Beam ripple minimization: influence of plasma instability

PS-ESS flexible magnetic system The magnetic system was thought as a flexible structure to explore different solutions in terms of plasma heating efficiency and beam quality optimization.

(1)Standard MDIS design  slightly overdense plasma at moderate RF power (2)Magnetic beach  suitable for under-resonance generation of EBW, high densities with low RF power levels BUT…. Beam emittance could be critical!! (3)Simple mirror: it should ensure the maximum proton fraction and also beam ripple solution. Never attempted (as 2), it will be specifically employed to moderate plasma “effervescence” (1)Standard MDIS design  slightly overdense plasma at moderate RF power (2)Magnetic beach  suitable for under-resonance generation of EBW, high densities with low RF power levels BUT…. Beam emittance could be critical!! (3)Simple mirror: it should ensure the maximum proton fraction and also beam ripple solution. Never attempted (as 2), it will be specifically employed to moderate plasma “effervescence”

Sources of beam ripple… Extrinsic causes: fluctuations in the pumping rate, unstable input gas flux, unstable extraction voltage, ripple in the mw generator Intrinsic causes: plasma breathing, ion waves formation, plasma sheath fluctuations, enhanced ion heating

Main causes of plasma instability (i.e. beam ripple) 1)Plasma effervescence (due to the unconfined high density plasma) 2) High plasma potential values (due to strong particle loss fluxes) 3)Formation of ion sound waves Strategies to reduce instability 1)Use of insulators on the walls of the plasma chamber (reduction of the plasma potential, reduction of loss rates) 2)Mixing of gases Causes and solutions

Modifying plasma diffusion process When the isotropic diffusion dominates, ions and electrons loss fluxes compensate locally But for a magnetized plasma in a chamber with conducting walls, anisotropic diffusion arises

Allumina Al 2 O 3 Boron nitride BN Cylinders and/or disks of insulating materials

Loss fluxes Insulator embedded the walls Influence of Allumina and BN to VIS performances The stop to Simon fluxes (due to charged insulators on the walls) reduce the plasma losses, then plasma oscillations and finally beam ripple Ion fluxes are reduced due to breakdown of Simon current fluxes along the chamber walls

Influence of Allumina and BN to VIS performances Stability and beam ripple reduction: insulators improve beam stability (ripple reduced from 5% to <1.5%) and allow operations on a wider combination of pressure and RF power

Ion waves measurements Formation of ion waves seems to be linked to the B-profile and RF power level Preliminary studies already carried out: strategy for their damping could be gas mixing.

Ion waves damping Ion Sound waves in a Deuterium plasma with Ar supply P=3.8E-4 mbar Damping of ion sound waves after tuning of the background pressure (fluxing Ar) P=4.5E-4 mbar