Study of ultra-high gradient wakefield excitation by intense ultrashort laser pulses in plasma 高強度超短パルスレーザーによる、プラズマ中のウェーク場励起に 関する研究
Study of ultra-high gradient wakefield excitation by intense ultrashort laser pulses in plasma
Recently laser-driven plasma accelerators using laser wakefields have been conceived to be the next-generation particle accelerators, promising ultrahigh field particle acceleration and compact size compared with conventional accelerators. For this purpose, it is first important to investigate the mechanism of the nonlinear optical phenomena in laser-plasma interactions; optical field ionization (OFI), self-channeling, ionization induced spectral shift, and excitation of laser wakefield. These phenomena are deeply related to laser wakefield acceleration of particles. We investigate the laser wakefield excitation and acceleration of bright electron beams due to laser-plasma interactions to clarify the feasibility of a laser wakefield accelerator (LWFA). We present the results of studies on the nonlinear optical phenomena and the laser wakefield excitation.<br /> Generation of intense laser pulses and measurements of the nonlinear optical phenomena induced laser-plasma interactions are presented. OFI and self-focusing depend on the gas density are observed in the measurement. Measurements of side and forward scattered radiations indicate self-channeling which is filamentation and a long propagation of the intense laser pulse. The gradient of a refractive index in an ionization front causes a spectral frequency shift of the short pulse laser. The spectral blueshift has been observed experimentally in the propagation of intense ultrashort laser pulses through a gas medium. This generic blueshift strongly depends on the laser intensity and the gas density. In our experiments, a different type of blueshift has been discovered. In this blueshift, a whole spectrum of the laser pulse shifts to a fixed value without regard to the laser intensity and the gas density. We call this effect an "anomalous blueshift". We present a physical interpretation of this puzzling effect.<br /> The gas density measurements and the laser wakefield measurement are presented. The gas-jet has been used for many applications to supply a plasma source in the vacuum chamber. Since a gas adiabatically expands through a nozzle at a sound speed, the density distribution changes in space and time at the same rate as the sound velocity of the gas. Therefore it is necessary to measure the time-dependent spatial distribution of gas density for controlling the plasma precisely. For this purpose, we have made a time-resolved measurement of the gas density distribution produced by the gas-jet. These results visualize dynamics of a neutral gas ejected from the gas-jet nozzle.<br /> Following the gas density measurements, a direct measurement of the plasma density oscillation can be performed by means of the ultrafast time-resolved frequency domain interferometry (FDI). The FDI measurement is based on the pump-probe technique consisting of an intense ultrafast pump pulse and two ultrafast probe laser pulses. In FDI, the plasma electron density oscillations excited by the pump pulse can be detected as a phase shift of the frequency domain interferogram in the spectrum produced by two probe pulses. The measurement of the phase shift as a function of time gives direct information of the amplitude and phase of the wakefields. Several measurements have been made with FDI to demonstrate wakefield excitation by ultrashort laser pulses in an underdense plasma. These measurements have been done for a relatively low density plasma in a gas filled chamber using laser pulse durations around 100 fs and pump peak powers less than 1 TW. In these measurements the probe pulse width limits the highest measurable density to ～4 × 10<SUP>17</SUP>cm<SUP>-3</SUP>, and the pump pulses were tightly focused to enhance the plasma wave excitation due to 2D effects. In the 2D dominant regime, where the pulse width is longer than the spot size, the radial wakefield is higher than the longitudinal one. Therefore a shorter pulse is preferable to generate a more 1D coherent planar wakefield at the higher resonant plasma density. The measurement of laser wakefields has been made in less 2D dominant regime. The measured wakefield is compared with 1D Particle-in-Cell simulation results. <br /> Hence, from the point of view of applications for particle accelerators, it is crucial that an ultrashort particle bunch with an energy higher than the trapping threshold should be injected with respect to the correct acceleration phase of the wakefield to produce a high quality beam with small momentum spread and good pulse-to-pulse energy stability. The trapped phase space of the wakefield accelerations are typically less than 100 fs temporally and 10 μm spatially, respectively. Therefore it is essential to inject a very short pulse and a low emittance electron beam into the wakefield. Electron beam injection triggered by an intense ultrashort laser is proposed to an injector of ultrashort electron beams as "optical injection". We present the numerical simulation an optical injection scheme based on the FDI system and the anomalous blueshift. <br /> In conclusion we have investigated the extraordinary nonlinear phenomena manifested via interactions of ultrashort laser pulses with gas and plasma; optical field ionization, ionization induced self-focusing and filamentation, an anomalous spectral shift and a large amplitude wakefield excitation. This study reveals that these phenomena occur in a consecutive strong field process through mutually correlated mechanism generated above a certain threshold intensity and that they can be controlled with femtosecond optical pulse technique in order to generate a relativistic bright electron beam with high quality in a laboratory table-top scale. As a result of particular observations of nonlinear optical phenomena in strong field, we found the anomalous blueshift that shows a coherent frequency upshift of the whole laser pulse to a fixed frequency independent of the plasma density and the laser power. We clarify that this phenomenon results from a complex mechanism of the ultrafast optical field ionization and filamentation to cause acceleration of the whole laser photons due to a steep gradient of the refractive index change from neutral gas to plasma. In the wakefield measurement, we have made the first direct observation of 20 GeV/m of coherent ultrahigh gradient wakefields excited by an intense ultrashort laser pulse in a gas-jet plasma. The experimental results agree with the 1D PIC simulation results and the linear theory. In the numerical simulations based on the results of these measurements, we verify generation of a relativistic electron beam accelerated by laser wakefields to be optically controlled with two colliding injection pulses of which one pulse can utilize a frequency up-shifted pulse due to the anomalous blueshift effect. This synthetic study on laser wakefield excitation illuminates physical mechanisms of complex ultrafast nonlinear phenomena generated by interaction of ultraintense laser pulses with plasma and gives prospects of next generation particle accelerators for applications to a wide range of sciences; such as material science, nuclear science, high energy physics, chemical science, biological science, and medical science.