The high-intensity proton linac with a beam energy of 1.5GeV and a maximum current of 10mA has been proposed for the Neutron Science Research Program (NSRP) in JAERI. The NSRP is aiming at exploring new basic researches and nuclear waste transmutation technology based on spallation neutron. The R&D work has been carried out for the components of a low energy part of the proton accelerator and conceptual design study on superconducting accelerating cavity as a main option for a high energy part (high b linac) above 100MeV.
The proposed plan for accelerator design and construction
will be composed of two consecutive stages. The first stage will
be completed in about 7 years with the beam current of 1mA. As
the second stage, gradual upgrading of the beam current will be
made up to the final maximum value of 10mA.
In 1980ís, research activities have been made for high intensity proton linacs to be applied to the nuclear fuel breeding and high level radioactive waste transmutation. After the OMEGA (the partitioning and transmutation research) program was proposed by the Japan Atomic Energy Agency, JAERI started the work to study an accelerator-driven transmutation system of minor actinides. In addition to the development of the OMEGA program, new basic neutron researches on material science, neutron irradiation, neutron physics and many other potential applications for applying the intense linac have been also discussed. Those include meson/muon production and spallation RI beam (mainly for nuclear physics studies) and radio isotope production.
JAERI had originally planned to build the pulsed
linac with an energy of 1.5GeV and a peak current of 100mA with
10% duty factor[1]. The design study has been intended to apply
the accelerator to the engineering test for the transmutation
system and obtain the technical validity to accelerate high peak
current from the beam dynamics point of view. In this accelerator
development, the R&D work has been continued on high brightness
ion source, radio frequency quadrupole linac (RFQ), drift tube
linac (DTL) and RF source, as well as the conceptual design of
the whole accelerator components. In the beam test, the current
of 70mA with a duty factor of 10% has been accelerated from the
RFQ at the energy of 2MeV. A hot test model of the DTL for the
high power operation with high duty factor was fabricated and
tested[2]. The conceptual layout of the NSRP-LINAC is shown in
Fig. 1.
General Concept
Recently, JAERI has modified the original plan by proposing an option of superconducting (SC) linac to meet requirements for a variety of basic researches mentioned above and an ultimate goal for waste transmutation. This SC linac will be operated in pulse as a first stage for the spallation neutron source and gradually upgraded toward CW by increasing duty factor. The SC linacs have several favorable characteristics as follows; the length of the linac can be reduced, which can meet the rather stringent requirement from the limited area of our laboratory site, and high duty operation can be made for simultaneous experiments. The possibility to inexpensive operation cost may be found in comparison with normal conducting (NC) option.
A preliminary specification for the NSRP LINAC is
given in Table 1. The final value of the energy/current, accelerating
frequency and pulse time structure etc. will be determined from
further discussions based on the user requirement and the cost
estimate. In particular, because the relationship between the
energy and the current is complimentary, the reduction of the
energy value can be compensated by increasing the beam current.
Neutron scattering facility will require more strict pulse time
structure. The beam chopping capability with about 400ns intermediate
pulse length will be needed to compress the beam width by the
storage ring. Three major R&D items are presently carried
out. 1) the beam dynamic calculation including the high b
linac. 2) the development of the negative ion source[3] and the
fabrication of high power test models for CW-RFQ and CW-DTL. 3)
the SC cavity development with the KEK electron SC group[4].
Table 1 A preliminary specification of the JAERI NSRP-LINAC
| Energy
Accelerated particle Average current: Low energy part High energy part
Pulse structure Repetition rate Macropulse width Intermediate pulse width Chopping factor: Peak current | 1.5GeV
Negative and positive hydrogen ion First stage; 1mA Second stage; Maximum 10mA Normal-conducting linac: 200MHz Super-conducting linac: 600MHz First stage ; Pulse mode operation Second stage; CW/pulse mode operation maximum 50Hz 2ms (at1mA operation) -> maximum CW 400ns (interval 270ns) 60%: nominal 17mA |
Low Energy Accelerator Part
In the case of a high intensity accelerator, it is
particularly important to maintain the good beam quality (low
emittance; small beam size and divergence) and minimize beam losses
to avoid damage and activation of the accelerator structures.
The R&D work for the low energy portions has been made as
a first step in the NSRP-LINAC development. Table2.1 gives the
preliminary specification of negative ion source which will be
necessary for the injection into the storage ring.
| Accelerated particle
Energy Current Emittance(rms) Type | H-
70keV 30mA 0. 2pmm.mrad Single /multi-aperture Volume type |
| Energy
Current Frequency Vane voltage Length Number of cells Bore radius Synchronize phase Total power | 70keV - 2MeV
nominal 17mA 200MHz 88kV 3228mm 183 5.93mm -30º 280kW(60%Q) |
Because the superconducting accelerator has been selected for the high b linac, the low energy part should be capable for the CW mode operation. The design study has been started to develop the CW-RFQ (at 200MHz) cavity in the range of 20Å`30mA. From the experience of the pulse RFQ operation, the maximum electric field will be reduced to be 1.43 Ek (Kilpatric Limit) compared to the previous value of 1.63Ek. The calculated transmission for the CW-RFQ is 97% for 20mA and more than 90% expected for the wider range of 0 - 60mA. Because the most important problem for the R&D-RFQ was the RF contact between vane and tank, the CW-RFQ will be made as integrated type by brazing without any RF contact between vane and tank. In 1996, the high power test model of the CW-RFQ of 50 cm in length is fabricated and tested in order to establish the manufacturing and assembling techniques. Table 2.2 gives the preliminary specification of the CW-RFQ.
The parameters for the CW-DTL are also re-evaluated
to match the CW operation for the new superconducting design concept.
The frequency of the CW-DTL is chosen to be 200MHz. Accelerator
gradient may be lowered to be 1.5MeV/m in order to reduce the
RF consumption and the RF heating. The expected maximum magnetic
field gradient for the focusing magnet is about 60.1T/m using
the hollow conductor type Q-magnet. The end point energy for the
DTL is 100MeV which will be determined from the beam dynamics
and mechanical consideration of the high b
structure.
|
| |
| Cavity configuration
Average synchronize phase Accelerating length (m) Total length (m) Number of cavities Number of cells Output emittance (50mA) x:pcm.mrad (rms) y:pcm.mrad (rms) z:pdeg.MeV (rms) Total wall loss (kW) |
|
|
Ep:Maximum
peak field
| Energy
Current Frequency Accelerating gradient Synchronize phase Number of cells Length Focus gradient Total wall loss | 2-100MeV
nominal 17mA 200MHz 1.5MV/m -35º - -25º 239 90.17m 60.1T/m - 26.6T/m 3.16MW (100%Q) |
High Energy Accelerator Part
Superconducting cavity is selected as main candidate for high energy portion. In the CW electron accelerator, technologies of SC accelerators are established. Long design and operating experiences are accumulated and routinely used for the operation such as KEK-TRISTAN and other many accelerator laboratories. In the proton accelerators, however, the proton velocities b gradually change from 0.43 to 0.92 corresponding to the energies for 100MeV and 1.5GeV. Accordingly, the length of the cavity also has change. Main concern is the strength of the cavity under the vacuum load for the low b region. The mechanical structure calculations with the ABAQUS code have been made to determine the cavity shape parameters as well as electromagnetic ones with the SUPERFISH code[4].
In order to determine the layout of the SC accelerating structure, two typical cases of the SC linacs, which are composed of 4 different b sections and 8 different b sections, respectively, have been studied. The cavities in each b section will be made identical with 4 cells and designed at the specific beam energy but also can be operated at slightly different beam energy with lower efficiency. The structure of the cryomodule, input/HOM couplers and tuning devices etc. are being designed based on the KEK-TRISTAN experiences. Using these parameters, preliminary calculation for the beam dynamics has been made with the modified PARMILA code. Preliminary data is given in Table. 3.
The test stand for a superconducting cavity development
with the cryostat 80 cm dia. x 350 cm long and a clean room is
under preparation and the first SC test cavity will be fabricated
and tested within 1996.
The R&D work for the prototype linac structures (ion source, RFQ, DTL and RF source) has been performed. The good performance of the components has been achieved.
Since 1995, the basic specification for the accelerator
has been changed such as negative ion acceleration, SC cavity
option and storage ring. The new design modification has been
started. The test stand for the SC cavities is under preparation.
For the injector of the SC cavities, continuous-beam or much longer
duty operation will be required. The design work on the RFQ and
DTL as well as SC cavities for the CW operation is being performed.
The authors would like to thank Drs. S.Noguchi, K.Saito
and E.Kako of KEK for discussion and help on the SC cavity development.
They also thank Drs. T.Kato and Y.Yamazaki of KEK and Dr. R.A.Jameson
of LANL for valuable suggestion about the beam dynamics calculations
and accelerator system optimization. The PARMILA calculation carried
out by Mr. Y.Honda of MHI is greatly appreciated.
[1] M.Mizumoto et al, ìDevelopment of High Intensity Proton Acceleratorî, 2nd Int. ADTT Conf. 2-7 June 1996, Kalmar, Sweden
[2] K.Hasegawa et al., ìR&D Status on the High Intensity Proton Accelerator in JAERIî, Proc. of the 10th Symposium on Accelerator Science and Technology October 25-27, (1995), Hitachinaka, p233 and ìTHP09î in these proceedings.
[3] H.Oguri et al., ìDevelopment of a high brightness negative hydrogen ion sourceî, Rev. Sci. Instrum. 67 (3), p1051 (1996) and ìTHP10î in these proceeding.
[4] N.Ito et al., ìDevelopment of a Superconducting Cavity for the High Intensity Proton Linac in JAERIî, ìTHP11î in these proceeding.