The interaction between the hydrogen plasma and the divertor wall is yet an unresolved issue in the design of ITER. Especially the erosion rates and retention of tritium (the fuel of a fusion reactor) presently foreseen for the ITER divertor are critical issues for its prolonged operation. The large linear plasma generator Magnum-PSI, is presently being build at our institute to study the underlying processes. This requires intense hydrogen plasma jets to mimic the plasma conditions of the ITER divertor. The research described in this thesis was carried out at Pilot-PSI, the fore- runner of Magnum-PSI, and focused on the e±cient production of such intense hydrogen plasma jets with a wall-stabilized cascaded arc operated in a strong magnetic ¯eld. The cascaded arc that was used here operates at a relatively high pressure (» 0:1 bar). Coupled to a vacuum vessel, the hydrogen plasma is observed to ex- pand in a way similar to gas expansion. The investigations start with Langmuir probe measurements to determine the plasma density and temperature under these conditions. The results con¯rmed the importance of molecular assisted recombination. The performance of the arc operating on hydrogen was characterized and compared with argon operation by power measurements. An important aspect observed in these measurements is a decreasing discharge voltage for increasing discharge currents for hydrogen operation. This is a consequence of an increasing e®ective plasma channel with increasing power input resulting in a decreased average resistivity. Such an IV-characteristic is not observed for argon. Results on argon for di®erent diameters of the discharge channel demonstrate that the resistivity of the arc (i.e. its resistance divided by the area of the channel cross section and the channel length) scales predominantly with the discharge current density as ´ / ¹j¡0:6 and is independent on the diameter. For hydrogen, this relation is ´ /¹j¡1:3. The independence on the channel diameter is not entirely clear for hydrogen and at most true for channels wider than 4 mm. A model is developed from a power balance for the discharge channel. It explains the experimental data on the basis of the higher heat conduction of hydrogen, which leads to a smaller hot plasma channel compared to argon. The model predicts higher e±ciencies if higher discharge currents are combined with larger channel diameters. Pilot-PSI o®ers magnetic ¯elds up to 1.6 T, which is unique for a linear plasma generator. This con¯nes the otherwise expanding plasma into an intense jet and reduces the recombination losses that are typical for hydrogen plasma. Thomson scattering was applied to determine the record plasma parameters, unprecedented in a linear plasma generator: an electron density ne = 7 ¢ 1020 m¡3 and temperature Te = 2 eV (at B=1.6T). The average forward velocity of the plasma was determined from the Doppler shift in the light emission and amounts typically » 3 km/s at the position of the Thomson scattering experiment. Together with the measured electron density, this yields a proton °ux density that is expected in the ITER divertor: ¡H+ = 2 ¢ 1024m¡2s¡1 The magnetic ¯eld also e®ects the operation of the source. This is concluded from the signi¯cant increase of the potential di®erence at the exit of the source induced by the magnetic ¯eld. The additional potential is dependent on the inner diameter of the nozzle (with respect to the discharge channel) and amounts up to 90 V for an 8 mm nozzle at B=1.6 T. The potential increase from a wider nozzle causes a signi¯cantly improved source output, up to a factor of 2, as was quanti¯ed by Thomson scattering. The e®ect is explained in a physical picture where a signi¯cant part of the discharge current continues outside the source before it attaches to the nozzle. The consequence of current continuing into the free jet is that a large poten- tial is built up, which gives rise to an appreciable radial electric ¯eld. The radial electric ¯eld is perpendicular to the axial magnetic ¯eld and causes strong ro- tation of the jet via E£B drift of the plasma particles. High-resolution optical emission spectroscopy (HiRES) was performed to investigate this rotation from the Doppler shift in the atomic light emitted perpendicular to the plasma jet. The measured line shapes were asymmetric, which was explained by the ex- istence of two populations in the radiating atoms. One is coupled to rotating ions and has the ion temperature and velocity. The other is coupled to colder background gas and rotates at most slightly. This picture was implemented in a ¯tting procedure that yields the ion temperature and rotation velocity, the back- ground gas temperature and rotation velocity, and the electron density. In this way, peak rotation velocities up to 104 m/s were determined, probably limited by ion-neutral friction to below the thermal velocity. These rotation velocities correspond to electric ¯elds larger than 104 V/m. The rotation frequency of the central part of the plasma jet was observed to scale with the potential di®erence between the last plate and the nozzle, which con¯rms the physical picture on currents continuing beyond the source. The ion temperature Ti that followed from the ¯tting procedure was found to be systematically larger than Te. Al- though the accuracy in the temperature determination is expected to be too limited to quantify the ratio Ti=Te, we do conclude that it is larger than unity. This is in line with additional viscous heating of the ions due to the rotation of the jet. On the basis of the results presented in this thesis we conclude that the cascaded arc can serve as the basis for a future Magnum-PSI source. Scaling of the arc will be based on an enlargement of the discharge channel diameter (to » 10 mm) in combination with an increased discharge current (to 1 ¡ 2 kA).
|Qualification||Doctor of Philosophy|
|Award date||12 Jun 2006|
|Place of Publication||Eindhoven|
|Publication status||Published - 2006|