LOW BETA STRUCTURE IN LINAC

s. Inagaki
National Laboratory for High Energy Physics
In LASL the studies of low beta structure W < 0.04) of linac are triggered
by PIGMI (Pion Generator for Medical Irradiation) and FMIT (Fusion Material
Irradiation Test) project.
The focusing is the most serious problem in low beta region. D. Swenson( 1 )adopted t~e alternating ph~sy focused linac for PIGMI. The idea goes back toM. Good( 2 or Ia. Fainberg 3 in fifties, but was not payed much attention tobecause of the expected narrow stable region, though B. Murin reported thesuccess to accelerate 2 mA protons from 50 keV to 575 keV. D. Swenson devided
the PIGMI proto-type into four; APF, QUAD RAMP, PHASE RAMP and DTL sections(Fig.l). The APF structure has seven superperiod, each of which consists of FDDF
radial rf focusing structure. The synchronous phase of the first gap decreases
from - 78° to - 66° by 2° step in every superperiod. The author showed that thestructure has a better space charge current limit yampared to a simple FD struc( )ture i f the designed electric field is attained. ItThe idea to give up the axial symmetry dates back to the early sixties. 5In 1969 Kapchinskii and Teplyakov proposed to generate an rf field by a configuration of four conducting wires. Soon various versions were proposed as a fourchamber resonator, a double-R resonator{ y four-vane cavity (Fig.2) and a cloverleaf cavity. The papers by Kapchinskii 6 gave a comprehensive survey of thetheory. In 1978 the vorkshop for RFQ structure were held for FMIT facility inLASL. In the case of RFQ the focusing is rather easily attained and the problem
is how the four vanes should be scalloped to get the acceleration, how it should
These studies
be manufactured and the four chamber should be excited ~tc.
indicate that the injection energy can be reduced to as low as 50 keV at 400 MHz
for protons. The first cell length, which is defined by SA/2, is 3.52 mm. This
gives the byproduct that the beam has very low ingidity defined by?''15E; r )-:S-

R = 2 rr. ( :l TL e'!1\oC.

(')~sTherefore the structure can be designed to have a good bunching effect. By a
suitable combination of synchronous phase and electric field the capture efficiency over 98 % are easily obtained. Fig.3 shows an example of phase oscillttion and energy and Fig.4 gives the capture efficiency-length charactristics. 7 )The distance between vanes a and the modulation factor m can be inversely derived
by the following relations.
.
('1>\'"-1)/c, I,t-l<.r),1.
V [ .::Lr(..l.J
E. t =- "3:" To vo-o J. T + -m:'· r. u•• ~L> -r r. t "lr~.,.IL) M- i. ,*· J

Eo'f = - V ( ~ )"'M....21f,E.ol =(3)('»1.2 - ,> r.t-k .. t')v-;:-· ~2. r .. l-t..o..J -t r.(~.""a.) -le, c..&-o /.,l.

{_f-)

The auther congratulates their success early in this year.
References
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D.A. Swenson, "Alternating Phase Focused Linacs", Part. Accel., 7, 61 (1975).
M.L. Good, "Phase-Reversal Focusing in Linear Accelerators", Phy-:- Rev. 21_,538 (1953).
Ia.B. Fainberg, "Alternating Phase Focusing", Proc. CERN Simposium on High-233-Energy Accelerators and Pion Physics, CERN> p.91 (1956).
S. Inagaki, "Space-Charge Limit for an Alternating-phase-Focusing Structure",LA-8287-MS (1980).
G.M. Anisimov and V.A. Teplyakov, "Focusing by an Acceleraing Field",
Pribory i Tekhnika Eksperimenta 1, 21 (1963).
I.M. Kapchinskii, "Ion Linear Accelerator with Strong High-Frequency
Focusing-Beam Dynamics in the Section with Spacially Uniform Focusing
Part I", IHP-INZH-72-29 (1972).
S. Inagaki, "Longitudinal Capture in the Radio-Frequency-Quadrupole Structure",LA-8290-MS.

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FOCUSING..

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Fig.l PIGMI prototype focusing effects.

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210

Fi.g.2 Four-vane cavity.

t.en.r.ee-3e.~ee~

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UIJ•.SIS!II.U~

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o.o3

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Fig.4 Capture efficiency vs. length characteristics for PHI = 20 em.

Fig.3 Phase oscillation and energy.

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