|Plasma Physics Seminar ( Phys 769)|
| Dr. Balram Prasad, Visiting Senior Research Scientist, IREAP
Counter-streaming large diameter two electron beams interacting with ionized gas plasma in steady state laboratory experiments were observed (Hershkowitz et. al, Phys. Fluids, 1984) to generate intense coherent microwave radiation at twice the plasma frequency in a quadrupole-radiation pattern. The radiated microwave power density was measured in a series of steady state beam-plasma experiments where the two opposing beams average energy was always kept equal. The measured power density in all cases increased exponentially with the input average energy of the electron beams and it further increased with higher ion masses of the ionized gas plasma.
These two spectacular experimental results were published in 1984, and together they characterize a MASER like action in the colliding two electron beam plasma system. The experimental results have not, as of yet, been explained by a single comprehensive theory. However, a quasi-quantum formalistic theory (QFT) for a weak plasma turbulence condition for astrophysical plasmas was developed by Tsytovit (1972), and was extensively applied by Melrose in (1980) and Cairns (1987) to explain the characteristics of radio burst spectra having harmonic structure. The TMC model involved three-wave scattering processes which occur in astrophysical plasma under solar active conditions. The same model using QFT theory has been adapted by Dr Balram Prasad and applied to the two opposing beams-plasma system. All the experimental results obtained earlier have been explained both qualitatively and quantitatively. From the latter model, an analytic expression for output power at twice the plasma frequency is derived that predicts generation of coherent radiation in the microwave to Terahertz (THz) spectral region. RF frequency selection is controlled by proper scaling/optimizing of electron beam and plasma parameters, constrained by beam-plasma volume kinematic effects as well as the weak plasma turbulence condition imposed by the TMC model. The expression allows numerical calculation of the THz output power predicting enormous levels exceeding several Kilowatts.
By operating the triple plasma machine (DOLI II) at optimized beam and plasma parameters, 30 to 50 kilo watts of Terahertz wave power in the 0.1 to 3 THz spectrum can be produced and used as a hyper-band (very wideband) THz radiation source. An important application of the new hyper band THz radiation source is to detect, image, and selectively kill human cancer cell structures. A principal feature of the talk will be an overview of a recent paper by the author, entitled "Remote THz wave Resonance Imaging (TRI) of Human Breast Tumors", originally presented by the author at the URSI 2007 Radio Science International Meeting at Ottawa, Canada in July 2007. That paper is a conceptual construct of a model to remotely detect, image and also kill the tumor cells selectively by application of two THz illumination pulses successively. The first pulse is a hyper-band emission intended to measure the unique cancerous cell resonant frequency, and the second pulse then is a very narrow band pulse modulated by the characteristic resonant signature measured by the first pulse. How the two THz pulses are implemented in a practical procedure to execute the three intended functions of the conceptual/theoretical model shall be discussed.
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