Generation of Short Electromagnetic Wave via Laser Plasma Interaction Experiments N. Yugami, Utsunomiya University, Japan US-Japan Workshop on Heavy Ion Fusion and High Energy Density Physics Sep. 28-30,2005 Department of Energy and Environmental Science, Graduate School of Engineering, Utsunomiya University 7-1-2 Yoto, Utsunomiya, Tochigi 321-8585, JAPAN, TEL: +81-28-689-6083, FAX: +81-28-689-7030
Outline A proof-of-principle experiment demonstrates the generation of the radiation from the Cerenkov wake excited by a ultra short and ultra high power pulse laser in a perpendicularly magnetized plasma. The frequency of the radiation is in the millimeter range(up to 200 GHz). The intensity of the radiation is proportional to the magnetic field intensity as the theory expected. Polarization of the emitted radiation is also detected. The difference in the frequency of the emitted radiation between the experiments and previous theory can be explained by the electrons' oscillation in the electric field of a narrow column of ions in the focal region.
Oscillating Current in Plasma Wave Electron Current Skin depth ;d c/wp n=1.0x1016 cm-3 f=0.9 THz Frequency ; w wp (plasma freq.) n0 = 1016 cm3 lp = 300 mm d 50 mm  J. Yoshii et al., Phys. Rev. Lett. 79, 4194 (1997).
Typical Example of Emitted Radiation Expected Pulse Width Experimental condition Laser Power: 0.5 TW B0: 8.5 kG, He 375 mTorr
Radiation intensity was proportional to the strength of B field Laser Power ; 0.5 TW N2 750 mTorr
Plasma cavity for radiation Wavelength of the radiation is satisfied the matching condtion, Strong radiation is expected to observe. The data suggests the wavelength(frequency) depends on the strength of B field. plasma boundary boundary Plasma column works as “cavity” for radiation
Damping at the plasma-vacuum surface boundary Ratio
Typical Waveform of Radiation--- 2 peaks were observed --- Cut-off freq. of waveguide fc = 31.4 GHz Pulse width 200-250 ps
Freq. Spectrum measured by TOF method--- Each peak has different freq. --- Flight length L = 1.2 m 1st peak ~74 GHz 2nd peak ~40 GHz
Freq. of 1st peak does NOT depend on B field.Freq. of 2nd peak depends on B field.
1st peak of Radiation • Freq. : 74 GHz． • Polarization of radiation // B field • Frequency of radiation dose not depend on the strength of B field Electrons' oscillation in the electric field of a narrow column of ions in the focal region. Electron B x E + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + w z ion column
2nd peak pulse • Freq. : ~40 GHz． • Polarization of radiation B field • Frequency of radiation depends on the strength of B field Bernstein mode 4 3 2 1 0 1 2 3 4
Conclusion • The radiation from the interaction between laser and magnetized plasma was observed. • Higher freq. andlower freq.components were observed. • Higher Freq. Component • Freq : 74 GHz • Polarization …. Parallel to B field • Frequency does not depend on the strength of B field. • Due to the electron motion parallel to the B field. • Lower Freq. Component • Polarization …. Perpendicular to B field • Frequency does depends on the strength of B field. • Bernstein mode?