T. Kawakubo

A Proof-of-principle Experiment of Laser Wakefield Acceleration

A principle of the laser wakefield particle acceleration has been tested by the Nd : glass laser system with the peak power of 30 TW and the pulse duration of 1 ps. The particle acceleration up to 18 MeV/c has been demonstrated by injecting 1.0 MeV/c electrons emitted from a solid target by an intense laser impact. The corresponding field gradient achieves 1.7 GeV/m.

A table-top X-ray FEL based on the laser wakefield accelerator-undulator system

Laser wakefield acceleration makes it possible to build a compact electron linac. While acceleration is attributed to longitudinal wakefields, transverse wakefields simultaneously generated by a short laser pulse can serve as a plasma undulator with a very short wavelength equal to half of the plasma wavelength. We propose a new FEL concept for X-rays based on a laser pulses delivered from a table-top terawatt laser. The system is composed of the accelerator and undulator stages in a table-top size. A low energy electron beam is accelerated due to laser wakefields in the accelerator stage. A bunched electron beam travelling to the opposite direction of driving laser pulses produces a coherent X-ray radiation in the undulator stage. A practical configuration and its analysis are presented.

Recent results of laser wakefield acceleration in KEK/U. Tokyo/JAERI

We report recent results of the laser wakefield acceleration (LWFA) project in KEK/U. Tokyo/JAERI. The project aims at achieving high energy particle acceleration to energies more than 1 GeV on a table-top scale owing to a channel-guided LWFA scheme by the use of 100 fs, 2 TW table-top-terawatt (T3) laser system. We have demonstrated the self-channeling of ultrashort laser pulses with a relativistic intensity over a few cm. We have achieved synchronization of a 10 ps electron beam with a 100 fs laser pulse within a few ps in order to accelerate injected electrons firmly due to wakefields induced by laser pulses in the rate of 10 Hz. Recently we have observed laser wakefield acceleration of an electron beam, injected at 17 MeV, up to 100 MeV.

Nonlinear acceleration caused by a plasma wakefield in an underdense regime

Acceleration greater than an order of magnitude larger than that predicted by a linear theory was observed in a moderate underdense plasma wakefield when the time delay between the drive and test bunches was less than one plasma oscillation period.

Production of micro‐bunches by a laser wakefield in a plasma and its application: X‐ray generation

Laser wakefields driven by the ponderomotive force of a laser in a plasma accelerate charged particles. The particles are bunched during the course of acceleration. The bunch spacing is equal to the plasma wavelength, which can be less than 10 μm in a plasma with density higher than 1019 cm−3. The energy and phase spectra of the accelerated particles are presented. X‐ray generation based on a plasma undulator is introduced as an example of applications of the laser wakefield.

Formation of self-channeling and electron jet in an underdense plasma excited by ultrashort high intensity laser pulses

Experimental investigations on interactions of a tera-watt laser (0.5–2.5 TW) with an underdense plasma are reported. Formation of self-channeling longer than the vacuum Rayleigh length is observed when ultrashort (100 fs in FWHM) high power (∼2u2009TW) laser pulses are focused into a gas (He, Ar, N2) filled chamber. In addition to the formation of self-channeling along the laser axis, a bright radiation in the transverse direction of the channel was observed for the highly excited dense gases. This sideway radiation was observed for all three gases and especially for N2, sideway jet-like radiation (we call it sideway electron jet) was monitored. The energy of the emitted electrons was estimated to be ∼10u2009keV from the range of electron trajectories in N2.

Plasma wakefield acceleration experiments using twin linacs

A collinear wakefield test facility using two linacs with a common test section is constructed. Beams from one linac excite wakefields in a test medium such as a plasma, while beams from the other linac witness the wakefields. The time interval between the two beams is controllable with an accuracy of ~1 psec. Plasma wakefield acceleration experiments were conducted using this facility.

Injection and induction acceleration of Ar 3+ in the KEK Digital Accelerator

Four microseconds long Ar 3+ beam with injection energy of 15xa0keV/u has been injected into the Digital Accelerator of the High-Energy Accelerator Research Organization. Beam production, transportation, and injection are described as well as machine properties. Results of a free running experiment under static magnetic field and longitudinal confinement and acceleration under a fast ramping magnetic field are presented in detail with a brief discussion on the beam lifetime.

Nonlinear and collective phenomena observed in plasma wakefield acceleration driven by multiple bunches

Electron acceleration by plasma wakefields driven by multiple bunches was experimentally studied. It is expected that a wakefield driven by each bunch builds up to result in a large acceleration gradient. However, the build-up of wakefields was not ideal, due to the peaked plasma-density distribution in a small plasma chamber. Two puzzles were encountered. First, the amplitude of the wakefield observed was larger than that calculated based on a linear model. The observed wake structure was not sinusoidal. These results suggest that the plasma wave was nonlinear in spite of the fact that the perturbation of a plasma by the wave should be far smaller than unity. Second, a quadratic deformation was found in the shape of test beams; when it was focused transversely, it was defocused longitudinally, and vice versa. This contradicts the theory that the plasma-lens effect is uniform.

Laser Wakefield Accelerator experiments using 1 ps 30 TW Nd:glass laser

The peak power of 30 TW and the pulse width of 1 ps produced by the Nd:glass laser system is capable of creating a highly-ionized plasma of a moderate density gas on an ultrafast time scale and generating a large amplitude plasma wave with an accelerating gradient of the order of GeV/m. We are going to demonstrate particle acceleration, injecting electrons of a few MeV emitted from a solid target by intense laser irradiation.<<ETX>>