Quantum Beam Science Research

Large Increase in the Maximum Energy of a Laser-Driven Proton Beam

10010 µm

10-2

Intensity (arb.)

(a)

10-4
10-6
10-8

1.0MainPulse

36 fs

0.0-2000(b)200Time (fs)
Prepulse Level(Noise Level)

10-10
10-12(c)Maximum Proton Energy (MeV)=40 MeV Proton Generation Using a Ti:sapphire/OPCPA Hybrid Laser System=

Intensity (arb.)

5-1

Prepulse-400-2000200

Time (ps)Fig.5-3 Focal spot and temporal profile(a) Typical spatial intensity distribution at the focal spot, (b) measured
third-order cross correlation in the 50 TW operation mode (the crosscorrelator temporal resolution of 120 fs causes up to a 3-foldunderestimation of the contrast on the ps and ns time scales), and (c)Transient-grating frequency-resolved optical gating (TG-FROG)
measurement of the main pulse temporal profile.

It is well known that when a laser beam is focused on ametal foil or a plastic foil target, hot electrons are produced
and ejected from the back of the foil target. When these hotelectrons leave the thin foil target, a strong electric field isgenerated owing to charge separation. Protons fromcontaminants, such as water or oil on the back side of the foil
target, are accelerated by this electric field. To apply thislaser-driven proton source to ion cancer therapy, etc., it isnecessary to produce high-energy protons by tightly focusing
the laser beam. A 70 MeV proton beam was reportedly
produced by a single-shot base large glass laser system. In
addition, a 25 MeV proton beam was produced using arepetitive Ti:sapphire laser system.
In this study, we used the J-KAREN laser system (JAEAKansai Advanced Relativistic Engineering Laser). To
efficiently focus the laser beam on a target, deformation of
the wavefront of the laser beam transmitted from the laser
system to the target was suppressed as much as possible. We
focused a laser beam with an energy of 7.5 J onto a target in
a 2 µm spot (Fig.5-3(a)). The laser intensity on the target
reached 1021 W/cm2. As far as we know, there have been noreports of the observation of any physical phenomena

200100

CR-39
NuclearTrack Detector
Protons10 mm〜40 MeV (d)10Lasers that are notsuitable for applications
Lasers that are suitable
for applications

10.1

110100Laser Energy (J)

(e)
1000

Fig.5-4 Results for this experiment and other studies(d) Example of tracks formed by protons. (e) Maximum
proton energy vs. the on-target laser pulse energy indifferent experiments. The star denotes the result for thepresent study.

corresponding to a 1021 W/cm2 interaction, although thegeneration of over 1022 W/cm2 has been achieved. Using an
optical phase control technology for the laser system and apulse-shape measurement technique, we produced a low
prepulse level of 10-10 (Fig.5-3(b)) and a pulse width of 36 fs(Fig.5-3(c)). If the prepulse level is very high, it affects thefoil target before the main pulse arrives. In this experiment,
because the contrast was approximately 10-10, the foil target
was not damaged before the main pulse arrived. As a result,
we could irradiate the foil in spite of high intensity. Thus, wegenerated a 40 MeV laser-driven proton beam (Fig.5-4(d)(e)).
The energy of the generated proton beam was estimated fromthe range of the protons using a CR39 nuclear track detector.
Because of the deep stopping range for small animals, thisnew proton source can now be used in preclinicalexperimental studies of in vivo proton therapy. We believethat this laser-driven proton generation technique reduces thesize of the particle accelerator system required for cancer
therapy. It is also expected that this laser-driven proton source
can be applied not only to other medical applications but also
to other proton acceleration applications.

Reference
Ogura, K. et al., Proton Acceleration to 40 MeV using a High Intensity, High Contrast Optical Parametric Chirped-Pulse Amplification / Ti:Sapphire Hybrid
Laser System, Optics Letters, vol.37, no.14, 2012, p.2868-2870.

66 JAEA R&D Review 2012