MEQALAC: A 1-MeV multichannel rf-accelerator for light ions

Nuclear Instruments and Methods in Physics Research B37/38 (1989) 5088511 North-Holland, Amsterdam

508

MEQALAC:

A I-MeV MIJLTICHANNJZL RF-ACCELERATOR R.G.C. WOJKE and J.G. BANNENBERG

W.H. URBANUS, Association

FOR LIGHT IONS

EUR.4 TOM-FOM,

FOM-Institute

H. KLEIN, A. SCHEMPP,

for Atomic and Molecular Physics, Kruislaan 407, 1098 SJ Amsterdam,

R.W. THOMAE

The Netherlands

and T. WEIS

Institute for Applied Physics, University of Frankfurt, Robert-Mayer-Strasse

2-4, 6000 Frankfurt/Main,

FRG

P.W. VAN AMERSFOORT Association

EURATOM-FOM,

FOM-Institute

for Plasma Physics, Edisonbaan 14, 3439 MN Nieuwegein,

The Netherlands

The MEQALAC (Multiple Electrostatic Quadrupole Array Linear Accelerator) project deals with multichannel acceleration of light ions in a resonator cavity. An advantage of the MEQALAC over the RFQ is its higher acceleration efficiency. The total accelerated current can be increased via an increase of the number of beams. The space charge dominated beams are focused by arrays of miniaturized electrostatic quadrupoles; the channel radius is 2.5 mm. In a proof-of-principle experiment we accelerated four He+ ion beams of 2.2 mA per channel from 40 to 120 keV. The objective for the next stage which presently is under construction is to accelerate four N+ ion beams from 40 keV to 1 MeV. With the same acceleration structure both the ion mass and the exit energy can be varied up to a factor of 2 via a variation of the resonator inductance. Performance tests on various components are presented.

1. Introduction At the FOM-Institute in Amsterdam a novel device for acceleration of intense ion beams to MeV energies is under construction. The device is called MEQALAC which stands for Multiple Electrostatic Quadrupole Array Linear Accelerator. In this approach, originally proposed by Maschke [l], ions are accelerated through parallel channels. The ions are accelerated in gaps which

4 Grid

carry an rf voltage. In between the gaps electrostatic quadrupoles are placed which oppose the space charge forces of the intense ion beams. The advantage of this setup is that the rf power is used only for the acceleration of the ions, while in an RFQ accelerator it is used both for acceleration and transverse focusing. Therefore the power efficiency of a MEQALAC system is roughly one order of magnitude better. The total accelerated current can be increased by the number of channels,

Extraction

MEQALAC

section

source

Turbo

pump

1

Turbo

pump

Fig. 1. Setup of the FOM-MEQALAC

0168-583X/89/$03.50 0 Elsevier Science Publishers B.V. (North-Holland Physics Publishing Division)

2

experiment.

W.H. Urbanus et al. / MEQALAC

which is also an advantage accelerator

in comparison

The experimental

system.

ion source

A Low Energy

[3] with 42 quad high pressure acceleration

with a 40 kV four-grid Beam Transport

ion source section.

experi-

shown in fig. 1. It consists

arrays In

transports region both

rf

approach.

setup of the MEQALAC

ment [2] is schematically multicusp

of a MEQALAC-type

with the RFQ

of a

extraction

(LEBT)

section

the ions from

the

to the low pressure sections

rf

electrostatic

arrays are used. Typical dimensions are: quadrupole length 2 10 mm, channel radius a = 3 mm, interbeam distance 12.9 mm with four beams in a square configuration. The cylindrically symmetric beams from the source are radially matched [4] to the periodic LEBT channel by means of a matching section which consists of five quad arrays. The accelerator is a modified interdigital-H resonator using a FODO grid. A particle is successively focused by a quadrupole lens (F), accelerated in a rf gap (0) defocused by a quadrupole lens (D), accelerated in a rf gap (0) etc. The longitudinal matching to the accelerating section can be accomplished by a buncher in the LEBT section. Our project consists of two stages. In Stage I [5], which was terminated in 1986, four helium ion beams were accelerated from 40 to 120 keV in a 20 gap rf accelerator. The accelerating structure was mounted in a cylindrical resonator, operating at 40 MHz. The accelerated timeaveraged current was 4 x 2.2 mA. The resonator dimensions are: diameter 40 cm; length 65 cm. In Stage II [6] of the MEQALAC experiment the first objective is to accelerate four N+ beams from 40 keV to 1 MeV. The resonance frequency will be 25 MHz because of the higher mass and thus lower velocity of the nitrogen ions. A second objective is to accelerate four N+ beams from 20 to 500 keV. The resonance frequency will be 17.7 MHz in this case. The same ion source, extraction geometry and LEBT section are used as in Stage I. However, the new resonator cavity has a rectangular cross section. The rectangular cross section offers the possibility to vary the resonator frequency. Without changing the accelerator geometry, ions with different mass M and/of different energy E can be accelerated, provided the expression E(A4f2)-’ is kept constant, where f is the rf frequency. quadrupole

509

structure and resonance frequency. The parameter which can be varied in the extraction geometry is the voltage on the intermediate electrode. The electrode is mounted in between the plasma electrode and the extraction electrode; the first one is approximately at the potential of the source, the latter is at earth potential. The voltage of this electrode strongly influences the extracted ion current and the beam emittance. For an extracted N+ beam of 5.1 mA at 40 keV energy, a minimum unnormalized emittance of < 20 71 mm mrad was obtained at an intermediate electrode voltage of 32-34 kV. An unnormalized RMS emittance of 20 71 mm mrad is sufficiently small; such beams can be transported through the LEBT section without severe losses. The beam current and its emittance have also been measured behind the LEBT section. Fig. 2 shows the transported current for nitrogen beams of 20 and 40 keV and a helium ion beam of 40 keV. The injected currents are 1.8, 5.1 and 10.1 mA for the 20 and 40 keV nitrogen ion beams and the 40 keV helium ion beam, respectively. These currents are slightly less than the theoretically maximum transportable current. The optimum transmission is always about 90% at a zero current phase advanced per cell (pa) of 60 O. At smaller values of pa the beam losses are more severe because of the corresponding larger beam envelope; at p0 > 90 o the beam is unstable and the losses are about 50%. A problem which was not present in the first stage of the project is that the ion source now produces two kinds of ions, i.e. N+ and NC. Because MEQALAC is, like all rf accelerators, a fixed velocity machine only either N+ or NC ions will be accelerated. Therefore the fraction of the N+ current has been measured for various source parameters like gas pressure, arc voltage and arc current behind the extraction system and behind the LEBT section, with a magnet system to separate the two

10 helium,

40 keV, 1,” = 10.1 mA

nitrogen,

20 keV, Iin = 1.8 mA

8-

2. Extraction and transport of nitrogen ion beams through the LEBT

section

The source used in our experiment is a so-called bucket-type plasma ion source. The diameter is 14 cm and the depth is 11 cm. Rows of small CoSm magnets are mounted on the source walls and give a line-cusp magnetic field thus preventing the electrons emitted by the filaments to get lost on the walls. The extraction voltage is fixed for a given particle mass, accelerating

20 40

80

60 u0

100

[deal

Fig. 2. The ion current transported through the LEBT section as a function of the zero current phase advance per cell (pa). V. NEW IMPLANTATION

EQUIPMENT

510

W.H. Urbanus et al. / MEQALAC

masses.

A thin wire, at a potential

secondary

electrons,

the ion currents fraction

are measured.

of the N+ current,

and behind

of 40 V to suppress

is swept through

the LEBT

the ion beams

The results

behind

section,

and

show that the

the extraction

system

is 60% of the total beam

current.

resonator and accelerator structure

The Stage II resonator

with dimensions

and a length of 180 cm is constructed steel.

PARMILA

severe particle

structure,

when

to the extraction

by energy

induced

blowup,

caused

beam

compression, opposed

of the cavity.

gap voltages overall

are

by mounting

fig. 3) three

accelerating ator

gaps

trance

low

fulfilled,

structure

when

blocks

[6]

at reasonaat the en-

for additional

in the full scale modifications

distribution

(see

plates

a voltage

plates

FRONT

fig. 4).

VIEW

way around. Shorting plates at the entrance of the resonator between the entrance plate and the meander support

do reduce

By perturbation The

resonator

instead

(a)

at the enof the other

the free area

from

that the anticipated

voltage

resonance

f0 is still 1 MHz

ipated

frequency

25 MHz,

is adapted

reduces

design

is

ramp

so that in a second

via a reduction

which

resoof the

along the acceler-

has a high voltage

and a low one at the exit,

cell number Fig. 4. The measured voltage distributions, normalized to the maximum gap voltage, along the accelerator gaps for different structures: (a) conventional setup; (b) with shorting plates; and (c) tuned to the final resonance frequency by a reduction of the gap capacitances at the entrance.

the

slightly

increased

are investigated.

is obtained

gap

on a scale model

shorting

different

the voltage

shorting

average

20

10

be

structure.

a set of dummy

measurements without

can

at the en-

demands

while providing

free space in the meander (see

effects

ramp can be obtained

of the accelerator,

With

charge

longitudinal

a high

Rf measurements

that a voltage

ble loss power

nator

by strong These

are due

space

A high transmission

demands

is chosen.

trance

are com-

losses

spread,

demands

e

that

in the first gaps. On the other hand, a short

Both

indicate

shown,

of the gap voltage

structure-length

voltage.

These

and rf defocusing.

by a reduction

have

the gap voltages

caused

i

50 x 100 cm

in the first gaps of the

voltage.

to a mismatch

ramp

50

from copper-plated

simulations

losses are expected

accelerating parable

trance

F P

3. The MEQALAC

mild

& d

100%

to 5%, so

is obtained below

(b). The the antic-

modification

(c) /c

of the size of the first blocks,

resonator modified;

capacitance. the

number

The of

final

gaps

is

from 30 to 32. such that the electrical

field

strength

The gap width is chosen

is the same for all gaps. The field strength

is 13

kV/mm

which is equal to one time the Killpatrick

The

gap voltage

block

increases

length is proportional

69.47 mm), corresponding

from

9 to 47.5

kV,

to the ion velocity

limit. and the

(15.31

to

to 40 to 1000 keV ion energy,

respectively.

SIDE

VIEW

Fig. 3. Accelerating structure. The B-lines surrounded the meander supports, leading to radial currents. Shorting plates force the field lines through the free area, leading to additional axial currents.

W.H. Urbanus et al. / MEQALAC

511

first cells a maximum power loss of 2 kW per cell is expected. The work described here was performed as part of the research program of the association agreement between the Stichting voor Fundamenteel Onderzoek der Materie (FOM) and EURATOM, with financial support from the Nederlandse Organisatie voor Wetenschappelijk Onderzoek (NWO), the Nederlandse Ministerie van Onderwijs en Wetenschappen and EURATOM.

n

cell number Fig. 5. The estimated distribution of the power dissipation P in the meander structure of the final design of Stage II along the axis. P is the sum of the power dissipation of the radial rf current Prad, the power dissipation P, of the axial current, and the power dissipation Pcaused by particle losses.

The perturbation measurements predict a maximum loss power of 60 kW including beam loading. This rf power and additionally the beam losses have to be handled safely by the cooling system. An estimation of the local power dissipation shows that roughly half of the power (27 kW) is expected on the accelerating structure. There are three currents of interest: the axial current, the radial current due to the axial magnetic field, and the beam current on the meander structure due to the particle losses. The axial current is caused by the shorting plates forcing the magnetic field through the meander structure. An estimated distribution of the total power dissipation along the accelerating structure is given in fig. 5. It is seen that the additional heating by radially lost particles Pbeam is only of minor influence for the meander structure. The sum of Prad, Pax and P be_ amounts to the already mentioned 27 kW. In the

References [1] A.W. Maschke. Brookhaven Nat. Lab. Rep. BNL-51209 (1979). [2] E.H.A. Granneman, R.W. Thomae, F. Siebenlist, P.W. van Amersfoort, H.J. Hopman, J. Kistemaker, H. Klein, A. Schempp and T. Weis, Proc. 3rd Int. Conf. on Production and Neutralization of Negative Ions and Beams, BNL, Upton (1983) ed. K. Prelec, AIP Conf. Proc. III (AIP, New York, 1984). [3] F. Siebenlist, R.W. Thomae, P.W. van Amersfoort, F.G. Schonewille, S.T. Ivanov, E.H.A. Granneman, H. Klein, A. Schempp and T. Weir., Nucl. Instr. and Meth. A256 (1987) 219. (41 F. Siebenlist, R.W. Thomae, P.W. van Amersfoort, F.G. Schonewille, E.H.A. Granneman, H. Klein, A. Schempp and T. Weis, NucI. Instr. and Meth. A256 (1987) 207. [S] R.W. Thomae, F. Siebenlist, P.W. van Amersfoort, E.H.A. Granneman, H. Klein, A. Schempp and T. Weis, J. Appl. Phys. 62 (1987) 4335. [6] P.W. van Amersfoort, R.G.C. Wojke, W.H. Urbanus, A.E. van Putten, R.J.J.M. Steenvoorden, H. Klein, A. Schempp and T. Weis, Proc. IEEE Part. Act. Conf., Washington (1987) eds. E.R. Lindstrom and L.S. Taylor (New York, 1987) p. 340.

V. NEW IMPLANTATION

EQUIPMENT