Feasibility study for Heavy Ion Beam Probing (HIBP) project on stellarator W7-X

The feasibility study for HIBP on the stellarator W7-X was done to provide the measurements of the radial profiles of plasma potential, density and their fluctuations. Calculations of probing Tl+ beam trajectories were done for previously chosen port combination A11-N11 and the various W7-X magnetic configurations with B0 = 2.5 T. They show that satisfactory access is possible for this port combinations. The trajectory optimisation aiming for the maximal plasma observation was done for chosen port combination. The initial calculation shows that HIBP allows us getting radial profiles of plasma parameters. The detector line of equal energy ETl+ = 2.0 MeV allows us to obtain a series of radial profiles for outer half of the plasma column (0.45 < ρ < 0.95) during single shot by changing of the entrance angle with the scan of the control voltage. There was found a possibility to get quite good spatial resolution 0.012<Δρ<0.015 for 2 cm beam diameter. The special beam-line for the primary particles is necessary to transfer them from the accelerator to the plasma through the area of magnetic field. The special beam-line for the Tl++ secondary particles is also necessary to transfer them from the plasma through the area of magnetic field to the ion energy analyser. The beam-lines are also plan to be used to control the beam trajectories and to drive them to energy analyzer with optimized entrance angle. It was found that toroidal focusing of the secondary trajectories will lead to construction of rather complex secondary beam-line. The size of toroidal correction plates of the secondary beam-line may be comparable to the size of the electrostatic analyzer. Such electrostatic beam control looks to be the necessary elements of the HIBP hardware for large optimised stellarators like W7-X. The beam attenuation of the primary and secondary probing beams due to collisions of the ions with plasma particles is acceptable for the measurements in all the typical magnetic configurations. The work of Kurchatov team was supported by RFBR, grant 05-02-17016. (author)