11-1 Ion Acceleration from Near-Critical Density Plasma Produced by Lasers

- Proposal of a Novel Ion Acceleration Method for Cancer Therapy -

Fig.11-3 Laser ion accelerator
Fig.11-3 Laser ion accelerator

A laser pulse is focused onto a thin-foil target with intensity exceeding 1018 W/cm2, which generates high-density plasma at the focal spot. This plasma produces a MeV-scale proton beam.

 

Fig.11-4 A novel mechanism of ion acceleration

Fig.11-4 A novel mechanism of ion acceleration

A laser pulse can invade into a near- critical density plasma target, and the laser energy is efficiently converted into electron kinetic energy.

Fig.11-5 Near-critical density plasma production method

Fig.11-5 Near-critical density plasma production method

A solid foil target is converted into near-critical density plasma by the amplified spontaneous emission (ASE) light precedin the main laser pulse.

 

Fig.11-6 A proton energy spectrum

Fig.11-6 A proton energy spectrum

We have successfully obtained protons of 3.8 MeV maximum energy.

When a laser pulse is focused onto a thin-foil target with intensity exceeding 1018 W/cm2, high-intensity plasma is induced at the laser focal spot, generating several kinds of radiation including high-energy ions. By using this phenomenon of laser-ion acceleration, conventional ion accelerators, that have lengths of tens of meters, can be replaced by a compact apparatus as shown in Fig.11-3. The Photo-Medical Research Centre (PMRC) aims to develop a compact and novel laser-driven 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 can reflect 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 into electron 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 part of 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 generated by a single laser shot. Higher-energy protons will be achieved by 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 of Education, 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.

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