2-RFQ for light ion acceleration

Nuclear instruments and Methods North-Holland, Amsterdam

FOUR-ROD-~/2-RFQ

in Physics

FOR

Angewandte

Physik

BlO/ll

der Universitiit

831

(1985) 831-834

LIGHT ION ACCELE~TION

A. SCHEMPP, H. DEITINGHOFF,

Insrrrurftir

Research

*

M. FERCH, P. JUNIOR and H. KLEIN Frankfurt/Main,

Robert-Mayer

-Str.

2 - 4, D - 6000 Frankfurt/Main.

West Germany

A simple type of RFQ structure with circular rods as electrodes has been developed in Frankfurt. The improved design uses a linear arrangement of supporting stems on a massive common bar. This linear rf structure consists of a chain of X/2-line pairs and leads to an advantageously simple but nonetheless effective RFQ structure. With this stable cheap type of RFQ resonator preaccelerator prototypes have been built for light ions. New results of electrode and structure optimization and beam measurements are presented.

1. Introduction

2. Electrode design The radiofrequency quadrupole structure is widely used as replacement for bulky C~kroft-Walton injectors. Because of the spatial homogeneous rf focussing used, high currents can be accelerated from low ion source extraction energies. There are many applications besides injectors for high energy accelerators [l], e.g. designs for light and heavy ions for a variety of nuclear physics applications, for fusion driver and fuel breeder injectors as well as cheap structures directly following the ion source for highly charged ions or decelerating efficiently ions which are fully stripped f2-41. In most projects [l] the TE,,&vane resonator is used. As part of the HIF program in Germany [5] we have started in Frankfurt the development of the X/2 RFQ as an alternative structure, which can easily be operated at low frequencies, high duty cycles and has the advantage of easy manufacture and tuning. Another feature is the feasibility of circular rods as electrodes giving very good mechanical as well as very good beam dynamic properties.

Conventional RFQ designs are based on a two term potential from which the axial fields are derived. This leads to a complicated 3D milling procedure for the electrode surface. Improvements intend the simplifications of the manufacture. Our solution uses cylindrical electrodes with cones and cylinders of variable diameter [6] as illustrated in fig. 1. The deviation from the ideal profile is obvious and the effects of higher field moments have to be investigated. We have done an expansion of the potential function, a 3-dimensional leastsquares fit (7,8] with up to 30 harmonics. Our proton RFQ (9,101 has been an example, and the results show a significant increase in acceleration rate compared to the ideal profile with nearly unchanged focussing strength as demonstrated by fig. 2. We are designing a short H; linac (parameters in table I). Fig. 3 demonstrates as example the 22nd cell of the accelerator (no. 22: (p,= 58.13*, p= 6%, PX = 1.632 cm, A = 0.0857, x = 0.8369). Of course, numbers are much too precise for manufacture, however, the list should demonstrate changes within a BX cell.

Table 1 H l RFQ parameters

Fig. 1. Scheme of RFQ electrodes. * Work supported

Input, output energy Frequency Current limit 28 mA Length 60 cm Minimum aperture Electrode voltage Rf power

7.1% keV/N 108.5 MHz 0, = 53” 33 cells 3mm 54 kV 9kW

by the BMFT.

0168-583X/85/$03.30 0 Elsevier Science Publishers B.V. (North-Holland Physics Publishing Division)

VIII. ACCELERATOR

TECHNOLOGY

A. Srhempp ef 01. / Four-rod-X/Z-

832

RFQ

f

OkA

’

0.5

9

0.6

0.7

0.8

A

4

i

LP

Fig. 4. Scheme of a linear h/2-RFQ.

Fig. 2. Phase advance cost, as a function of acceleration coefficient A for the ideal profile (11, the electrodes of our proton linac (2) and our new profile with steeper cones (3).

3. The four-rod-A/24ructure

Unlike the 4-vane structure the h/2-RFQ consists of a coupled transmission line, which can theoretically be treated with lumped circuit elements. The basic cell of the X/2-RFQ structure can be considered as two coupled h/2-transmission lines excited in transverse n-mode. The capacitive coupling via the RFQ electrodes and the inductive coupling of four rods as the current loops of the radial stems form a very compact, efficient and flexible rf structure consisting of a chain of strongly coupled cells driven in axial O-mode. Even for 15 MHz the resonator can be built very compact with onty 50 cm tank diameter. For 100 MHz the structure has approximately 20 cm diameter and the length of a cell is between 8 and 20 cm. The mechanical design of our RFQ resonators has been further simplified by arran~ng the straight stems linearly 1111. The RFQ is brazed on a massive rail and

~-~

Fig. 3. cm, S, cm, R, cm, R,

----

Rotational

Dimensions of cell no. 22 of the H;-RFQ: S1 ==0.237 = 0.317 cm, S, = 0.490 cm. S, = 0.336 cm, S, = 0.252 R,=0.549cm.Rq=0.471 = 0.470cm.Rz=0.5500cm, = 0.551 cm, R, = 0.469 cm.

can be aligned and tuned outside the cavity on a bench and then screwed into the tank, which acts only as a vacuum vessel; the resonance frequency remains within 2% the same as outside the cavity. Compared with the earlier 90°-arrangement [9,10] the RFQ is now composed of a row of parallel dipoles perpendicular to the axis. Fig. 4 shows two such X/2-cells, the arrows indicating the current flow. Each electrode pair is supported by a common leg, so dipole modes being a problem in TE,,,-cavities are very unlikely. Fig. 5 shows the measured electrode voltage i& dist~bution for the first three modes for an untuned &cell structure, fig. 6 shows the dispersion curve. In principle any kind of electrodes (e.g. vanes) can be inserted into this rf arrangement. Even multibeam electrode systems have been tested (5,121. The electrode length per cell is small compared to the vacuum wavelength, so for the calculation of frequency and shunt impedance L, C-circuit description can be used. The impedance R, (U$‘rf power per length) can be calculated as a function of geometric parameters assuming constant current in all stem loops charging the electrodes: R,=X= N/L

2(n+

1)BfL

R,w2C$(2Hi

D)K+

NQ’

0 0

50

XXI

Icml

Fig. 5. Measured voltage distribution along the electrodes for the 3 lowest modes of an untuned 6-cell A/2-RFQ.

A. Schempp et al. / Four-rod-X/2-RFQ

The electrode impedance wCo determines the relation of R, and the Q value. In a wide range of parameters calculations agree very well with experimental results. Fig. 7 shows the optimized impedances as a function of the operating frequencies together with data from operational RFQs.

833

A prototype of such a linear h/2-structure has been built and operated (parameters: Y = 108.5 MHz, L = 55 cm, aperture 10 mm, H = 8 cm, D = 5 cm, R, = 165 kL?. m, Q = 3500). In rf power tests high fields could be achieved (cw: IS kW. U, = 67 kV; 25% dc: 40 kW, 110 kV). The linear structure allows simple tuning, because the magnetic fields are concentrated in the current loops formed by the stems. A tuning plunger was tested and a tune off 0.5 MHz could easily be obtained during operation.

4. Experiments Beam experiments have been done with a 108 MHz, 300 keV proton accelerator structure [lo,111 and a maximum current of 1.1 mA has been obtained. With the linear structure a short 155 keV/N Hi-structure is under construction replacing the straight electrodes of the rf test module. Because the beam current of the 300 keV proton linac has been limited by the injection system, we now use Hi, extracting a higher percentage of “wanted” ions from the duoplasmatron and still using our compact electrical einzel-lens system. Table 1 shows parameters of this short linac. Fig. 8 shows a cross section. We hope to go into operation by Spring ‘85. This high power cw structure forms a prototype for light ions. In a next step this compact resonator will also be screwed directly to the extraction system, which we think will simplify the system for industrial applications.

>

04

n

E 4

2

Fig. 6. Dispersion relation for a 6-cell x/2-RFQ.

RP IkR~ml soo400.

+

200

300

vIMHz1

Fig. 7. Normalized impedance R, for optimized h/2-RFQ function

5. Conclusions

P

1Ml

of frequency.

Fig. 8. Four-rod-X/2-RFQ

as a

The 4-vane and X/2-RFQ can use the same kind of electrodes. We prefer our cylindrical electrodes, which can simply be manufactured on a lathe.

cross section. VIII. ACCELERATOR

TECHNOLOGY

834

A. Schempp et ai. / Fiw-mod-h/2-RFQ

The basic cell of the 4-vane resonator consists of 4 azimuthally weakly coupled cavities. Longitudinally it is a homogeneous zero-mode cavity. A severe problem are azimuthal asymmetries, which lead to dipole components in the quadrupoie field. The additional sensitivity can be overcome by additional stabilizing schemes ff3f. In case of the X/&RFQ one electrode pair is driven by one common leg. so dipole modes are impossible, the operating mode is more or less determined by the cells inductance and capacity. The structure is also operated longitudinally in O-mode, however, at stronger coupling. Because there is a really separated function t, C, the structure can simply be tuned by changing the support legs like a trombone tuner. The impedances are comparable but especially for lower frequencies the X/ZRFQ is very compact. The X/2-RFQ structure can be properly tuned and aligned on a bench outside the tank. There are no demountable contacts and cw operation is easily possible, because all current conducting parts consist of dire&y cooled copper tubes and bars. Because of the geometry there are no problems with conditioning and multipacting= And last but not least, because the structure is simpler and mechanical tolerances are less sharp, the cost of a X/2-RFQ is by a factor of 5-X0 lower, based on industrial offers. Another project in our group is a 202 MHz, 18-750 keV H--RFQ using the 4-rod-X/2-RFQ principle. It will be exchangeable with the 4-vane-RFQ we are building for HERA in Hamburg ft4,fSl.

References

111H. Klein, IEEE Trans. Nucl. Sci. NS-30 (1983) 3313. 121R.H. Stokes, T.P. Wangter and K.R. Crandall. IEEE Trans. Nucl. Sci. NS-28 (1981) 1999. I31M. Olivier et aL. IEEE Trans. Nucl. Sci. NS-30 (1983) 1463. Inst. f. Angew. Physik. 141 A. Schempp, Univ. Frankfurt/M., ht. Rep. 84-l. (51 ;t-i.Klein et al.. GSI-Report. GSI-82-8 (1982) p. 150. I61P. Junior et al., IEEE Trans. Nucl. Sci. NS-28 (1981) 1504. 171I.M. Kapchinskij and V.A. Tepiyakov, Prob. Tekh. Eksp. 2 (1969) 19.

@I P. Junior et al.. Proc. Lin. Act. Conf., Seeheim (1984). GSI-84-11, p. 97.

Trans. Nucf. Sci. NS-30 (1983) 1425. WI A. Schempp et al.. IEEE Trans. Nucl. Sci. NS-30 (1983) 191 A. Schempp et al., IEEE

3536.

1111A. Schempp et al., Proc. Lin. Act. Conf., Seeheim (1984), CSI-84-11, p. 100.

1121H. Klein et al., Los Alamos Nat. Lab.. LA-9234-C (1982) D3f

P. 96. A. Schempp, Proc. Lin. Act. Conf.. Seeheim (19843, GSI84-11, p. 338.

(141 Project Study for the 50 MeV HERA Linac, Hamburg

(1984). 1151 A. Schempp, Univ. Frankfurt/M..

11%Rep. 84-14.

Inst. f. Angew. Physik,