Transition radiation from a beam of hot electrons generated in ultraintense laser plasma interaction is theoretically studied. The total radiation is separated into two parts: one is incoherent transition radiation ͑ITR͒, the other is coherent transition radiation ͑CTR͒. The spectrum of ITR just depends on the particle velocity distribution in the beam. The angular distribution of ITR varies from sin 2 , and approaches the angular distribution of the beam when the hot electron temperature increases from the nonrelativistic limit (TӶmc 2) to the ultrarelativistic limit (Tӷmc 2). The spectrum of CTR is dependent on the particle configuration as well as their velocities. Any microbunching in the beam can greatly enhance the CTR intensity at the microbunching frequency, from which the dominant heating process can be inferred. The effects of target thickness and hot electron temperature on CTR intensity are also calculated. The simplified model shows that the CTR intensity decreases with the increase of the target thickness, and increases with the hot electron temperature. The divergence of the beam can broaden the CTR spectrum.

Freeman, R. R.; Gu, P.; Hatchett, S. P.; Hey, D.; Hill, J.; Key, M. H.; Izawa, Y.; King, J.; Kitagawa, Y.; Kodama, R.; Langdon, A. B.; Lasinski, B. F.; Lei, A.; MacKinnon, A. J.; Patel, P.; Stephens, R.; Tampo, M.; Tanaka, K. A.; Town, R.; Toyama, Y.; Tsutsumi, T.; Wilks, S. C.; Yabuuchi, T.; Zheng, J. Citation Physics of Plasmas.

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A double-line terawatt beat laser (BEAT) is developed for exciting beat wave oscillations. BEAT consists of two oscillators and an amplification system including optical parametric chirped-pulse amplification (OPCPA) in which two individual pulses with wavelength separations of 10-35 nm are amplified, recompressed, and focused as a single beam. The recompressed pulse trace shows that a 150-fs pulse duration full width at half maximum was modulated at a beating period of 72 fs. This beating period matches a resonant excitation of plasma wave with an electron density of 2.5 9 10 18 cm -3 , resulting in excitation of a beat wave in hydrogen plasma with wave amplitude of 15 GV/m. The multiple beating oscillations can amplify the plasma wave and improve its structure. This scheme would be ideal for stabilizing the plasma wave strength in the plasma cavity and for realizing a practical laser plasma accelerator.