The 13th Symposium on Accelerator Science and Technology, Suita, Osaka, Japan, October 2001

New Source System of Intense Heavy Ion Beams by Using Direct Plasma
Injection Method
T. Takeuchi and T. Katayama, Univ. of Tokyo, JapanM. Okamura, RIKEN, Japan
T. Hattori, N. Hayashizaki, and S. Okada, Tokyo Institute of Technology, Japan
R. A. Jameson, Univ. ofFrankfult
Abstract
We have developed a new source system, based on
Direct Plasma Injection Method (DPIM), and havesucceeded to produce intense Carbon ion beams of thebeam current 8 emA for Carbon ion beams with theenergy of 2I4 ke V/u. By a charge state analysis with abending magnet, we found that the accelerated beamincludes C4+ and c3+ of 4.0 emA and 0.8 emA,
respectively. From a comparison with simulation, it issuggested that DPIM has effects to suppress the strong
space charge effect by intense low energy beam at theentrance ofRFQ LINAC.

1. Introduction
Recently, Laser Ion Source (LIS) is being remarked
as an initial stage of accelerators for high energy physics
and medical science because LIS can produce heavy ion
beams with highly charge and intense beam current [1,2].
However, LIS has serious problems concerning beam
matching to the subsequent stage accelerator. Theseproblems are followings. The extracted beam from LIS
has a wide energy spread, a strong space charge effect,
and a rapid time-variation of beam current and beamprofile [3]. Therefore, it is difficult to control a low
energy beam transport (LEBT) between the LIS and thesubsequent stage accelerator. As a result, intense beams
from the LIS suffer a large beam loss. In order to resolve
such a difficulty, new principle of "Direct Plasma
Injection Method (DPIM)" has been developed with acollaboration of RIKEN-CNS-TIT which combined alaser ion source and a Radio Frequency Quadrupole
LINAC (RFQ linac).

2. Basic Concept of Direct Plasma Injection
MethodA schematic view of DPIM is gi:ven in Fig. 1. Atarget is positioned in a target chamber, which is justconnected to an RFQ linac, but electrically isolated by anisolator tube. The laser beam is focused onto a surface ofthe target, and produces the ablation plasma which flows
to the direction of RFQ linac. At the entrance of RFQlinac, ions in the plasma are extracted by RF vane voltage
in RFQ linac. Immediately, extracted ions are captured byRFQ focusing force and accelerated up to the design
output energy ofRFQ linac.

Fig. l. Basic concept ofDPIM

3. Experiment for Direct Plasma Injection
MethodThe experiment for Proof of Principle (POP) ofDPIM has been carried out for Carbon ions. A fig. 2
shows the overview of experimental setup.

F. C. 1

Tripletmagnet\Fig.2. DPIM experimental setup

The laser was a 4.1 J TEA C0 2 laser with 38 ns pulse
duration. The measured total power was 1.1 x I 08 W. The
diameter of laser beam from the resonator is about 50 mm
and has the hollow at the central axis with the diameter ofabout20 mm.
TITech RFQ [4] at Tokyo Institute of Technology, wasused in the experiment. The main design parameters of theRFQ are listed in Table I. The laser beam from the laser
resonator is transported by two cupper mirrors, passing
through a NaCl window, and is injected into the target
chamber as shown in Fig. 3. At the inside of the target
chamber, the laser beam is reflected and focused onto theCarbon target (~ = 30 mm, thickness = 10 mm) by ahollow focusing mirror. The laser power density on thesurface of the target is estimated as 3.35 x I 0 12 W/cm2 .
The laser-produced plasma expands passing through eachholes of the focusing mirror and a small aperture, and go- 37-

The 13th Symposium on Accelerator Science and Technology, Suita, Osaka, Japan, October 2001

into RFQ linac channel. The distance between theentrance of RFQ and the target is 264 mm. In order toadjust the initial beam energy of the Carbon ions to thedesigned input energy of the RFQ, the target chamber is
supplied to the high voltage and isolated to RFQ by theisolator. Faraday Cup 1 was installed at the close location
after RFQ and the second Faraday Cup 2 was set
downstream a triplet quadrupole magnet and an analyzing
dipole magnet and was 3.65 m after the exit ofRFQ linac.

rnA
24

Accelerated C ion beams

20

16
12JlS
Fig.4. Accelerated Carbon beam signal on F.C.l

Fig.3. Target Chamber

Table 1 The main parameters of the TITech RFQ
Designed Values
Charge to mass ratio~1116
Operating frequency (MHz)80
Input energy (keV/amu)
5Output energy (keV/amu)
214
Normalized emittance (100%)(cm· mrad)0.05 nVane length (em)422
Total number of cells
273
Characteristic bore radius, r 0 (em)
0.466
Synchronous phase-90° to -20°
Inter-vane voltage (kV)78.9Transmission for q/A=l/16 beam (10 rnA) 6.84 rnA

Fig.5. Analyzed C4+ beam signal on F.C.2
rnA

c 3+ion
1.2

0.8
0.4 ,1.1

l1'"'~~.-J.~~~~--e4. Experimental Results of DPIM for Carbon
Ion Beams
The accelerated Carbon ion beam was observed at theF.C.l. The measured peak current has reached 25 rnA as
shown in Fig. 4.
Since a distance between the RFQ and the Faraday Cup 1is very short, the bunched structure appears in the beam
signal. After averaging data points for 1 period of 80 MHz
and replotting it, we can obtain average peak current of 8emA with the pulse time width of 1.24 !-LS. This beam isexpected to contains of several charge states such as C3+,C4+, C5+ and C6+ because Carbon ions with these charge
state can achieve RFQ input energy of 5 ke V/u under thecondition of a target chamber potential (16kV) in thisexperiment.

/Fig.6. Analyzed C 3+ beam signal on F.C.2

The analyzed beam can be measured by F.C. 2 located atthe 3.65-m downstream the exit ofRFQ. Obtained C4+ andC 3+ ion current signals are shown in Fig. 5, 6, respectively.
The measured peak current were 4.0 emA and 0.8 emA
for C4+ and C 3+ with the pulse width of 0.41 11s and 0.35!-LS, respectively. The C 6+ and C 5+ ions could not bemeasured. The distance between the F.C. 2 and the exit ofthe RFQ is enough to get the de-bunched beam. We
confirmed that the Carbon beam can be accelerated withthe DPIM.

5. PA~TEQ-HI Simulation
To investigate this new method, beam simulation in theRFQ linac has been done. The standard beam simulation
code in RFQs, P ARMTEQ-HI was modified to be able to- 38-

The 13th Symposium on Accelerator Science and Technology, Suita, Osaka, Japan, October 2001proceeded to the RFQ entrance. Thus, it is expected thatDPIM has some effects to suppress the strong spacecharge effect by high charge state and intense ion beam.
Further consideration and verifications by means ofdetailed numerical calculation should be needed.

8:?
.§.

~s7. Conclusion
Direct Plasma Injection Method, suppressing thestrong space charge effect during the flight path of the low
energy highly charged intense ion beams, has been
proposed. The acceleration test using DPIM has been
done for Carbon ions. The accelerated peak current was 8rnA, which was analyzed as 4.0 mA and 0.8 mA for C4+
and C3+, respectively. The observed total current
exceeded designed current of the RFQ. The new principle,DPIM, was proved successfully.

4u0200100300cellFig. 7. PARMTEQ-HI simulation result for C4+, C 3+, and C2+

simulate various charge states ions simultaneously[5]. In
the simulation, we have tried to reproduce theexperimental results of the acceleration test. Assumed
initial conditions of the injected particles, such asemittances and currents, are determined based on plasmaexperiment at RIKEN and are summaried in Table2. To
simplify the calculation, only three kinds of the particles,
C2+, C3+ and C4+, were tracked. A result of the modified
PARMTEQ-HI simulation is shown in Fig. 7. This graphindicates accelerated currents as a function of the cellnumbers. The calculated output currents are 0.14, 0.75
and 1.32 mA for C2+, C3+ and C4+ respectively.
Table 2 The initial beam parameters for the simulation
Assumed Values
93.9
Total beam current (rnA)
C2+, C3+ and C4+Assumed particles14,31,47
Fraction(%)
13.1, 29.1, 43.1
Current (rnA)
32,48,64
Total energy (keY)
9.8, 9.8, 9.8
Emittance (1t mm mrad)

Acknowledgement
The authors are grateful to the staff members of RIKEN
Beam Physics and Engineering Laboratory for theencouragement and suggestions during the work. They
acknowledge to the members of Research ·Laboratory for
Nuclear Reactors in Tokyo Institute of Technology for the
many assistance in the operation. They would like tothank Dr. Sergei Kondrashev (ITEP, Moscow) for
valuable comments on the properties of the laserproduced plasma.

References[1] S. Kondrashev, et al., Proc. EPAC2000, (2000), p.
551.
[2] T. Takeuchi, et al., Proc. EPAC2000, (2000), p. 1622.
[3] P. Fournier et al., Rev. Sci. Instrum., 71, (2000), p.
924.
[4] M. Okamura, et al., Nucl. lnst. and Meth., B 99,(1995) p. 694.
[5] R. A. Jameson, RIKEN internal report. (2001)

6. Discussion
The comparisons with the experiment and the simulation
are listed in Table 3. All the experimentally measured
values are higher than the simulated currents. The most ofbeam losses are observed within first twenty cells. It
seems that there is neutralization effect due to presence ofelectrons at the entrance of the RFQ. The target chamber
was supplied at positive high voltage, however the vanetips of the RFQ were oscillating with higher voltage. The
electron in the plasma can move to the RFQ entrance
within some RF phase. Also, some electrons in the plasma
can be pulled by positive charges of ions, which firstly
Table 3 Output beam currents
Simulation
Experiment
2.21
8.0
Total (rnA)
cz+ (rnA)
0.14
3(rnA)+0.75
C
0.841.32
C + (rnA)
4.0-39-