beam ripple minimization: influence of plasma instability
TRANSCRIPT
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.
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”
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
B
Loss fluxes
e
Insulator embedded the walls
-
+ + +
+ +
+
++
H
H
e
-
-
-
--
-
-
-
e
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
0 200 400 600 800 1000 1200 14000
10
20
30
40
50
Microwave power [W]
Ext
ract
ed
cu
rre
nt [
mA
]
Pressure= 2 10-5mbar
No BN in ext. sideBNAl
2O
3 and BN
0 200 400 600 800 1000 1200 14000
20
40
60
80
100
Microwave power [W]P
roto
n fr
act
ion
[%]
Pressure= 2 10-5mbar
No BN in ext. sideBNAl
2O
3 and BN
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.