The Energy Technology Research Institute (ETRI) of the National Institute of Advanced Industrial Science and Technology (AIST), an independent administrative institution, succeeded in generating monoenergetic electron beam of high energy and ultra short pulse first in the world, in collaboration with the Department of Accelerator Physics and Engineering (DAPE), Research Center for Charged Particle Therapy (RCCPT) of the National Institute of Radiological Sciences (NIRS), another independent administrative institution.

Monoenergetic electron beam of high energy and ultra short pulse is expected to be an indispensable tool for medical treatment including cancer diagnosis and therapy, as well as for understanding and control of ultra fast phenomena related to chemical reactions. Up to now, monoenergetic electron beam has been generated by use of a conventional accelerator based on high voltage or microwave for experiments in high accuracy medical treatment or fast chemical reaction. However, the conventional accelerator requires a gigantic facility comparable to an airfield with high voltage and enormous power, making it difficult to realize full-fledged applications in the scenes of medical treatment and industry.

The laser plasma accelerator is based on a principle entirely different from that underlying the conventional devices, and expected to down-size the high energy accelerator drastically. In the laser plasma accelerator, electron beam is accelerated by a high electric field of a plasma wave, and can create an acceleration gradient, that is, acceleration energy per unit length, 100~1000 times as high as that in the conventional accelerator, owing to the lack of limitation on acceleration gradient resulting from the electrical breakdown. This makes it possible to reduce the accelerator length by a factor of 1/100 to 1/100 and a lot of R&D efforts are being paid in many countries. Previous works on the laser plasma accelerator, however, failed to generate monoenergetic electron beam, which has specific energy only. For the medical and industrial applications, it is necessary to provide some accessory devices to the accelerator for limiting energy spectrum of electron beam, such as filter to select necessary energy and block unwanted one, and shading device to cut off incidental high energy radiations if the beam consists of various energies. This makes it hardly possible to reduce the system size for the practical application of laser plasma accelerator.

Through an intimate collaboration, the ETRI-AIST and the RCCPT-NIRS has succeeded in generating monoenergetic electron beam of 7 MeV energy from a 0.5mm-long plasma by irradiating a laser pulse of 2 terawatts (1TW = 10¹²watts) for 50 femtoseconds (1 fs = 10^-15 seconds) to a plasma of electron density as high as 10²⁰cm^-3 (equivalent to the density of 10 atm air at the room temperature), which is an order of magnitude higher than that for the previous experiments. An evidence to demonstrate electron being accelerated by plasma wave has been obtained by diagnosing the laser plasma interaction. While the generation of monoenergetic electron beam with a laser plasma accelerator has been regarded unavailable up to now, the success in this study will provide a great step toward the practical use of the laser plasma accelerator.

While accelerators are useful not only for basic sciences but also for medical and industrial applications, they are hardly available to everyone and everywhere because of expensive cost and needs of enormous facility. If an accelerator were operated on the basis of an innovative concept, it might be possible to build up a high energy accelerator in a drastically reduced size.

One of such innovations is a laser plasma accelerator, where metal electrodes in the conventional system are replaced with ionized gas, plasma, and in place of microwave, laser excited plasma wave of much higher frequency is used to eliminate restriction caused by electrical breakdown. This allows the laser plasma accelerator to create an acceleration gradient 100~1000 times as high as that in the conventional device. In this way, for generating electron beam of identical acceleration energy, the accelerator length can be reduced by a factor of 1/100~1/1000.

The driver laser has also been down-sized, and at present, a 10-TW laser can be set up in an area of a few meters square. With the miniaturization, the development cycle of accelerators has been shortened, and it is expected that the cost will be reduced drastically and the utilization demands will be expanded quickly.

The latest experiment of laser plasma accelerator is reported to have achieved an acceleration energy of 600 MeV in maximum, but the output electron beam is “white”, containing every spectrum of energy from lower end to higher limit. In the practical applications, it is often necessary to provide a monoenergetic electron beam, by selecting a beam of necessary energy and discarding unwanted beams. This is not only very wasteful, but also emits dangerous high energy secondary radiation. As the implementation of a laser plasma accelerator capable of generating monoenergetic electron beam will makes readily available high energy electron beam of high quality, active R&D races are under way in the United States and other countries in the world.

The ETRI-AIST, former Electrotechnical Laboratory (ETL), has been engaged in basic research on the interaction of ultra-high intensity laser pulse with plasma and its applications since 1993, aiming at the development of ultra miniaturized accelerator. The work has been carried out under the Nuclear Energy Research Program of the Ministry of Education, Culture, Sports, Science and Technology (MEXT), titled “Study on high energy particle and radiation sources based on ultra-high intensity laser (FY2000~04) and under a collaboration project “Advanced Compact Accelerator Development Program” involving ETRI-AIST, NIRS, University of Tokyo, Kyoto University, Osaka University, Hiroshima University, High Energy Accelerator Research Organization, and Japan Atomic Energy Research Institute.

The laser plasma accelerator works on the basis of the following principle: when a laser pulse is shot into high density supersonic gas jet, the laser pulse pass through while turning gas into plasma and generating a compression wave of electron (plasma wave) which propagate at high speed. Electrons captured by plasma wave are pushed forward and accelerated to be emitted as electron beam (See Fig. 1).

Fig. 1. A schematic diagram of a laser plasma accelerator, and the principle of acceleration. When high intensity laser pulse is shot into high density gas jet, the laser pulse propagates while creating a plasma to generate compressive wave of electron (plasma wave) in the plasma. Electrons captured by plasma wave are accelerated forward to emit an electron beam.

It is inevitable, however, that monoenergetic electron beam can hardly be obtained with the laser plasma accelerator, because the acceleration energy varies depending upon the timing of “surf-riding” or where individual electrons are located in the slope of plasma wave and the output beam consists of a mix of energy spectra.

In the present study, the successful generation of monoenergetic beam owes to irradiation of 2-TW laser pulse for 50 fs to a plasma of electron density an order of magnitude higher than that used in the previous works, that is, 10²⁰ electrons per cm³ (equivalent to the density of air under about 10 atm. pressure at room temperature). The monoenergetic electron beam was obtained from a plasma of about 0.5mm length. It was also found that the monoenergetic electron beam had 7 MeV energy (Fig. 2), and occurred only when signals of plasma wave were recognized (Fig. 3).

Fig. 2. Energy spectrum of electron beam obtained in an acceleration experiment with a laser plasma.

Fig. 3.Spectrum of forward scattering light by a plasma. A peak around 1040 nm represents a component generated through the modulation by a plasma wave. This component appears only when monoenergetic electron beam is accelerated.