RFX-mod
Programme Planning Workshop, 9 Feb 2011
Influences of main gas choice on RFP plasmas
M Valisa with contributions of F Auriemma ad M Spolaore
RFX Programme Meeting
Feb-2010
Motivation
• In Tokamaks various beneficial isotope effects (IE)
and also certain advantages using He
• In RFP some isotopic/mass effect have also been documented.
Can we conceive experiments to improve the RFP performance
based on our knowledge on IE /mass effects?
RFX Programme Meeting
Feb-2010
Note
This is a collection of thoughts and results
that is far from being complete
Rather, this is an opportunity to trigger once
more the discussion in RFP’s
The isotope effect has been and still is an important subject in
Tokamak and Stellarator research and a vast literature exists
on the subject.
RFX Programme Meeting
Feb-2010
Outline
- Confinement / Heating / Turbulence
- LH transition
- QSH physics
- Density Limit
- Plasma wall interaction
RFX Programme Meeting
Feb-2010
Confinement / Turbulence
RFX Programme Meeting
Feb-2010
IE in Tokamaks: Confinement improvement
In JET Te and Ti at the top of the edge pedestal
JET
increase systematically as the isotope mass in increased
(J. G. Cordey, Report No. JET-P (98)53, 1998).
-With type I ELMs, the edge pedestal
- pressure increases with the
ion mass of the plasma
( Saibene NF 39 (1999) ,1133).
RFX Programme Meeting
Feb-2010
E. Righi et al NF 39 (1999) 309
IE in Tokamaks: ELM’s
E. Righi et al NF 39 (1999) 309
JET
Isotope Effect on ELM frequency
RFX Programme Meeting
Feb-2010
IE in Tokamaks: Confinement improvement
In AUG “deuterium plasmas always surpass hydrogen plasmas in all
respects: in energy, particle and momentum confinement. “
BESSENRODT-WEBERPALS WAGNER et al. NF 33 (1993) 1205
With D :
- more peaked and Te profiles
- higher Te
- more impurity production = more edge
radiation
- higher momentum confinement
- easier L-H transition
In He 20-30% lower confinement wrt D
(because of lower density due to Z=2).
RFX Programme Meeting
Feb-2010
AUG
IE in Tokamaks: Confinement improvement
 Asdex
AUG
:
BESSENRODT-WEBERPALS et al. NF 33 (1993) 1205
RFX Programme Meeting
Feb-2010
IE in Tokamaks: Confinement improvement
TFTR Global tE thermal Increased, cI Decreased in Core of
c toi t
(including conv ection) (m
2 / s)
DT Supershots Compared to D (R. Haryluk,, 2007)
7
D
D-T
1
factor of 2
P NBI = 18MW
I p= 1.6MA
0.1
0.0
0.5
Minor Radius / a
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Feb-2010
S. Scott, M. Zarnstorff
Fluctuations n TJ-I Tokamak
Fluctuations from Hard X-Rays
hygher in Hydrogen
H
runaway electron kinetic effects
induced by magnetic turbulence
D
H
D
L Rodrıguez-Rodrigo and F Castejon
Plasma Phys. Control. Fusion 38 (1996) 681
RFX Programme Meeting
Feb-2010
IE and Zonal Flows
Why confinement improves with deuterium not yet fully clarified.
ZF= self-generated shear flows out of a turbulent sea
In helical plasmas, an equilibrium-scale Er ( driving poloidal rotation
of trapped particles= reduced radial displacement of orbits)
enhances the ZF response.
An isotope effect occurs if Ti and Er are the same for all the different ion masses.
(This happens to be the case for neoclassical transport in helical plasmas).
Hence neoclasical transport generates an equiibrium Er, which enhances ZF
response that is larger for D than for H.
T H Watanabe et al FEC Daejaeon 2010 THC/6-1
RFX Programme Meeting
Feb-2010
RFX ( MH regime) H vs He: effect on transport:
•Lorenzini, Auriemma, Canton, Carraro PoP 2006
Particle diffusivity at the edge
• At the edge, in high collisionality
regimes, ( transport driven by
electrostatic fluctuations) : DH/DHe~2
• Effect disappears when n < 61019 m−3
H
RR and Harvey theory:
RFX
Hedensity profile
•Evidences of stochastic transport in the
plasma core: isotope effect fits with RR&H
theory
RFX Programme Meeting
Feb-2010
RFX: H and He: effects on the egde momentum
Different flow profile
at the edge.
With He less shear at the edge.
Spolaore et al JNM 2001
RFX Programme Meeting
Feb-2010
Fast ion confinement in RFP (MST)
In RFP: weak Impact of a Stochastic Magnetic Field on
Fast-Ion Confinement Fiksel et al PRL 2005
Magnetic field perturbations and passing fast ions can satisfy
resonance conditions unlike thermal particles Gobbin et al Nucl.
Fusion 48 (2008)
20 ms
1ms
Possibility of exploiting neutron diagnostics in D plasmas
Even with small D2 doping Diagnostic NBI would help.
RFX Programme Meeting
Feb-2010
RFP’s Mass effect on rotation
MST : momentum confinement time somewhat larger in D2
plasma.- based on a beam-based measurement
Customary explanation: the lower central neutral density in D2
plasmas results in lower drag on the plasma rotation and on
reduced loss of momentum due to CX.
- B.E. Chapman et al., Phys. Plasmas, Vol. 11, p. 2169 (2004).
MST : in He discharges plasma and modes rotate faster than in D2
( D Den Hartog pc – only few He experiments due to high
sputtering)
This effect was searched in RFX but never replicated (neither in
RFX nor in RFX-mod.
Possible explanation. In RFX mod modes rotation is controlled by
feedback system
RFX Programme Meeting
Feb-2010
Ion heating in MST
Fiksel et al PRL 103, 145002 (2009)
Non collisional ion heating increases approximately as the square root of the ion mass.
A simple model based on stochastic ion heating consistent with data:
A large cross-field radial transport of ions – caused by magnetic stochasticity through a strong fluctuating radial electric field causes fast random changes
in the ion perpendicular EXB drift velocity, which results in perpendicular
heating.
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Feb-2010
QSH physics
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Feb-2010
RFX-mod QSH and plasma mass/charge
No major differences in QSH persistency or duration in H and He plasmas
H and He
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Feb-2010
RFX-mod QSH and plasma mass/charge
Neither with a selection in current
H and He
Selezionati F e Ip
RFX Programme Meeting
Feb-2010
RFX-mod QSH and plasma mass/charge
Nor with a selection in n/nG
Is the QSH physics dominated by electron channel ??
Selezionati F e n/ngw
RFX Programme Meeting
Feb-2010
H and He
LH Transition in Tokamak
RFX Programme Meeting
Feb-2010
IE in Tokamaks: L-H power theshold
Complex dependence of LH transition on main plasma ions , but in general
PTh_H> PTh_He> PTh_D and PTh has a minimum when plotted vs density

DIII-D
He plasmas have
significantly higher P-TH
than D at low densities for
all heating schemes,
but similar P-TH as D at
high densities except for Hneutral beam injected
heated discharges, which
are still higher.
Gohil FEC 2010 Daejeon
RFX Programme Meeting
Feb-2010
IE in Tokamaks: L-H power theshold
NSTX
 NSTX
the L-H threshold power for He
is 20 to 40% greater than that for D
Li wall conditioning can ease access to the
H-mode significantly.
Potential complication with utilizing an
ELM suppression system which applies
low-n magnetic perturbations to the plasma
edge in a
preventative mode prior to the transition,
as this can result in at least a 50%
increase in P-Th
Kaye FEC 2010 EXC/2-3Rb
RFX Programme Meeting
Feb-2010
IE in Tokamaks: L-H power theshold
JET
RFX Programme Meeting
Feb-2010
Density Limit
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Feb-2010
Density limit in Tokamak
Greenwald PPCF 2002
ASDEX, NSTX and other machines have reported significantly
higher density limits for He discharges compared to D,
particularly at low current
The hydrogen ion isotope is not
found to be particularly important
for the density limit.
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Feb-2010
Density limit in RFX, higher in He than in H
H
H + pellet
He
In MST: limit slightly overcome in D. No experience with He. (B Chapman, p.c.)
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Feb-2010
Plasma Wall Interaction
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Feb-2010
PWI: erosion yield
Very complex processes that depend
on ion energies, experiment history
Wall materials, deposited layers etc.
Deuterium produces larger physical
Sputtering
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Feb-2010
PWI : H vs He
For low energies ( <200 eV) Strong Chemical erosion for H
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Feb-2010
PWI : H vs He
H induced erosion mainly chemical ;
DEPENDS ON SURFACE TEMPERATURE
He induced erosion is physical
DOES NOT DEPEND ON SURFACE TEMP.
He plasmas more reproducible
RFX Programme Meeting
Feb-2010
Edge gas pressure
In NSTX lower edge gas pressue with Li
Conditioning
Gas pressure dependence
on mass of main fuel?
If so . Consequences on
- Momentum losses
- Energy losses
- Beam reionization before
beam enters the plasma
RFX Programme Meeting
Feb-2010
Summary
Deuterium features
- higher confinement ( particle, energy and momentum) in Tok
- favourable PTh for L-H transition in Tok
- also higher sputtering ,but associated to higher radiation
- higher Ti than H after reconnection events
- Extended diagnostic capabilities ( neutrons)
- Possible IE on Zonal Flows and turbulence regulation in helical plasmas
He
-more favourable PTh for L-H transition than H in Tok
-- better reproducibility
Shoudl RFX_TOK work in He to search for H-mode?
- lower confinement
in RFX , apparently, no major impact on QSH physics
Effects on mode control and RMP (through viscosity) ?
RFX Programme Meeting
Feb-2010