TH E 1 GeV PROTON LINAG FOR THE JAPANESE HADRON FACILITY
Y. Yamazaki, T. Kato and M. Kihara
National Laboratory for High Energy Physics
Proposed design of the 1 GeV proton linac for the Japanese Hadron
Facility is described together with rationale for the design parameters. Thelinac is composed of a volume production type H ion source, a 432 M Hz RFQ
linac (3 MeV), a 432 M Hz D T L (150 MeV) and a 1296 M Hz high-|3 linac (1 GeV).
Problems expected in each part are discussed and possible remedies are alsopresented.
1 . Introduction
A 1-GeV proton linac w ill be constructed to inject proton beams tovarious ring accelerators of the Japanese Hadron Facility.1) Parameters of thebeams to be delivered by the linac are listed in Table I. The linac has three
distinctive fe atu re s:1 ) high energy, 2) a high average current and 3) a highduty factor of an rf system, that should be carefully taken into account indesigning the linac. Also, it is required that the linac can b e operated in
extreme stability.
Tabic I

Design parameters of the HlinacEnergy1

GeV Total length-5 0 0Peak current
Pulse length

20 mA Repetition rate
400 |HS
Average current

50 Hz>200 |iAm

A high energy proton linac accelerating an intense beam with a limited
length immediately requires very high rf power. Necessary rf power is further
increased for the following reason. In the high intensity and high energy
proton linac no beam loss is allowed at the high energy region of theaccelerator, since radioactivity caused by the beam loss becomes a serious
problem in a long term operation. Then sufficiently large beam acceptances arerequired for all of accelerator tanks. This requirement tends to increase bore
radii of the accelerator tanks and shift synchronous phases further from thephase of the highest rf field, resulting in lower acceleration efficiency. Thus,
the requirement of no beam loss also implies increase of the rf power.
A fundamental scheme of the proton linac was proposed as shown in Fig.
1 . The linac will be composed of an ion source, an RFQ linac, a drift-tube linac(DTL) and a high-p linac. Negative hydrogen beams will be accelerated, since
the injection efficiency is higher than the multi-turn injection of proton
beams.

83 m

411 m

Ion SourreHRFQH DTL й(432 MHz)t夺

50keV 3MeVF ig .1

Hiqh-g Linac(1296 MHz)

♦150MeVThe scheme of the proton linac.

tt

IGeV

Among various parameters of linacs, an accelerating frequency is
one of the most important parameters, since cost and performance of the linacs
w ill he strongly dependent upon the frequency. In general both of shunt
impedances and possible accelerating fields of cavities with the same figure
increase proportionally to a square root of the resonant frequency. However, as
the frequency increases, sizes of accelerating cavities and klystrons decrease,so that cooling of these R F parts becomes difficult and beam acceptances of thecavities decrease, Thus, the nearly highest frequency should be chosen, so far
as the cooling of the R F parts is feasible and the beam acceptance is large
enough. Also, it is advantageous to choose the frequency, at which commercial
klystrons arc available. Taking these factors into account, we have chosen
1296 M H z for the frequency of the high-p linac and 432 MHz for those of theR F quadrupole and drift-tube linac.
2. Ion Source
一We are planning to use a volume production type H
ion source rather
than a surface plasma type multi-cusp ion source whose possible cesium vapor
flow w ill reduce the breakdown voltage of the following RFQ. We havedeveloped a test model of the ion source, whose typical parameters are listed
in Table II.
一Table II Typical parameters of the test Hion source of the volume-production type
Arc current
Filament current
Anode bias

14075+5AArc voltage
A x 4
Hydrogen gas flowVAcceleration voltage

150 V5seem
30 kV

In order to increase the beam current density we are planning toinvestigate its dependence on the volume of the ion source. It is expected thatthe density of the excited hydrogen molecules would be increased bydecreasing the volume of the ion source at the same arc condition, resulting inincrease of the H
beam current.
3. RFQ Linac
The H beam from the ion source will be injected to a 432 M Hz fourvane type R FQ linac at the injection energy of 50 keV and will be accelerated
to 3 MeV. Design parameters of the RFQ linac are listed in Table III.
RFQ (four-vane)
3 MeV
Output ene rgy
Number of cells
3050.340 cm
0.236 cm
15.4 cm10%)
7cmm»mrad1.50/V

Table III Parameters of the 432 MHz HFrequency
432 MHz
Input energy
0.05 MeV
Vane length
269 cmMean bore radius
Minimum bore radius
Cavity diameter
Normalized acceptance%)

10)
(9%%)

響

Normalized emittance at 3 Mev

1.0
2.6
1.30

(9Kilpatrick factor
Maximum surface field
Focusing strength
Transmission

361(Es,max)(0 mA)98 %1.8(20 mA)

4.094 %

kV/cm

The parameters were determined to obtain a normalized acceptance of
1.5 7cmm«mrad for the beam current of 20 mA with the maximum electric fieldEs,max of 1.8 times of the Kilpatrick limit • The RFQ linac with the small bore
radius w ill require rigid tolerance limits on dimensional errors, and it isdifficult to machine, assemble and align the long vanes accurately.
To test a
feasibility of the R FQ linac with these parameters a prototype R FQ linac isgoing to be fabricated. Modification of the parameters of the RFQ linac to makemanufacturing of the R FQ easier, for example, by reduction of the acceleratingenergy and/or frequency, may require change of parameters of the following
drift-tube linac.
4. Drift-tube Linac
A 432 M Hz drift-tube linac (DTL) will accelerate the beam from 3 M eV
to 150 MeV. Parameters of the drift-tube linac are listed in Table IV. The linac
w ill be separated into 13 tanks, each of which w ill be driven by a 1 M W
klystron. Shunt impedances of the drift-tube linac are shown in Fig.2 as afunction o f p(v/c). Three parameters of an inner diameter of tanks, an outer
diameter and a comer radius of drift-tubes were approximately optimized toobtain the highest shunt impedances, keeping the maximum surface electricfield within 75 percent of the Kilpatrick limit •Table IV

Parameters of the drift-tube linac and high-p linac

Frequency
Input energy
Output energy
Accelerating field

DTL
432
3148
3

High-p linac
MHz1296
148
MeV
1017
MeV
3.6 〜 4.4 MV/m

Synchronous

-26°

-30°

75.7
83.3
o.5
342
139.0
2.9
11.9
13
1.0

303.0410.9
1.53568
152
81.7
17.4
99.1
36
3.0

phase

10.0MVMWMVMV

Input phase spread

88°

Output phase spread

24°

3.08.8

MeVMeVо
230.301.447cmm®mrad

Tonm.mrad
7tmm.mrad

о78

Acceptable
Input energy spread
Output energy spread
Acceptable7cmm®mrad

43

8.8
10.0

62(10Q%)

8.9

63( 100%)
Ay,n (90 %)

mm
cm

92

Tank length
Total length
Bore radius
Number of cells
Number of tanks
Total wall loss
Beam loading
Total power
Number of klystrons
Klystron power
Normalized acceptanceAx,n (90 %)The 88° phase acceptance of the D T L will be large enough for the 30。
phase emittance of the RFQ even with the drift space between the RFQ and DTL.
Even if the frequency of the R FQ is halved to increase the bore radius of theRFQ, the D T L can longitudinally accept the beams from the RFQ. Therefore, it
is worthwhile to investigate the possibility of the 216 MHz RFQ linac, althoughanother type of a 216 M Hz RF power source is required in this case. We areplanning to use permanent magnets made of SmCo, since the permanent magnets
require neither of wiring nor water-cooling for the magnets, that is, become
maintenance-free and SmCo can produce a strong magnetic field, being rather
stable against effects due to radiation。However, it is difficult to seal the drift
tubes containing the SmCo, since the SmCo cannot stand the high temperatureused for the silver brazing and the strong magnetic field produced by the SmCo
inhibits conventional use of the electron-beam welding (EBW). At present
attempts to seal the drift-tubes containing the SmCo are in progress by a fewmethods including EB W and laser welding, but results are not yet satisfactory.
It w ill be another method to expose the SmCo to vacuum, and outgassing
measurement of the SmCo in vacuum is in progress.

^fi (v/c)

Fig.2
Fig.3
Shunt impedances of the drift-tube linac (Fig.2) and the side-coupledlinac (Fig.3) as a function of p. The values in M£2/m as calculated with
the SUPERFISH are plotted.
5. High-p Linac
Standing wave linacs are more advantageous than traveling wave linacs,
if R F pulse widths are longer than filling times ( 〜 a few \is typically) . The 冗/2
mode operation of a multi-cell cavity is necessary to keep a high degree ofstability of the accelerating field against effects due to heavy beam loading
and manufacturing imperfections. Then, possible candidates for the high-pcavity structure are alternating periodic structure (APS) without nose cones orcoupling slots, alternating periodic structure with nose cones and couplingslots, side-coupled structure (SCS), disc-and-washer structure (DAW ) andannular-coupled structure (ACS).
For the annular-coupled structure it was reported that serious
depression of a quality factor is arising from excitation of a coupling-cell
quadrupole mode.2)
Although a possible remedy was proposed，
2) extensive
study will be necessary to solve the problem. In the disc-and-washer structurea TM1 passband crosses the accelerating frequency. A method to keep the TM1
passband away from the accelerating frequency decreases a shunt impedanceseriously.^)Both of the alternating periodic structure and side-coupled structure
are free from these troubles. Moreover, in the alternating periodic structure
coupling cells are located on the beam axis, consuming space for accelerating
cells, and space for the coupling cells is relatively limited compared with theside-coupled structure. Therefore, a shunt impedance of the alternating
periodic structure is lower than that of the side-coupled structure, and aquality factor of the coupling mode of the APS is lower than that of the SCS
resulting in more strict requirement for manufacturing accuracy 4). On theother hand the axially symmetric structure o f the APS, in particular, without
coupling slots has the following advantages. The field of the TMO mode of theAPS is more symmetric. Machining and assembling of the APS are easier,allowing a more variety of assembling methods. Also, we have lots ofexperience in manufacturing and operation of the APS that is used in theT R IS TA N rings. Therefore, we decided to develop both of the side-coupledstructure and alternating periodic structure in parallel for the time being. In
the following paragraphs, however, results of computation with the SCS arepresented as an example. Shunt impedances of the SCS calculated with acomputer program SUPERFISH are shown in Fig.3 as a function of p. A rather
large bore radius of 1.5 cm as 1296 M Hz structure was chosen to obtain a largetransverse acceptance. The gap length was adjusted to optimize the shunt
impedance. The sizes of the coupling slot was determined to obtain a coupling
constant of 5 percent with a three dimensional computer program M A FIA 5).
A proposed typical configuration of the high-p accelerating tanks isshown in Fig.4. The drift space between two tanks have space enough for two
quadrupole magnets and cither of a steering magnet or a beam monitor. Shortertanks will be used in the low energy side to increase transverse acceptances.
Transverse and longitudinal acceptances thus obtained are shown in Table IV.

Fig. 4 A typical configuration of the high-p linac.
It is seen that the normalized transverse acceptances of the high-p linac
is about 3 times of those of the D T L and 10 〜 15 times of those of the RFQ.

However, it should be noted that the acceptances were computed only for thesynchronous beams. It is found by the computer calculation that thetransverse acceptances are reduced to 60 percent of those of the synchronous
beams, if the beam phase is located near the separatrix. Thus, more detailed
study will be necessary, including study of effects due to possible imperfectalignment of the cavities and quadrupole magnets, to decrease the bore radiusof the cavities.
For detailed design of the high-p linac including tuning
method and so on it is required to estimate machining and assembling errors
and to find the best method of welding or brazing. Thus, attempt is being madeto fabricate the cavities using silver-brazing, electron-beam welding,electroplating welding and diffusion welding.
6. Modulator
One of the most difficult parts of the proposed 1-GeV proton linac will
be a high power modulator that drives the 3 M W klystron with a long pulse(600 p.s) and a high duty factor (3 %). Also, it should be operated with extreme
stability and reliability.
We decided to develop a line-type modulator rather
than the othertypes, for example, a hard-tube modulator for the following
reasons. First, we have some experience for line-type modulators. Second,line-type modulators provide stable pulses with relatively simple circuits
whose behaviors can be easily understood. Third, efficiencies of line-type
modulators are
better than hard-tube pulsers. Finally, running costs w ill beless expensive,
since no replacement of the tubes is necessary. Power-gridtubes used in hard-tube modulators are very large and expensive, while theirlife time cannot be expected to be long.
We have designed a prototype modulator for a 6 M W klystron rather
than the 3 M W klystron for the following reason. In order to obtain klystrons
that can be stably operated at 3 M W in unsaturated region, it is necessary todevelop klystrons With the power capability of 5 or 6 MW. Development of the6 M W klystrons requires a modulator with an output power of 15MW for aklystron efficiency of 40 %, and successful development of the modulator will
directly lead to development of stable modulators for 3 M W klystrons at thesame time. Parameters of the modulator designed for the 6 M W klystrons arelisted in Table Y.
TableVParameters of the test modulator for a 6 M W klystronAжu%.
54) Y. Yamazaki, T. Higo and K. Takata, Part. Accel, in print.
5) T. Weiland, Part. A c c e l . П , 227 (1985).

s0a.
7(short term) <0.2%(long tenn) <0.5 %hRefcfences1 ) M. Kihara, K E K Preprint 86-106 and references therein.
2) R. K. Cooper et a l . , Preprint LA-UR-83-95.
3) S. Inagaki, Nucl. Instr. Meth. A 2 5 1 . 417 (1986).

w

04575
2652
<0к

Average power
Output current
Load impedance
600 jis (flat top)
Flatness

оPeak power
15 M W
Output voltage
20 kvPFN impedance
26.7Q
Pulse width (half value)
Pulse rise time
<30 usRepetition rate
50 ppsPulse voltage stability