Photo-Medical Research Cooperation

11-1 Ion Acceleration from Near-Critical Density Plasma
Produced by Lasers–Proposal of a Novel Ion Acceleration Method for Cancer Therapy–Electrons

10cm

Magnetic
field

Ion Beam
Magnetic
field

Main pulse

direction

Solid-foil target

Electrons

Laser

ASE

Laser

Near-criticalDensity plasma

TargetNumber of protons (1/MeV/msr)

Fig.11-3 Laser ion accelerator
A laser pulse is focused onto a thin-foiltarget with intensity exceeding 1018 W/cm2,
which generates high-density plasma at
the focal spot. This plasma produces aMeV-scale proton beam.

Fig.11-4 A novel mechanism of ionacceleration
A laser pulse can invade into a nearcritical density plasma target, and thelaser energy is efficiently converted into
electron kinetic energy.

Fig.11-5 Near-critical density
plasma production method
A solid foil target is converted
into near-critical density plasma
by the amplified spontaneousemission (ASE) light preceding
the main laser pulse.

Maximum energy: 3.8MeV
Number of protons: 1010(E>1MeV)

Fig.11-6 A proton energy spectrum
We have successfully obtained protons of
3.8 MeV maximum energy.

Energy (MeV)

When a laser pulse is focused onto a thin-foil target withintensity exceeding 1018 W/cm2, high-intensity plasma is
induced at the laser focal spot, generating several kinds ofradiation including high-energy ions. By using this phenomenonof laser-ion acceleration, conventional ion accelerators, thathave lengths of tens of meters, can be replaced by a compact
apparatus as shown in Fig.11-3. The Photo-Medical ResearchCentre (PMRC) aims to develop a compact and novel laserdriven ion accelerator for cancer therapy.
Recently, we proposed and demonstrated experimentally a
new method of laser-ion acceleration by which laser pulse energy
can be converted more efficiently into ion kinetic energy. The
method is based on creation of a near-critical density plasma
target. In the case of a usual solid target, the target surface canreflect a large part of the laser pulse energy. By contrast, a laser
pulse can penetrate into a near-critical density plasma target(Fig.11-4), where laser energy is efficiently converted intoelectron kinetic energy. The vortex of moving electrons thus
created generates a magnetic field on the target rear surface
and sustains an electrostatic ion-accelerating field. A solid foil
target is converted into near-critical density plasma by the partof the light pulse that precedes the main high intensity laser,
amplified spontaneous emission (ASE) (Fig.11-5).
In experiments we have generated protons with maximum
energy of 3.8 MeV. Fig.11-6 shows the proton energy spectrum,
which also shows that approximately 1010 protons are generatedby a single laser shot. Higher-energy protons will be achievedby optimizing the target density through control of the ASE
intensity and duration.
This work is supported by Special Coordination Funds
for Promoting Science (SCF) provided by the Ministry ofEducation, Culture, Sports, Science and Technology of Japan.

Reference
Yogo, A. et al., Laser Ion Acceleration via Control of the Near-Critical Density Target, Physical Review E, vol.77, no.1-2, 2008, p.016401-1-016401-6.

JAEA R&D Review 200889