Proceedings o f the 21st Linear Accelerator Meeting in Japan(Septem ber 3 0 -October 2，1996, Tokyo, Japan)〔P 1 一 2了〕

IFMIF Accelerator RF systems Conceptual Design
M. A. Chernogubovsky
Japan Atomic Energy Research In stitu te (JA ER I)
Tokai-mura, N aka-gun，Ibaraki 319-11, JapanABSTRACT
Analysis of segmented RFQ electrodynamical properties and the conceptions of RF systems design with di­rectional selective coupling principle for excluding different type transmission lines long feeders，undesirable
modes eliminating under intense beam acceleration, matching of RF source, producing of RF control system
supporting signals, accelerating structures fine tunings with electro-optical method are considered.

1Introduction

The IFM IF [ 1 ]high duty deuteron linear accelera­
tor specifications (40 MeV for intense 125 mA beam)cause special requirements on accelerating structures
and all the RF systems design. A part from the or­
dinary constructing of high power feeders, matching
RF source nonreciprocal devices and coupling ele­m ents design the application of four-vane RFQ for
initial acceleration sets the problem [2,3] of undesir­able modes eliminating (stabilizing) and the struc­ture precise tuning. At the advanced segmentation
principle [4,5] application for relatively long RFQ
structure design the operating mode special features
m ust be also considered. Presenting RF system con­ception allows to obtain simultaneous solution ofthese problems for RFQ and for the second stage
DTL accelerators instead of traditional separate con­sideration.

2

RFQ segmentation

According to the method of RFQ modes analysis [6 ]all quadrupole modes are delivered at electric (idealm etal) walls setting in II’ and II" symm etry planes,
and the longitudinally equidistant arrangement of
two identical cuts of the vanes yields the symmetry
conditions for electric or magnetic walls placing inthese c u t’s m idpoint z ニ zs , F ig .1 ;operating mode isdelivered at electric walls1. The mode properties do
not depend on presence of thin coupling plate (F ig .1in [4]) in this position, however, the second mode
(magnetic walls in the m idpoints” ones will be de­pendent greatly. In contrast, the influence of the cut
size on the mode frequencies splitting is rather weak,
2It is indicated by [4] description of m agnetic field config­
uration in undercuts region.
2 This m ode is considered as the operating one in [5].

vanishing for long segments. Radial quasi-TEM wave
in four identical shortcircuited inhomogeneous coax­
ial (quasi-rectangular) transmission lines, loaded byelectrode end - inter-wall corner capacitances, forms
operating field; the voltages between the ends of thevanes in any radial cross-section do not depend on
the cut size and are equal to inter-vane ones, so it
is alternating signs mainly longitudinal field in the
midpoint region with zero field on the axis.
IFF ig .1 . Inter-segment gap in segmented RFQ.
Minimal distortion of the beam dynamics by the gap
field can be obtained at the symmetrical field dis­tribution, th a t leads to the gap design in electrodes
without m odulation region: p intervals on Fig. 2.

Fig. 2. Gap forming elements.
In this case the focusing quadrupoles will be added
before and after the gap with alternating mainly lon­
gitudinal field in accelerating bore ; p intervals can bedifferent in the first and the second gap for the differ­ent accelerating conditions. Requirements on electricstrength constrain desirable minimization of the gap
size and the same strength as in the main RFQ partcan be realized under spherical shape of the vane cor-

ners and rounded edges at 6 = ((r 0 + R ) / V 2 ) — R.
Transverse field orientation in p interval at thebeam aperture bounded region consideration allow
to place magnetic walls M l and M 2, Fig. 2, so thegap field is continuous extension of the sphere in cube
field in x < r 0 R, zg < z < zs space. Mirror image’s m ethod for this electrostatic problem solution
at symmetrical set 一one charge in the center with
six symmetrically disposed on x f , y 、? axes at S dis­
tance ones - provides the desired continuation. The
cube walls images potential evenness and per m uta­
tive invariance refer to x f, y’, z' allow to use boundary
face for determ ination of S and charges values; withthe equivalence of the sphere and uninterrupted elec­
trode potentials condition it yields g interval field
result, which is convenient for PARM TEQ code [7]modification.

3

Directional selective cou­
pling

3.1

R F sy ste m

Four loops feeding of RFQ segments gives the pos­sibility for the directional selective coupling princi­ple [6 ] application: the RF system contains three
3 db directional couplers D 1 —jD3, equal length feed­ers 込 一 , 4 and I5 — Iq to attain equal am plitudes
cophased excitation at equal coupling loops spaces
in each segment; feeders-loops m atching on operat­
ing mode results in full decoupling of m atched loads
cl —c3, Fig.3.

Lowering of the undesirable modes quality factors iseffective for intense beam ，[6 ], so the dipole stabi­
lizer rods (F ig .1 in [4]) can be excluded. Under thering bridges installation on each segment in threecross-sections the m ajority of the different types longfeeders will be exclude cl also. B ridge’s realization on
square coaxial line is a sim plest, centra! diam eter
will be about 0.8 M and there will be enough space
between RFQ and the bridges for the square linewith accordant electric s tr e n g th ;I 5 ,lg feeders will
be about 1 M length, and l i . . . 14 lengths — 20 cm,Fig.3. At the beam excitation the loads remain de­coupled at operating mode but undesirable modes
are loaded on appropriate loads c l … c3 (the max­
imum loading is attained ju st at 3 db transient a t­tenuation of the couplers), while RF source is de­coupled from these modes; the nearest nonexcluded
mode will be 9 longitudinal alternation’s quadrupole.
In addition, the system assures RF source m atching- e.g., in damage case in one of / 1 . . .I 4 feeders or aloop breakdown the source feeder SW R will be 1.4
for vanishing li .. .Iq lengths and correlation of thehigh level signals び1 一ひ3 determines the damaged
channel; in operating conditions any deviations fromam plitude’s equality of these low signals detect theoperating mode asymmetry.
Real system loops can not be absolutely equal, so
the signals are in proportion to the resonator re­flectance; び1 ，
ひ2 and US phases form supporting
for autom atic control system phase-lock loop. For
all th at, any frequency differences between operat­
ing and eliminating modes and even their coinci­
dences are admissible at least over the directional
coupler’s frequency band, the couplers central fre­quencies must be on the quadrupole mode resonancefor additional loading minimization.
A high degree of precision is not required for thesystem components realization even for long /! . •. Iq
feeders case; undesirable modes levels proved to be
not more than —36 db at [6 ] experiments. Supple­
m entary lowering of the quality factors ( 1 0 — 2 0times) is obtained at mismatched loads c l … c3 ; e.g.,
for short l\ . .. , 4 feeders the c l , c 2 resistance m ust be
greater than the wave impedance and usually applied
circulator will assure the m atching in damage cases.

余每杂
杀
44

3.2

L1L3L2L4Fig. 3. RF system (not uniform longitudinal scale,
L —total length).

R F tu n in g

The strong advantage of [4] principle is possibil­
ity of fabricating of relatively short segment RFQs,
those can be tuned separately under the real short cir­
cuiting plates setting in the segment c u t’s m idpoint: ニ ，F ig .1 . At accordant turn of the system Fig. 3
loops any desired well-definable mode will be excitedand the segment neid sym m etrization can be carriedout by the adjacent dipole modes degeneration fea­tures [6 ]; in this case decoupled tuning elements atthe vane’s undercuts are to be provided.
Determ ination of the quadrupole field symmetry
axis positions (the [6 ] m ethod accuracy is about
1 0 mkm) gives the means for exact connection of thetuned segments; under the precise tuning m ethod[8 ] application the accuracy will be about 4 mkm.
The p spaces on Fig. 2 can be used for muster gauges
or contact optical targets installation, which gives a
convenient way for precise forming of the datum axis
at RFQ segment field fine tuning and especially forthe desired setting in coincidence the sym m etry axes.
Connected segments field sym m etry can be also ex­
amined by the degeneration features and the exact­
ness of each coupling loop tuning can be examinedon quadrupole mode resonance at matched-loaded
the rest eleven loops by SW R=23 value verification.
Electro-optical m easurements for desired imple­
m entation of RFQ vane ends m odulation can be com­bined with the axis determ ination procedure underequipotentials detecting principle by the means ofdoubled length lighted strip rotation on the roundphotosem iconductor plate, F ig .1 in [8 ]. This princi­
ple at DTL tuning for precise measurements of ac­
celeration period positions is realized by full light­
ing of the same round plate at minimal lighted —unlighted frequency deviation positions detecting in­side the drift tubes. Autoreflection method with [8 ]optical system gives several m icrom eter’s accuracy
at the tubes geometrical sym m etry axis determ ina­tion, and the field measurements in acceleration gaps
and in ramp-field regions can be carried out by thinlongitudinal photosem iconductor cylinder with fulllighting.

4Conclusion

Conception of directional selective coupling principle
application for segmented RFQ accelerating struc­ture has been developed. The same conception canbe applied for Alvarez-type DTL feeding or especiallyfor advantageous H-type DTL.
Analysis of segmented RFQ operating mode al­
lowed to estim ate the accelerating bore field distri­bution and the electrodes possible shapes in inter­segment gape regions. However, it is necessary toprove the proper beam particles dynamic possibility
for this quadrupole with mainly longitudinal fields
in the gaps. Electrostatic approxim ation of thesefields for com puter simulation of the beam dynamics
is proposed.
The devised RFQ and DTL precise tuning m eth­ods are provided by projections forming and mea­suring optical system under electro-optical principle
application.

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