M. Madert, R. Cee, M. Grieser, R. von Hahn,
C.-M. Kleffner, S. Papureanu,
H. Podlech, R. Repnow and D. Schwalm
Max-Planck-Institut für Kernphysik, Heidelberg, Germany
A. Schempp
University of Frankfurt, Germany
and
D. Habs
Ludwig-Maximilian-Universität, München, Germany
Many experiments at the Test Storage Ring TSR are limited by weak ion beam intensities [1], delivered from a tandem-postaccelerator combination. A new high current injector, consisting of a CHORDIS ion source, 2 RFQ and 8 seven-gap resonators, will deliver 1-3 orders of magnitude higher intensities of singly charged ions. The final energy of 1.8 MeV/u is well adapted to the acceptance of the postaccelerator. By adding an ECR-source in a second phase the system will be able to deliver heavy ion beams up to uranium with energies above the coulomb barrier of the heaviest elements. The CHORDIS is already operating in cw-mode, in sputter mode the pulsed intensity has still to be optimized. By means of an optimizing algorithm it was possible to lower the electrode voltage of the RFQ-accelerator from 71 kV to 60 kV maintaining a particle transmission of about 80% with ion currents of 10 mA. All of the 8 seven-gap resonators have been power tested successfully and performed as expected. This paper describes the status of the project.
In its first phase the high current injector consists of a commercial
CHORDIS ion source [4], 2 RFQ-accelerators [3] and
eight 7-gap resonators [2] delivering 
Li 
or
Be ion beams with 1-3 order of magnitude higher intensities.
In a second phase an ECR- or EBIS-source will be added to increase the
currents for highly charged heavy ions because some experiments are
frequently limited by low beam currents due to stripping losses.
Figure 1: Schematic layout of the new high current
injector. A, M, Dou, Tri: magn. dipoles and lenses, Re: rebuncher,
D: beam diagostic.
In fig. 1 the schematic layout of the new injector is
shown.
The accelerator will be placed parallel to the Tandem.
The Li - or a Be -beam will be injected directly into the
postaccelerator acting as a transfer line.
For a second phase stripping will be used behind the last seven
gap resonator and the proper charge state will be selected by an
achromatic separator consisting of four 60 
-magnets. Like the existing
post accelerator the new injector operates at 108.48 MHz.
The ion velocity of 
=v/c=6% after the high current injector is
well adapted to the post accelerator and final energies higher than 5 MeV/u
can be reached for all ion species in a pulsed mode operation with up to
25% duty cycle.
For the production of high currents of Li and Be with low duty factor
(5 Hz, 500 
s) the commercial ion source CHORDIS [4] is used.
The construction of the ion source section consisting of the source on
a platform, a 60 
-magnet for isotope selection and a quadrupole
triplet to match the beam to the RFQ section has been finished.
The CHORDIS ion source has been in operation on its test-bench for several
hundred hours [see Table 1].
Table 1: List of ion species and current intensities
already produced with the CHORDIS source.
in stable operating
conditions. Higher currents were reached and could be stably produced
by using an additional cooling equipment of the sputter cathodes.
For the Be -source an alloy with a Beryllium contents of only
2% was used with which an intensity of 0.2 mA was satisfactory for all tests.
Higher currents can then be achieved with cathodes made from pure Beryllium.
Improvements were made with respect to diagnostic methods for source
operation and particle beam optimization.
The pulsed mode operation has been established for the gas version,
however, some improvements are still necessary in the sputter mode.
The emittance of the CHORDIS of 35 
mm mrad has been measured
to be within specifications.
The second section of the high current injector consists of two 4-rod-RFQ
resonators [3] operating at an eigenfrequency of 108.48 MHz. With
a rf-power of 80 kW (25% duty cycle) an electrode voltage of
71 kV should be reached in order to accelerate ions with
a charge to mass ratio q/a 
1/9 as required for Be .
The electrodes with 3 m length are milled out of a hollow profile from a
copper-tin alloy to provide sufficient cooling (35% of the rf power
is dissipated at the electrodes) as well as mechanical stability.
However, the maximum diameter of the rods is limited by the capacity
between the electrodes to preserve a high shunt impedance [3].
To provide optimal electrical conductivity the electrodes were copper
plated at the GSI. The first RFQ-resonator was constructed and tested in
full length this year.
Figure 2: Calculated (line) and measured (dots) energy distribution at
an electrode voltage of 15.8 kV
mm) measured after installation is
satisfying.
Massive copper plates were installed between the stems to adjust the
eigenfrequency to 108.48 MHz. The resonator has a Q-factor of 3800.
Power tests up to 20 kW in cw-mode were carried out without any
problems. Weak ponderomotoric oscillations - which could be observed in
pulsed high power operation - could easily be eliminated by means of
mechanical decoupling between the cryo-pumps and the resonator-tank.
Moreover, a mechanical stabilization was used at the long ends of
the electrodes to suppress mechanical oszillations. 
of the structure the energy
gain of an accelerated H 
-beam behind the first RFQ-resonator was
measured. The set up consists of a penning ion source with electrostatic
lenses and an analyzing 90 -double focusing bending magnet with
two diagnostic boxes.
Figure 3: Comparison between the calculated (solid line) and measured
(dots) energy spread 
to determine the shunt impedance.
=15.8 kV, design voltage for an H 
-beam.
Figure 4: Diagram of the optimizing algorithm.
= 101 k 
m.
With this shunt impedance it is not possible to reach an
electrode voltage of 71 kV (design value) with a rf power
of 80 kW. Therefore the electrodes are redesigned with a
lower voltage of 60 kV by means of an optimizing algorithm. This new method
is based on random variations of the design parameters
synchronous phase 
, modulation m and
focussing parameter B and governed by the value of a scalar
function b(T,L) which takes only
two criteria into account: the length L of the resonator for a fixed final
energy and the calculated particle transmission T
[see Fig. 4].
Fig. 5 shows the parameter behavior of the old 71 kV-design
and the re-designed 60 kV electrodes.
Figure 5: Comparison between old (dashed line) and re-designed
(solid line) electrodes: m = modulation, = synchronous phase,
B = focusing parameter, l 
= bunch length, 
= long.
and 
= trans. phase advance.
0.2mm misalignment
of the rods and is therefore tolerable.
With increasing ion velocity, RFQ acceleration becomes less efficient and
other accelerating structures such as seven-gap resonators are more
economical. Therefore the third part of the high current injector
consists of 8 seven-gap resonators with an eigenfrequency of 108.48 MHz
operating at 80 kW rf power with 25% duty cycle.
To simplify the construction, the resonators are designed as
four pairs of identical resonators for synchronous velocities of
=3.7, 4.5, 5.1 and 5.7% [5].
All 7-gap resonators have been calibrated with a particle beam with
synchronous velocity.
Table 2: Measured resonator voltages with beam tests.
It is a pleasure for us to thank the technicians of the Max-Planck-Institute for their excellent work.