T. Kumita

0.56 GeV Laser Electron Acceleration in Ablative-Capillary-Discharge Plasma Channel

A high-quality electron beam with a central energy of 0.56 GeV, an energy spread of 1.2% rms, and a divergence of 0.59 mrad rms was produced by means of a 4 cm ablative-capillary-discharge plasma channel driven by a 3.8 J 27 fs laser pulse. This is the first demonstration of electron acceleration with an ablative capillary discharge wherein the capillary is stably operated in vacuum with a simple system triggered by a laser pulse. This result of the generation of a high-quality beam provides the prospects to realize a practical accelerator based on laser-plasma acceleration.

Observation of the nonlinear effect in relativistic thomson scattering of electron and laser beams

Thomson scattering of high-power laser and electron beams is a good test of electrodynamics in the high-field region. We demonstrated production of high-intensity X-rays in the head-on collision of a CO2 laser and 60-MeV electron beams at Brookhaven National Laboratory, Accelerator Test Facility. The energy of an X-ray photon was limited at 6.5 keV in the linear (lowest order) Thomson scattering, but the nonlinear (higher order) process produces higher energy X-rays. We measured the angular distribution of the high-energy X-rays and confirmed that it agrees with theoretical predictions.

Construction of polarized slow-positron beams using a compact cyclotron

Abstract We have constructed a polarized positron beam using positrons provided from the β+ decay of 27Si with the half-life of 4.1 s and the maximum β+ energy of 3.85 MeV. This isotope is produced via the 27Al(p, n)27 Si reaction caused by proton irradiation using a compact proton cyclotron. The intensity of this beam is measured to be 5 × 105 e+/s for protons with the energy of 18 MeV and the current of 30 μA. A pair of Monte Carlo simulation programs has been developed to calculate depolarization of positrons in our beam channel. One program simulates spin motion of the positron under electric and magnetic fields, while the other simulates depolarization due to multiple Coulomb scattering in matter. Using these simulators, we designed a special polarimeter by means of magnetic quenching of ortho-positronium and measured a quenched lifetime of ortho-positronium under the magnetic field of 0.4, 2.5 and 4.0 kG leading to the average polarization of 33.4 ± 5.3%, which is consistent with the estimation based on the Monte Carlo simulations.

Study on laser cooling of ortho-positronium

Abstract We have been theoretically and experimentally studying details of laser cooling of ortho-positronium. Experimental apparatus consists of a pulsed positron beam generator, a long pulse wide bandwidth ultraviolet laser and a time-of-flight system to measure kinetic energy of positronium atoms is constructed and tested. Using this apparatus, production of thermally activated positronium is confirmed. Its production rate is larger as the target temperature increases and ratio of thermally activated ortho-positronium to all γ-ray annihilation events is estimated to be 32.3±6.7% for the target temperature 1000 K.

Construction of a time-of-flight measurement system to study the low energy positronium production

Abstract To investigates the low energy positronium (Ps) production in terms of target temperatures and materials, surface condition and so on, we adopt the time-of-flight (TOF) method and perform extensive Monte Carlo simulation to design an apparatus. Under reasonable assumptions in the simulation with respect to an initial energy distribution of Ps, we examine how cleanly ortho-positronium (oPs) is observed without being subject to large backgrounds of 2γ decay from e+e− annihilation. The simulation finds that the contribution of 2γ events is suppressed by 10−6 smaller than the generated Ps by using the 2.5-cm thick W and 4.6-cm Pb collimators with slit gap of 1 mm. We inject slow positron beams on the W target installed in the TOF system and move the position of the target. Then we expect the position resolution of 1.5 mm resulting the accuracy of determining the oPs velocity 20%, corresponding to 1.1×106 cm/s for thermal Ps with 30 meV.

Transmission of high-power CO2 laser pulses through a plasma channel

A 5 J, 180 ps CO2 laser pulse is channeled by a 17 mm long capillary discharge. Plasma dynamic simulations confirm occurrence of optical guiding conditions along a plasma column of a quasiparabolic radial profile with the minimum axial free-electron density ∼1017 cm−3.

Bunched beam generation of polarized positrons in TOPPS-II

Abstract We have constructed a bunched beam generator of polarized slow positrons, namely TOPPS-II (Tokyo Metropolitan University Polarized Positron Beam System) using the magnetic transportation. The positrons emitted from 100 mCi 22 Na are implanted on the tungsten moderator of 6 μm thickness. Reemitted positrons, accelerated to 200 eV and guided with 100 G magnetic field of solenoid coils, are chopped to be 40 ns every 200 ns and bunched into 2 ns time width. In this paper, we present the results of the overall performance of TOPPS-II.

Studies of a proximity focusing RICH with aerogel radiator for Belle II experiment

For the Belle II experiment, we are developing a proximity focusing ring imaging Cherenkov counter using a silica aerogel as a radiator. Recently, a new aerogel fabrication method, called “pin-hole drying”, was invented and remarkably improved optical transparency has been realized in the region of refractive index larger than 1.05. For the photo-detector, we have developed a 144-channel Hybrid Avalanche Photo-Detector with Hamamatsu Photonics K. K. The HAPD has excellent single photon detection performance, 5 × 5 mm2 position resolution and immunity to magnetic field. The tolerance to the neutron irradiation up to 5 × 1011 neutron/cm2 has been confirmed. In November 2009, we have performed a test beam experiment at KEK for a prototype A-RICH counter with 6 HAPDs and aerogel tiles produced by the “pin-hole drying” method. We successfully observed a clear Cherenkov ring image and obtained the single photon Cherenkov angle resolution of 13.5 mrad and 15.3 detected photoelectrons. This result corresponds to more than 6σ K/π separation at 4 GeV/c and fulfills the requirement for the Belle II.

Laser cooling system of ortho-positronium

Abstract We have studied laser cooling (Doppler cooling) of ortho-positronium to achieve Bose–Einstein condensation of positronium atoms. We have been developing a long-pulse laser with the wavelength of 243 nm , a bunched slow positron beam to realize the proposed experiment and a Monte Carlo simulation program for theoretical understanding. The Monte Carlo analysis clarified that we were able to obtain 7% of ortho-positronium atoms which were cooled down to 1 K and confined within a small volume without using a trap system.

Development of a positron trap for the laser cooling experiment of positronium

A cloud of ortho-positronium produced at the room temperature can be cooled down to 0.6 K utilizing a long pulse (180 ns) laser with 243 nm wavelength. We developed a time bunching system of a slow positron beam to synchronize production of positronium atoms to the laser pulse. After a theoretical examination using a Monte Carlo simulation, an experiment was made for confirmation.