JAERI-Conf 95-021

PB44
Experiments of Beam-Induced-Plasma for Basic Study of HIF by RFQ-Linac
Kimikazu SASA, Takashi ITO, Noriyosu HAYASHIZAKI and Toshiyuki HATTORI
Research Laboratory for Nuclear Reactors, Tokyo Institute of Technology, 2-12-1,0-okayama, Meguro-ku, Tokyo, 152, Japan

Abstract
2. Intense ion beams from an RFQ-Linac
An intense heavy ion linear accelerator system
is being constructed for basic researches on HIF andheavy ion pumped laser at TIT. This accelerator system
consists of an RFQ-Linac, an ECR ion source, a finefocusing beam transport system and a fast beam
pulsing system. At present, the obtained maximum
beam current from the TIT-RFQ is 1.6 rnA for 4He+.
By using this system, An ion beam of 4He+ with anenergy of 220 keV/amu was focused onto a small spot
area of about 1mm 2. This beam deposited a specificdeposition power of about 0.14 GW/g to the target.
The overall status of this linac system and preliminarye~""Periments with beams of 4He+ are discussed.

!.Introduction
Heavy ion beams are one of the favorable
drivers for Inertial confinement fusion because of their
good efficiency and effective energy deposition in thetarget. It is important for Heavy· Ion Inertial
Confinement Fusion (HIF) to make researches on aheavy ion accelerator and a beam-target interaction, AtTokyo Institute of Technology (TIT), a heavy-ioninduced plasma has been studied for basic researches on
HIF and heavy ion pumped laser. For theseexperiments, an energy driver is required to create ahigh intensity and brightness beam. An intense heavy
ion linear accelerator system is being constructed tomeet the requirements. Fig 1 shows the layout of theintense heavy ion linac system. This system consistsof a four vane type 81 MHz Radio-Frequency
Quadrupole Linac (TIT-RFQ), a 2.45 GHz Electron
Cyclotron Resonance (ECR) ion source, a fine-heavyion linac system. The first experiments have been
carrying out with 4He+ beams.
Solenodcoli

Elnzellens

The TIT-RFQ was designed to accelerateparticles with a charge to mass ratio (q(A) greater than
1/16 from 5 keV/amu up to 220 keV/amu. The
calculated beam transmission is 68.4% for 10 rnA witha beam of 16o+. The main parameters of the TIT-RFQ
are shown in Table 1 [1].
Table !.Parameters of the TIT-RFQ
Charge-to-mass ratio
Operating frequency (MHz)Input energy (keV/amu)
Output energy (keV/amu)
Duty factor (%)
Q factor
Cavity diameter (em)Cavity length (em)Expected beam transmission (%)
0 rnA input
10 rnA input

0?!1/1680.9

5220
1010635

72.5
425
91.8
68.4

At present, the beam experiments are being
performed with intense 4He+ beams by using a 2.45
GHz Electron Cyclotron Resonance (ECR) ion source.
This ECR ion source can produce a beam current of 2.5rnA for 4He+. The maximum productive ratio of 4He+
is more than 99%. The 4He+ beam is e:11.iracted by a
voltage of 20 kV from the plasma chamber and focusedwith an einzel lens, and injected into the RFQ. The
maximum obtained beam current was 1.6 rnA for 4He+
after the acceleration by the RFQ-Linac [2]. By
considering the beam acceptance, a transmission
efficiency is estimated to be 70%. This result is ingood agreement with the calculated beam transmission.

Gate

Electrostatickicker

Optical fiberceo

2.45 GHzECR

Beamdump

TIT-RFQion source
IOOOmmFine-focusing system

Fig.l. Layout of the intense heavy ion linac system at TIT-374-00

Streak camera

ceo camera

JAERI-Conf 95-021

marked the geometric structures (Fig 3). Table 2 shows
the comparison of calculated and experimental spot
size. TI1e area of experimental spot size is 1.06 mm2.
This value is in good agreement with the calculated
value. A power of 1.4 kW was transferred to thealuminum target. This corresponds to a specificdeposition power of 0.17 GW/g.

3. Fine-focusing systemThe fine-focusing system after the RFQ
should be designed to obtain minimum beam spot sizein order to increase a specific deposition power P(W/g). Because if the beam can deposit enough energy
to be evaporated in the target, then the specificdeposition power P is the appropriate parameter toestimate the temperature of the beam-induced plasma.
We designed the fine-focusing system thatconsists of a faraday cup, a gate valve, three magnetic
quadrupole lenses and an electrostatic beam kicker. The
magnetic quadrupole lenses were made with a bore
diameter and a maximum field gradient to be 60 mm
and 3 kG/em respectively. The parameters of RFQ
output beams were analyzed by using the computercode PARMTEQ-H. The momentum dispersion isabout 0.8 %. The initial phase space occupied by theion beam is 24.5 mm mrad in the x - and y directions for the maximum intensity of the RFQbeams. The beam envelope for the fme-focusing
system was calculated by using the computer code MSTRANSPORT (M. Sekiguchi at INS). The calculated
beam. envelope is shown in Figure 2. By the result ofthe simulation, the heavy ion beam of 16o+ with acurrent of 7 mA and an energy of 3.5 MeV can befocused onto a spot size of 1.0 mm2. From this spotsize, the beam power density of 2.5 MW/cm2 - during
the macropulse of the RFQ Linac - is e.'q)ected to betransferred to the target.
At present the ECR ion source for the TITRFQ can produce only 4He+ beam. After conditioning
the magnetic lenses, we focused 4He+ with a beam
current in the macropulse of about 1.6 rnA In this casethe beam current corresponds to the ion beam of 16o+with a current of 6.4 rnA. To measure the size of thespot, we stopped the 4He+ beam in an aluminumtarget (0.8 mm in thickness), where the 4He+ beam

Fig 3. Marked spot size structure by focused 4He+
beam in an aluminum target

Table 2. Comparison of calculated and experimental
spot size for 4He+ beam with a current of
1.6mA.
x (mm) y (mm) S (mm2)
1.00
1.22
0.26
Calculated value
1.06
1.12
0.30
Experimental value··X~p_latle:.: :::·: :: .. ·····

QMl

QM2

QM3

0.62 kG/em• 1.!!3

2.37
-,··!

--------------- ··---:.:X~p_lane......................................!..

: i......... ::::::::·::::::::: . ::: .:::.:~:::·:::::::·:·:~::::::::fl···.

Axis~~~~=====t~~~~~~~tw48~~ts~~j_ ;. .~. .g. .~. .~. ~. -~. .~. .~. .~~--tL~. .~--~· -~--~ .. :.:~~.:: .................. y z 1.22 nun··: .. ::.:.:j::............ :::::::::::::::::::::: ....~:·:· ·:::·:::::::/• 20;1} mr•d_.
----··
0'• I·----------------; .·······.:..:..::.
1000
1942 [mm]Fig 2. Calculated beam envelope for the fme-focusing system by the computer code MS-TRANSPORT. [160+:7mA]-375-JAERI-Conf 95-021

4. Expected Parameters of beam-induced
plasmas By Simple Scaling Laws

Table 4. Expected specific deposition power for various
ion beams in H2 gas target (1 atm).
[Beam spot size : 1.06 mm2]

At the first, the beam-target interaction
experiments to study the heavy-ion-induced plasma areplanned by using a solid target, and then .a gas pufftarget. The gas target is a cylindrical tube of 25 mm
length and 10 mm diameter. Table 3 shows the
expected parameters of ion-induced plasma for various
solid targets. Table 4 shows the expected specificdeposition power for various ion beams in H2 gas
target (1 atm). TI1e deposition power P (GW/g) isgiven

ION
BEAM

BEAM
CURRENT(mA)

BEAM
POWER(kW)

P(GW/g)

4He+
14N+
160+

2.0
6.1
7.0

1.76
18.9
24.6

0.76
9.00
11.55. Conclusion

with the total particle energy E (ke V), the particle
intensity I (rnA), the density of matter (g/cm3), theinitial radius of the target r (em) and the range of theion beam in matter R (em). The maximum temperature
is expressed

The intense heavy ion linac system for plasma
experinlents is currently set-up. By using this focusing
system, we succeeded that the 4He+ beam with acurrent of 1.6 rnA from the TIT-RFQ focused onto asmall spot area of about 1 mm2. Due to the shortrange of the ion beams of about 1 mg/cm2, the beam
power of 1.4 kW amounts to a specific deposition
power of 0.14 GW/g to be transferred into the target.
The first experiment of 4He+ beam-induced plasma
will soon be carried out by using a gas puff target.

13Pxr)[ev]5!!so x ( - - 2REFERENCES

IxE
p x;rr2 x RP=--~­

Tmax

(1)

2

(2)

for solid targets [3-4].
By the simulations of simple· scaling laws,
the beam power amounts to 24.6 kW, and a specificdeposition power can be obtained 11.5 GW/g to focus aheavy ion beam of 160+ with currents of 7 rnA and anenergy of 3.5 MeV in H2 gas target.
The parameters of ion-induced plasma such as
the electron density and temperature are measured witha CCD camera coupled to a streak camera.
Table 3. Expected parameters of ion-induced plasma for
the TIT-RFQ Beam
160+ ( 3.5 MeV, Imax=7 rnA, Wmax =24.6 kW)
Target(solid)R(10-4cm)P(GW/g)

Tma'l:(eV)

AlTiAg

2.96
2.54
1.73

2.93
2.02
1.28

0.98
0.66
0.53

-376-

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B89 38-41 (1994).
[2] K. Sasa, Y. Oguri, M. Okamura, T. Ito and T.
Hattori, "RFQ-Linac system for Intense Heavy Ion
Beams and Beam-Target Interaction Experiments",
Proc., 12th International Conf., Laser Interaction andRelated Plasma Phenomena.
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Prog. Phys, 50. 559-606 (1987)[4) J. Jacoby, D. H. H. Hoffmann, R. W. Muller, K.
Mahrt-Olt, R. C. Arnold, V. Schneider and J. Marohn," Hydrodynamic Motion of a Heavy-Ion-Beam-HeatedPlasma ", Physical Review Letters, Vol. 65, 20072010 (1990).