Ⅱ-8. Accelerator

RIKEN Accel. Prog. Rep. 49 (2016)

Response of an axial magnetic field to injection of laser ablation
plasmaS. Ikeda,∗1,∗2 M. Okamura,∗3 and K. Horioka∗2An ion source using laser ablation plasma can provide various types of heavy ion pulsed beam with relatively simple conﬁguration, such as the source operated
in the Brookhaven National Laboratory.1) In addition
to this advantage, the plasma is expected to become ahigh current ion source because of the high density andhigh drift velocity. To achieve this, guiding of the expanding plasma with axial magnetic ﬁeld is necessary
and the mechanism needs to be made clear. During
the above interaction, the magnetic ﬁeld is distortedby the plasma while the distortion generates magnetic
pressure aﬀecting the plasma dynamics. The distortion is determined by a competing process between theconvection by the plasma and the diﬀusion into theplasma. In the case of laser ablation plasma, both effects play some roles and vary within the plasma plumedue to the distribution of the conductivity and theplasma velocity. Therefore, the magnetic ﬁeld should
evolve within the plume while giving rise to a structure
within the plume. Hence, we investigated the responseof an axial magnetic ﬁeld against the plume injection
with a magnetic probe, and then compared the resultwith plasma behavior.
Figure 1 shows the experimental setup. We produced the plasma plume using a 4 × 108 W/cm2
Nd:YAG laser from an iron target in a chamber evacuated to 4 × 10−4 Pa. A coil 50 mm in diameter and 5mm in length was placed at a distance of 260 mm fromthe target to generate the magnetic ﬁeld that was 65 G
at the center. A disk with a 14-mm-diameter aperture
was placed between the target and the coil to deﬁne theplasma plume diameter. A magnetic probe was placedin the coil. A 20-turns pickup coil 6.4 mm in diameterand 2.0 mm in length was attached to the probe edge.
By integrating the voltage signal from the probe, wemeasured the variation in the longitudinal componentof the magnetic ﬁeld. We also measured the plasma ion
current density at a distance of 740 mm with a biased
ion probe.
From the current density measurement, we foundthat the head of the plasma plume is preferentiallyguided and directed. In addition, the plume was estimated to pass through the coil from 5 to 30 µs.
Figure 2 shows the ﬁeld response ∆Bz as a functionof the distance from the coil center z at various time(8, 10, 14 µs) from the laser pulse. At any given point
∗1
∗2
∗3

RIKEN Nishina Center
Interdisciplinary Graduate School of Science and Engineering, Tokyo Institute of Technology
Collider Accelerator Department, Brookhaven National
Laboratory- 153 -Fig. 1. Schematic of the experimental setup for magnetic
field measurement.

Fig. 2. Variation of magnetic field ∆Bz as a function of thedistance from the coil center z at various time (8, 10,14 µs) from the laser pulse.

of time, ∆Bz was negative in z < 0 and positive inz > 0. The absolute value of ∆Bz in z < 0 was larger
than in z > 0, and additionally, ∆Bz changed fromnegative to positive in z > 0. These results mean thatthe magnetic ﬁeld was distorted asymmetrically withrespect to the midplane of the coil. On the other hand,∆Bz in z < 0 in earlier time was smaller than that inlater time. This means that the head of the plumedistorts the ﬁeld to a greater extent. Because the headis faster than the tail, this indicates that the diﬀerent
convective eﬀect depending on the velocity leads to thediﬀerent distortion within the plume.
The larger distortion by the head and the preferential guiding of the head as mentioned above show thatthe diﬀerence of the distortion results in the diﬀerenceof the guiding eﬀect.
From the investigation, we found that the magnetic
ﬁeld evolves within the plume asymmetrically, whichleads to structure in the plume.
Reference1) T. Kanesue et al.: Proceedings of IPAC2014, Dresden, Germany, p 1890 (2014).