable for the acceleration of protons with energies greater than 200 MeV. It is a 4 MeV 25 mA electron linac using the LAMPF side coupled structure operating at 805 MHz and consists of one graded /3 and one constant ß tank . It operates cw and experiments have been run where greater than 80% of the rf power was transferred to an electron beam [181 . The ETA program had led to a program studying possible industrial applications of electron linacs . The accelerating structures differed from those at LAMPF in that they had additional cooling channels, but, as occurred in the Alvarez tank, power dependent
temperature distributions arose, resulting in frequency shifts - in this case different for each module . As with the Alvarez linac a programmed power run-up had to be executed . The rf drive is overcoupled at zero beam current and becomes undercoupled as beam current is increased. This results in reflected power and a standing wave in the drive line . Although this was tolerated in the ETA tests, a mechanism would be required to adjust coupling with beam current in an accelerator for a spallation source. For some runs the second tank (constant-/3) was operated at reduced power to increase the ratio of drive power delivered to the beam to power dissipated in the structure. Under these conditions greater than 80% of the power went to the beam, which did not create a control problem. This ratio is typical of what would be required in a spallation breeder. The beam can excite higher order modes in the linac structure, but this was not a problem for ETA [191.
DRIFT TUBE MOUNTING
zW pt 5 O U 2 W m
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200 TIME
300 BETNEEN
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400
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500 (MIN
1
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B. G. Chtdley / Cw accelerators for spallation neutron sources
BEAM
SET PHASE
Fig. 14. ETA control system.
Modifications of the ETA coupled cavity structure have been built whith improved cooling that are capable of cw operation at considerably higher gradients. The gradient as used in ETA is appropriate to minimize the total cost of linac structure plus rf power, but the improved cooling may ease the control problem by reducing the power dependent frequency shift. 3.6. Rf and control systems High power rf systems can be built at almost any desired frequency, although the market for such systems is small. A special development may be required for a suitable system . While this is no problem for a large project where the development cost can be spread over many units, for a small test study where the use of existing hardware is mandatory, the choice will be restricted . The control system requirements differ from low duty factor accelerators principally because the particle beam contains high power and can do a great deal of damage if it gets out of control. For this reason tolerances may be tighter and response time must be very fast . The ETA control system is shown in fig. 14 .
4. Conclusions All the components of an accelerator suitable for a cw spallation neutron source are designable using present technology . A cost estimate has not been given but using as guides the very rough estimates of $100000/m for structure costs and $1000/kW of rf power a cost of between $200M and $300M is obtained. References C.M . Van Atta, Proc . Information Meeting on Accelerator Breeding, Brookhaven National Laboratory, CONF770107 (1977) p. 9. [2] Design and Construction of the Livermore Linear Accelerator MTA Project, Livermore Research Laboratory, LRL-60 (1954) . The AECL Study for an Intense Neutron Generator, eds., G.A. Bartholomew and P.R . Tunnicliffe, Atormc Energy of Canada Limited, Report AECL-2600 (1966) . [41 ING Status Report July 1967, ed., T.G . Church, Atomic Energy of Canada Limited, Report AECL-2750 (1967) . [5] P.R . Tunnicliffe, Proc . Information Meeting on Accelerator Breeding, Brookhaven National Laboratory, CONF770107 (1977) p. 69 .
B.G. Chidley / Cw accelerators for spallation neutron sources [61 J.H . Ormrod, M.D. Snedden and J. Ungrin, Atomic Energy of Canada Limited, Report AECL-4224 (1972) . [7] B .G . Chidley, S.B . Hodge, J.C . Brown, J.H . Ormrod and J. Ungrm, Proc . 1972 Linear Accelerator Conf., Los Alamos Scientific Laboratory, LASL-5115 (1972) p. 218 . [8] J.S . Fraser, S.H. Kidner, J. McKeown and G.E . McMichael, ibid ., p. 226. [91 G.A . Bartholomew, Proc . 5th Meeting of the Int. Col-
[10] [11] [121 [131
laboration on Advanced Neutron Sources, All-Conf-45, Jülich (1981) p. 89. S.O . Schriber, Proc . 1984 Linear Accelerator Conf ., GSI81-11 (1984) p. 501. M.R . Shubaly and M.S . de Jong, IEEE Trans. Nucl . Sci. NS-30 (1983) 1399 . J. Ungrin, Atomic Energy of Canada Limited, Report AECL-6584 (1979) . G.C . Kelly and O.B . Morgan, Minutes of the 1964 Linear Accelerator Conf., UC-28, TID-4500 (1964) p. 456.
[14] R.M . Hutcheon, S.O. Schriber, J.C . Brown, D.W . Clements, H.F . Campbell, G.E . McMichael and M.S . de Jong, Proc. 1984 Linear Accelerator Conf ., GSI-84-11 (1984) p. 74. [15] B.G . Chidley, S.B . Hodge and J.D. Hepburn, Atomic Energy of Canada Limited, private communication (1978) . [16] J. Ungrin, B.G . Chidley, J.C . Brown, G.E . McMichael, D.W . Clements and H.F. Campbell, Atomic Energy of Canada Limited, Report AECL-8952 (1985) . [171 J. Ungrin, private communication (1985). [18] G.E . McMichael, J. McKeown and J.S. Fraser, Proc . 1979 Linear Accelerator Conf., Brookhaven National Laboratory, BNL-51134 (1979) p. 180. [19] J.-P. Labrie, K.C .D. Chan, J. McKeown, H. Eutcneuer and A.M . Vetter, Proc . 1984 Linear Accelerator Conf., GSI-84-11 (1984) p. 168.