Summary:This work describes an experimental study of the backscattering of electron beams with an energy range between 20 and 100 keV. The energy distribution of electrons backscattered by bulk gold and silicon samples at different take-off angles was determined with an original device. The different major sources of noise were studied and then the spectra obtained were processed. Theoretical interpretation of this phenomenon using Monte Carlo simulations will be presented in a future paper.
A summary of the second harmonic ECRH breakdown experiments performed in the TJ-II stellarator is given here. The goal of these experiments is to determine the dependence of second harmonic X-mode plasma breakdown efficiency on several relevant parameters as the EC power, wave polarization, beam injection angle, tokamak-like induced electric field, neutral gas pre-fill pressure and discharge creation time with respect to the ramp-up of the currents in the coils. For each different scenario, the breakdown properties are determined, as well as the initial pressure conditions and the level of initial radiation before the injection of the ECRH power, which plays an essential role in the breakdown process.
A study of the ionization dynamics in the dark space of a stationary cylindrical hollow-cathode discharge has been carried out. In a first-order approximation for the current densities, theoretical expressions have been obtained for the electric field and potential distributions. The dependence of the discharge current density on the forwards voltage drop, argon being the used gas, has been measured under the following experimental conditions: pressure 10-200 Pa, cathode radius 3-6 mm and current density 20-500 A . The thickness of the dark space has been determined from theoretical expressions and by a photographic method, allowing us to discuss the ratio between the effective number of ionizations in the dark space and their total number. Under our experimental conditions, most of the ionizations are produced in the dark space. In this case, the dependence of the electric field on the cathode distance is nearly linear.
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