Ⅱ-8. Accelerator

RIKEN Accel. Prog. Rep. 48 (2015)

Investigation of the eﬀect of solenoidal magnetic ﬁeld on Fe plasmaﬂux for application to laser ion source
S. Ikeda,∗1,∗2 M. Okamura,∗3 and K. Horioka∗2

Biased ion probe
Laser
Fe targeto30 50

ø2 mm
Aperture

72

Grounded mesh

2.5

Current density (mA/cm2)

Laser ablation plasma has been studied as a highly
charged ion source for nuclear physics experiments1)
and as a high ﬂux ion source for heavy ion inertial fusion,2) and it is used as an ion source at Brookhaven
National Laboratory3) . In a typical conﬁguration of
the source, plasma ﬂux at an extractor varies as a function of time. The time-dependent ﬂux results in thechanges in the ion beam current and its optics withina beam pulse. To prevent the changes, we propose toapply a pulsed magnetic ﬁeld. Enhancement of theﬂux after passing through a static solenoidal magnetic
ﬁeld was observed4) . The enhancement depended on
the magnetic intensity. Therefore, if we apply the fastrising magnetic ﬁeld in accordance with the transient
ﬂux level of the plasma, we will be able to make theﬂux level ﬂat.
To predict the optimal pulsed magnetic ﬁeld, weﬁrst investigated the eﬀect of a static magnetic ﬁeldthat was driven by quasi-stationary current during theplasma passing through the coil. We scanned a biased
ion probe detecting plasma ﬂux transversely or normal
to the plasma drifting direction by applying a magneticﬁeld.
Figure 1 is a brief schematic diagram of the experimental setup. A Nd:YAG laser irradiated an iron target with a pulse width of 6 ns and intensity of 4.0×108
W/cm2 . At this irradiation level, the laser mainly produces singly charged ions in the chamber evacuated to4 × 10−4 Pa. The coil was driven by a pulse circuit.
During the plasma passing through the coil, the decrease in magnetic ﬂux density was less than 10 % andwe can regard the magnetic ﬁeld as almost constantduring the interaction. A 2-mm-diameter aperture and0G40 G

21.5

10.5

0-40

-30-20-1001020Transverse distance from center x(mm)30Fig. 2. Transverse flux distribution with and without magnetic fielda metal mesh whose transparency was 90.3 %, weregrounded and placed in front of the probe.
Figure 2 shows the transverse distribution of plasma
ﬂux whose longitudinal velocity vz is 14 mm/µs at 870
mm away from the target. vz was estimated by division of the distance by the time of ﬂight. Horizontal
axis is the distance x from the center axis. Each pointis an average of 3 data. The red squares are data withno magnetic ﬁeld and green ones are in the presence ofmagnetic ﬁeld. The magnetic ﬂux density at the centerof the coil is estimated to be 40 G using a simulation
code(OPERA). When the magnetic ﬁeld was applied,
two peaks and decrease in the ﬂux near the center axis
were observed. The formation of two peaks around x =-15 and 20 mm means that the plasma collected aroundthe radius. The collection indicates that the magnetic
ﬁeld had focusing force that increases nonlinearly withincreasing x. On the other hand, the decrease in theﬂux around the center (x = 0 mm) may mean that themagnetic ﬁeld does not converge the plasma withina certain value of x. The diﬀerence in the magnetic
ﬁeld eﬀect on the plasma with respect to x may result from the shape of the magnetic ﬁeld. We need toinvestigate this diﬀerence in order to increase the ion
ﬂux with minimum emittance growth. The discussionof the enhancement mechanism will help optimize theparameters of pulsed magnetic ﬁeld.
References

Fig. 1. Schematic of experimental setup for transverse scanof ion flux distribution
∗1
∗2
∗3

RIKEN Nishina Center
Interdisciplinary Graduate School of Science and Engineering, Tokyo Institute of Technology
Collider Accelerator Department, Brookhaven National
Laboratory

1) P. Fournier et al.: Nucl. Instrum. Meth. A, 71, 924(2000).
2) J. W. Kwan, IEEE Trans. Plasma Sci. IEEE Nucl.
Plasma Sci. Soc., 33, 1901 (2005).
3) T. Kanesue et al.: Proceedings of IPAC2014, Dresden,
Germany, p 1890 (2014).
4) J. Wolowski et al.: Laser Part. Beams, 20, 113 (2002).

- 199 -完全版2014_本文.indd 199

15/10/16 17:53