The nonlinear interaction process between the electrons and the immobile ions in a plasma-filled diode has been studied via two different approaches: (1) solving the coupled analytical integral ...equations and (2) simulating the system by running a particle-in-cell code PDP1 (plasma devices planar 1D). The investigations have extended the analysis to the case where the effect of a magnetic field is considered in the diode. The plasma-filled diode can still operate in four distinct dynamical regimes - steady, oscillatory, chaotic, and unstable - if the magnetic field is relatively weak below the diode cutoff threshold.
A negative electron affinity (NEA) diamond surface is employed as an emitter electrode in a vacuum thermionic energy conversion device in order to mitigate the negative space charge effect. The ...motive diagram of an NEA device operating at the virtual saturation point is compared to a similar device with a conventional emitter material operating in the space charge limited regime in order to understand how NEA mitigates space charge. Output current characteristics are calculated for various NEA values, and the results are compared to an ideal (no space charge) model. Increasing the value of the NEA causes the output current characteristic to approach that of the ideal model. Motive diagrams for various values of NEA are calculated and used to explain this phenomenon. It is shown that an NEA device can achieve a maximum output power density equal to the maximum output power density of a similar ideal device.
Hysteresis of electron-emission current from filament cathode was observed in a 10-kW axial electron gun used for evaporation of metals. It was observed that in the forward direction of power change ...(i.e., increasing electron-beam (e-beam) power), the required filament-heating current for a specified e-beam current was significantly higher than the filament current required in the backward direction (i.e., decreasing e-beam power). With appropriate calculations and experimental methods, it is established that this hysteresis arises primarily due to the energetic metal-ion bombardment on the electron-emitting area of filament in the electron gun. Finally, results of modification of the electron-emitting filament, such as drilling of a hole at its center to check the uncontrollability arising due to the bombardment of ions, are presented
The feasibility of a magnetohydrodynamics (MHD) generator with frozen inert gas plasma (FIP) is examined numerically. The FIP is produced by pre-ionizing helium of inert gas without alkali metal seed ...at the generator inlet. Because the three-body recombination coefficient of helium ions is low at the electron temperature more than 5000 K, the ionization degree of inert gas is kept almost constant in the entire region of the generator channel and the relation between the electron temperature and the electron number density deviates from the Saha equilibrium. In a small-scale disk-shaped generator, the enthalpy extraction ratio of 22.7% and the isentropic efficiency of 54.8% are obtainable for the load resistance of 3.0 /spl Omega/ and magnetic flux density of 4 T under the pre-ionization degree of 4.79/spl times/10/sup -5/. The auxiliary power to sustain the pre-ionized plasma corresponds to about 2% of the thermal input. For high-magnetic flux density, the generator performance can be improved as well as a conventional-seeded plasma MHD generator.
A plasma-filled diode (PFD) technique is described for producing high dose-rate bremsstrahlung over small areas. A PFD was developed for this purpose using the Gamble II generator. For maximum ...dose-rate conditions, the short-circuit phase duration was 40 ns, after which the voltage increased to 1.8 MV and the total current in the diode was 0.5 MA. The X-ray pulsewidth was 8 ns, much less than the typical 50 ns pulsewidth with a vacuum diode. This PFD was adapted to the higher-current Saturn generator at 6-8 MA by making a 10-cm diameter circular array of 6, 9, or 12 isolated PFDs. Higher dose rates were obtained using fewer PFDs; about 1 /spl times/10/sup 12/rad/s with the 12-PFD array, 2 /spl times/10/sup 12/ with nine PFDs and 4 /spl times/10/sup 12/ with 6 PFDs. Analyses based on electrical and radiation diagnostics indicate that 40%-77% of the electrical current produces radiation at the time of maximum dose rate. The X-ray pulsewidth was typically 13-16 ns, less than the pulsewidth for standard (vacuum) bremsstrahlung diodes on Saturn. This system, with improvements in reproducibility, could provide a high dose-rate, small-area bremsstrahlung capability for high current generators.
Characterization of a self-magnetic-pinched diode Hinshelwood, D.; Cooperstein, G.; Mosher, D. ...
IEEE transactions on plasma science,
04/2005, Letnik:
33, Številka:
2
Journal Article, Conference Proceeding
Recenzirano
Self-magnetic-pinched diode behavior at 1.5-2 MV was diagnosed using a variety of electrical, radiation, and optical diagnostics. Results are compared with predictions of the LSP particle-in-cell ...code, and shown to be in good agreement. A practical diagnostic of electron incidence angles is demonstrated. A quadrature interferometer is shown to be capable of measuring the time-dependent position of the effective electrode-plasma boundaries. Both one-dimensional (1-D) and two-dimensional (2-D) interferometry show the importance of anode plasma expansion in such diodes with high anode-power concentration. Not only does the anode plasma contribute significantly to gap closure, but there is evidence that anode plasma expansion results in a distortion of the effective anode shape, which can significantly affect the diode performance.
Three-dimensional numerical simulation has been carried out to investigate the performance of a large scale disk magnetohydrodynamics (MHD) generator coupled with radio frequency (RF) electromagnetic ...field. The RF technique is verified to be useful for the improvement of performance and widening the actual operating conditions even for a large scale MHD generator. The skin effect of plasma influences the distribution of RF electric field, however, under the present plasma condition in the MHD generator, this effect will not induce the failure of the RF application. When the plasma does not achieve the fully ionized seed condition only by self-induced Joule heating, the plasma is unstable and nonuniform three-dimensionally. Coupling with the RF power can stabilize the plasma and improve the performance of MHD generator. An optimal value of the RF power under which the plasma structure becomes completely uniform in azimuthal direction, and the performance of MHD generator can be most effectively improved is evaluated. The energy cost of RF Joule heating is smaller than the power output increased by coupling with the RF power. The improvement of MHD generator performance by the RF power is efficient.
A plasma diode appears resistive to an external observer at the magnetized electron series resonance (MESR). The authors derive the MESR resonant frequencies from an equivalent circuit model using ...linear cold plasma theory. The noise current from a decaying plasma is predicted to exhibit the MESR resonant frequencies and is verified by observing the spectra of the noise currents in 1d3v simulations. The spectra compare well with the theoretical values, which depend on the (central) plasma density and the sheath thickness. The latter is obtained from the (warm) plasma simulations.
The possibility of the improvement in performance of a nonequilibrium disk magnetohydrodynamics (MHD) generator by externally applying a radio frequency (RF) electromagnetic field (RF preionization) ...was examined experimentally. The MHD power generation experiments were carried out using a shock-tube driven disk MHD generator with the RF induction coils for the two inlet stagnation temperatures of 2275(/spl plusmn/75) K and 2650(/spl plusmn/50) K. As a result, under the conditions of both high and low stagnation temperatures, the output power was increased by the RF preionization. The increase was markedly observed for the low inlet stagnation temperature, where the plasma could not be produced only by the Joule heating attributed to a self-excited electromotive force. For the high inlet stagnation temperature, the plasma state in the MHD channel was improved by the RF preionization, leading to the increase in the output power. For the low inlet stagnation temperature, the marked increase in the output power by the preionization was not only because applying the RF electromagnetic field triggered the plasma production by the Joule heating attributed to a self-excited electromotive force but also because the plasma with the high electrical conductivity was produced in the more upstream region of MHD channel by the Joule heating attributed to the RF electromagnetic field.
The results of the investigation of electrical breakdown in neon filled diode at 13.3 mbar pressure in function of distances between electrodes (0.1 < d < 20 mm), using electrical breakdown time ...delay method, are presented in this paper. The Paschen curve, as well as the electrical breakdown time delay mean values, and discharge formative time dependence of distance between electrodes are analyzed. The electrical breakdown time delay distributions and Laue diagrams, both as the functions of the distances between electrodes, were used in consideration. The shapes of the established distributions indicate the complex nature of the discharge formation mechanism. The results also indicate that on the right hand side of the Paschen curve (pd > 15 mbar · cm) the transition regime between Townsend and streamer mechanism of electrical breakdown is dominant.
