Akira Maekawa

Development of a laser-driven plasma cathode for medical applications

In this article, the present status of radiation therapy in Japan and updated medical accelerators are reviewed. In addition, the potential of laser plasma acceleration as a future medical accelerator is discussed. The updated results of laser plasma cathode experiment by the University of Tokyo are described.

Measurement of ultra-short electron bunch duration by coherent radiation analysis in laser plasma catode

We have performed measurement of electron bunch duration from laser plasma acceleration. Spectrum analysis of the transition radiation from the electron bunch was selected for measurements, using an IR (infrared) bolometer with different filters first. The bunch is estimated to be 30 fs in PIC simulation, but measured duration was 2 ps by the result of bunch elongation at 340 mm downstream from the plasma edge due to broadness of energy spectrum. And then measurement by In-Sb polychromator aiming shot-by-shot measurement is also done. But the signal of transition radiation cannot be detected by X-ray background signal even if shielding by Pb. So we have done optimisation of X-ray shielding and position of detector.

Manipulation of elctron beam generation with modified magnetic circuit on laser-wakefield acceleration

Electron beam injection triggered by intense ultrashort laser pulses, which is called as plasma cathode, is presented. We have studied generation of relativistic electrons by interaction between a high intensity ultrashort laser pulse and gas jet. When a static magnetic field of 0.2 T is applied, the modification of the preplasma cavity, and significant enhancement of emittance and an increase of the total charge of electron beams produced by a 12 TW, 40 fs laser pulse tightly focused in a He gas jet, were observed. And very high stability and reproducibility of the characteristics and position of well-collimated electron beams was detected. We performed an experiment with a magnetic circuit that has more intense magnetic field of 1 T.

Generation and characterization of electrons from a gas target irradiated by high-peak-power lasers

We present the results of electron generation experiments conducted at the Advanced Photon Research Center, Japan Atomic Energy Research Institute, using 23-fs relativistically intense 20-TW tightly focused laser pulses with underdense plasma. We observed electron energies up to 40 MeV characterized by a two-temperature Maxwell distribution. With the help of particle-in-cell simulations, we found that these are due to different plasma wave-breaking processes. A charge of 5 nC/shot was obtained at a small solid angle, which corresponds to high peak current generation.

Single-Shot Characterization of Plasmas Generated by 17 TW 37 fs Laser Pulses

Single-shot measurement of plasma parameters is very important in laser wakefield acceleration research aiming to generate compact high-quality femtosecond electron beams. To achieve the efficient and stable monoenergetic acceleration of plasma electrons, effects of laser prepulses, plasma density gradients, wave-breaking processes, laser pulse instability, and energy spectra of accelerated electrons have to be measured simultaneously, granting reproducible laser pulse parameters. A strong correlation between the laser prepulse forming the plasma cavity and electron acceleration has been experimentally confirmed.

CTR Bunch Length Measurement of Monoenergetic and Maxwellian Electron Beams from Laser Plasma Cathode

Recently, several plasma cathodes at universities and institutes can generate a monoenergetic electron beam. LBL measured the bunch length by the CTR (Coherent Transition Radiation) interferometer to determine it as about 50 fs (FWHM). We are trying to carry out a single‐shot measurement of the bunch length by a infrared polychromator. As the first step forward it, we measured the CTR spectra emitted at a 300microm‐thick Ti foil by the electron beams from our plasma cathode. The laser parameters are 12 TW, 50fs, 3×1019W/cm2. He gas‐jet is used and the electron density is 6×1019cm−3. By using the liquid He bolometer and several optical low‐pass filter, we measured the spectra of the CTR. We can clearly classify the difference of the spectrum and bunch length for the monoenergetic and Maxwellian beams. The numerical analysis of the bunch elongation due to the energy spread confirms the experimental results. The bunch length of the monoenergetic beam is less than 100 fs at FWHM with about 10 pC per bunch, wh...