25B02
Ion acceleration by the interaction between the high peak power femtosecond
laser pulses and the solid density targets
高強度フェムト秒レーザーによる固体薄膜ターゲットからのイオン加速
Mamiko Nishiuchi
西内満美子
1Advanced Photon Research Center, Kansai Photon Science Institute, Japan Atomic Energy Agency, 8-1 Umemi-dai,Kizugawa, Kyoto, 619-0215, Japan
関西光科学研究所 日本原子力研究開発機構 〒619-0215 木津川市梅美台8-1-7
先 端 基 礎 研 究 セ ン タ ー 日本原子力研究開発機構 〒 319-1185 茨 城 県 那 珂 郡 東 海 村 白 方 白 根 2-4
Ultra-high intensity laser interaction with a solid target can produce energetic ions whose energies up to
several tens of MeV. Related to this interesting observation, many experimental as well as theoreticalinvestigations have been carried out in all over the world in the past 10 years. Particle acceleration basedon this technique has achieved high quality particle beams that compare favorably with conventional
acceleration techniques in terms of emittance, brightness, pulse duration and compact source size.
However, for the actual applications, we need to overcome some issues, for example, relatively small
maximum energies of protons for some applications, the reduction of beam density according to thebeam transport, and so on. Introduction of the intensive investigations to overcome those issues inmany institutes are reported as well as the recent related activities at JAEA Kansai is also introduced.

More than 10 years have been passed since thediscovery of the energetic ions from theinteraction between the high peak power laser andthe target materials [1]. Laser-driven ions attract
many research fields, especially because those areaccelerated from a small spatial size acceleration
field of the order of ~µm with the extremely highfield gradient of ~TV/m, which will be neverachieved by other method. Therefore, thedownsizing of the conventional accelerator isachievable. Adding to the previous peculiar
characteristic, those laser-driven protons exhibit
small transverse emittance of ~10-4 π mm mrad [2],which is two orders of magnitude better value thanthat of the beam from the usual conventional
accelerator. More than 1013 protons are emitted
from a small source region within a pico-second
bunch duration at a source [3], thus high peakcurrent is achieved to be ~ GA, which is again,
amazingly high value.
Due to their quality, laser-driven ion sources
can serve many applications, which include
proton radiography [4], injectors for conventional
accelerators [5], and the potential fordevelopment of compact facilities for laser-driven
ion beam radiotherapy [6], producing high energy
density matter (warm density matter) byinhibiting hydrodynamic expansion (isochoricheating)[7], fast ignition [8].
However, since the proof of principle of thelaser-driven proton acceleration experiment, themaximum energy of the proton of ~60 MeV,
which is obtained with single shot based ~kJ classlaser pulse interaction with the thin-foil target is
not updated significantly. At least more than 80
MeV energy protons should be repetitively
obtained for the medical use [9]. It means thatsuch high energy proton beams should beproduced by compact laser system (e.g. sub-PWclass) which enables us to supply repetitively thelaser pulses.
A number of mechanisms of ion acceleration arediscussed for further understanding of thismechanism especially to know the mostinteresting and important issue, ‘energy scaling’.
Many theoretical models are proposed. The
information of the achievable maximum energy
with given laser intensity (or energy) is essential
for the feasibility study of the proposed
applications.
When the initial laser pulses have sufficiently
intense (for example, ≿ 1022 Wcm-2) so thatelectrons in the plasmas are stripped away
completely, the acceleration mechanisms takeplace are coulomb explosion (CE) mechanism[10] or Radiation Pressure Acceleration (RPA)
mechanism [11]. At this moment only a few
experiments appear [12], whose acceleration
mechanisms are dominated by such efficient
acceleration mechanisms. Because the achievedlaser intensity on target is just increase up to~1022Wcm-2[13].

In most of the experimental results reported in thepast 10 years the dominant acceleration occurs
therefore at charge separation field established at
the rear side of the target. Among the models,
which explain this physical picture, includinginteraction with underdense plasmas [14], two ofthem are quite well known; one of them is basedon the scheme, ‘plasma expansion into vacuum’[15], which is also known as Target Normal
Sheath Acceleration mechanisms. Ions areaccelerated by the charge separation field in anarrow layer at the front of the plasma, which isformed between the electrons and ion expanding
into vacuum. On the whole, the plasma iselectrically neutral. The other model is based on
the physical picture, ‘strong charge separation
field’ at the rear side of the target between theelectrons with certain temperature distribution andthe immobile ion sheet in the target [16].
The detail of the attempts to increase themaximum proton energies in all over the world will
be introduced in the presentation. The energy
scaling of those models and the comparison withthe data taken in many institutes will be reported inthe presentation. The resent experimental results at
JAEA where we succeed in accelerating protons up
to 40MeV energy with 250 TW, 40 fs, 8J laser
pulse interaction with the thin foil metal target isalso reported.
Another important issue to overcome is density
decrease in the ion pulse followed by the beamtransport. In contrast to conventional ion
accelerators currently used in medicine, ion
acceleration from flat foils typically produces
divergent proton beams with a broad energy spread( ~100% ) resulting in a potentially significant
decrease in number of particles delivered fortherapy. In order to achieve the same level of thebeam divergence and energy spread as in theconventional accelerators, it is desired that thelaser-driven beam at the source point (or at least at
the injection point which is near the source) shouldhave a quality comparable to what is given by
conventional accelerators.
Many attempts to overcome this issues carriedout in all over the world in many ways are
introduced, for example, using tailored target,
combining the conventional beam optics with thelaser-driven proton source, applying plasma optics
to the laser-driven proton source, and so on.
Detail of those attempts will be presented in the talkas well as the recent experimental investigations
carried out at JAEA where we achieve to obtain themonochromatize and collimated proton beam fromthe thin foil target by using sophisticated target

holder will be shown in the presentation.
Acknowledgments
We gratefully acknowledge the members of theLaser-driven
protonacceleration
Group,
Laser-driven Accelerator Systems and Medical
Applications group, Laser electron acceleration
Group, and J-KAREN operation group. I alsoProf. gratefully acknowledge T. Jitsuno and Prof. N.
Miyanaga for assisting us with the method ofwavefront correction. We also thank Prof. K.
Tanaka and Prof. H. Habara for the useful
discussion. This work has been financially
supported by the Special Coordination Fund (SCF)
forPromoting
ScienceandTechnology
commissioned by the Ministry of Education,
Culture, Sports, Science and Technology (MEXT)
of Japan. This work has been also supported by theMinistry of Education, Science, Sports and Cultureof Japan, Grant-in-Aid for Specially Promoted
Research No. 22740268.
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