Generation and application of energetic, broadband terahertz pulses (bandwidth ~0.1–50 THz) is an active and contemporary area of research. The main thrust is toward the development of efficient sources with minimum complexities—a true table-top setup. In this work, we demonstrate the generation of terahertz radiation via ultrashort pulse induced filamentation in liquids—a counterintuitive observation due to their large absorption coefficient in the terahertz regime. The generated terahertz energy is more than an order of magnitude higher than that obtained from the two-color filamentation of air (the most standard table-top technique). Such high terahertz energies would generate electric fields of the order of MV cm-1, which opens the doors for various nonlinear terahertz spectroscopic applications. The counterintuitive phenomenon has been explained via the solution of nonlinear pulse propagation equation in the liquid medium.
The mechanism of microwave guiding and plasma generation is investigated in a circular waveguide with a subcutoff dimension using pulsed microwaves of 3GHz. During the initial phase, gaseous breakdown is induced by the exponentially decaying wave. Upon breakdown, the refractive index of the plasma medium varies radially, with the plasma density reaching close to cutoff values in the central region. At lower pressures, the waves can propagate through the peripheral plasma with a reduced wavelength, due to the collisionally broadened upper hybrid resonance region. The intense narrow cross sectional plasma bears promise for multielemental focused ion beams.
The question of electromagnetic wave penetration and screening by a bounded supercritical ͑ p Ͼ with p and being the electron-plasma and wave frequencies, respectively͒ plasma confined in a minimum B multicusp field, for waves launched in the k Ќ B o mode, is addressed through experiments and numerical simulations. The scale length of radial plasma nonuniformity ͉͑n e / ͑ץn e / ץr͉͒͒ and magnetostatic field ͑B o ͒ inhomogeneity ͉͑B o / ͑ץB o / ץr͉͒͒ are much smaller than the free space ͑ o ͒ and guided wavelengths ͑ g ͒. Contrary to predictions of plane wave dispersion theory and the Clemow-Mullaly-Allis ͑CMA͒ diagram, for a bounded plasma a finite propagation occurs through the central plasma regions where ␣ p 2 = p 2 / 2 Ն 1 and  c 2 = ce 2 / 2 Ӷ 1͑ϳ10 −4 ͒, with ce being the electron cyclotron frequency. Wave screening, as predicted by the plane wave model, does not remain valid due to phase mixing and superposition of reflected waves from the conducting boundary, leading to the formation of electromagnetic standing wave modes. The waves are found to satisfy a modified upper hybrid resonance ͑UHR͒ relation in the minimum B field and are damped at the local electron cyclotron resonance ͑ECR͒ location.
Standing waves in the microwave regime are generated by a superposition of forward and backward moving waves induced by reflections from geometrical transitions in the plasma vacuum boundary. The waves are preferentially damped in the weakly collisional (νen∕ω≅10−4) plasma near the launch region (∼3−15cm), where the electron temperature has a higher than average value (Te>Teavg∼12eV). Typical e-folding damping lengths are of the order of 10cm, and depend upon the wave power and plasma collisionality. Fourier spectrum of the standing waves indicates about 23% downshift in the vacuum wave-number due to plasma dispersion. Electron trapping is observed in the potential troughs of the waves.
A miniature microwave electron cyclotron resonance plasma source [(discharge diameter)/(microwave cutoff diameter) < 0.3] has been developed at Kyushu University to be used as an ion thruster in micro-propulsion applications in the exosphere. The discharge source uses both radial and axial magnetostatic field confinement to facilitate electron cyclotron resonance and increase the electron dwell time in the volume, thereby enhancing plasma production efficiency. Performance of the ion thruster is studied at 3 microwave frequencies (1.2 GHz, 1.6 GHz, and 2.45 GHz), for low input powers (<15 W) and small xenon mass flow rates (<40 μg/s), by experimentally measuring the extracted ion beam current through a potential difference of ≅1200 V. The discharge geometry is found to operate most efficiently at an input microwave frequency of 1.6 GHz. At this frequency, for an input power of 8 W, and propellant (xenon) mass flow rate of 21 μg/s, 13.7 mA of ion beam current is obtained, equivalent to an calculated thrust of 0.74 mN.
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