7th Asia-Pacific Conference on Plasma Physics, 12-17 Nov, 2023 at Port Messe Nagoya

Effect of applying solenoidal magnetic field on laser ion source
Kazumasa Takahashi1, Toru Sasaki1, Takashi Kikuchi1, Shunsuke Ikeda2,
Takeshi Kanesue2, Masahiro Okamura2
1
Nagaoka University of Technology,
2
Brookhaven National Laboratory
e-mail (speaker): kazumasa@vos.nagaokaut.ac.jp
A laser ion source is an ion source that extracts ionsfrom an ablation plasma generated by irradiating a solid
target with a pulsed laser. It can generate plasmas athigh-density from various elements in solid state at room
temperature. The first laser ion source was proposed
within ten years of the invention of lasers in 1960 [1],and it was one of the earliest applications of lasers. Laser
ablation plasmas have the characteristic of having a large
drift velocity in the direction perpendicular to the target
surface. Due to the generation of high-density plasmas
compared to conventional ion sources, laser ion sources
can easily generate high-current ion beams. In addition, alaser ion source with the direct plasma injection scheme[2], which is a technique that extracts ions from a plasma
and injects them into a linear accelerator without low
energy beam transport, demonstrated to accelerate heavy
ion beams with much higher current compared to those
extracted from conventional ion sources [2, 3].
Owing to its capability of supplying ion beams withhigh current for various ion species, a laser ion source
has been in operation at Brookhaven National
Laboratory (BNL) in NY since 2014 [4]. The employedlaser is a Nd:YAG laser with nano-second pulse durationand the energy is on the order of 100 mJ. It is the firstlaser ion source in the world to supply stable ion beams
for users at a large accelerator facility. It is located at theupstream end of the heavy-ion accelerator complex at
BNL. Recently, laser ion sources are expected to be used
as ion sources that supply ion beams for medical
applications, neutron sources, and drivers for heavy-ion
inertial fusion [5-9].
The maximum beam current that can be transported
in an accelerator is limited by the space charge limiting
current and focusing force by magnetic or electric lens.

Figure 1. Schematic setup of a laser ion source with asolenoidal magnetic field.

Therefore, in order to increase the number of ions per
laser pulse, it is necessary to maintain the current below
the upper limit and extend the pulse duration.
In a laser ion source, the pulse duration of the plasma
can be easily extended by increasing the plasma drifting
distance. However, as the plasma expands
three-dimensionally, the plasma density decreases
drastically, leading to a decrease in ion current densitywith the increase in the drift distance. To suppress thedecrease in the current, a method was proposed thatguiding the laser ablation plasma with a solenoidal
magnetic field while transporting the plasma. In addition,
the effect of magnetic field has been investigated onother characteristics of ion beams such as emittance [11]and the distribution of ion charge-state [12].
In order to design an appropriate solenoid magnetic
field for guiding ablation plasma, considering thedifferences in required ion species and charge-states,
further understanding in the behavior of plasma inside
the solenoid is necessary. This study discusses the resultsof investigating the behavior of plasma and the extracted
ion beam when applying a solenoidal magnetic field asshown in Fig. 1 with the aim of developing high-current
laser ion sources.
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Particle Accelerator Conference (NAPAC2016),
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