rfx programme meeting feb-2010 influences of main gas choice on rfp plasmas rfx-mod programme...

RFX Programme Meeting Feb-2010

Influences of main gas choice on RFP plasmas

RFX-mod Programme Planning Workshop, 9 Feb 2011

M Valisa with contributions of F Auriemma ad M Spolaore


• 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?

Motivation


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 existson the subject.

Note


- Confinement / Heating / Turbulence - LH transition

- QSH physics

- Density Limit - Plasma wall interaction

Outline


Confinement / Turbulence


In JET Te and Ti at the top of the edge pedestalincrease 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).

IE in Tokamaks: Confinement improvement

E. Righi et al NF 39 (1999) 309

JET


IE in Tokamaks: ELM’s

E. Righi et al NF 39 (1999) 309

Isotope Effect on ELM frequency

JET


In AUG “deuterium plasmas always surpass hydrogen plasmas in allrespects: 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).


AUG


Asdex :


BESSENRODT-WEBERPALS et al. NF 33 (1993) 1205

AUG


TFTR Global E thermal Increased, I Decreased in Core of DT Supershots Compared to D (R. Haryluk,, 2007)


(in

clu

din

g c

on

vect

ion

) (

m2/s

)

tot i

7

1

0.1

D D-T

0.0 0.5

factor of 2

Minor Radius / a

PNBI = 18MW I p= 1.6MA

S. Scott, M. Zarnstorff


L Rodrıguez-Rodrigo and F CastejonPlasma Phys. Control. Fusion 38 (1996) 681

Fluctuations n TJ-I Tokamak

Fluctuations from Hard X-Rays hygher in Hydrogen

runaway electron kinetic effectsinduced by magnetic turbulence

H

D

H

D


IE and Zonal Flows

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 ZFresponse that is larger for D than for H.

Why confinement improves with deuterium not yet fully clarified.

T H Watanabe et al FEC Daejaeon 2010 THC/6-1


RFX density profileRR and Harvey theory:

Particle diffusivity at the edge

•Evidences of stochastic transport in the plasma core: isotope effect fits with RR&H theory

RFX ( MH regime) H vs He: effect on transport:

• At the edge, in high collisionality regimes, ( transport driven by electrostatic fluctuations) : DH/DHe~2

• Effect disappears when n < 61019 m−3 H

He

•Lorenzini, Auriemma, Canton, Carraro PoP 2006


RFX: H and He: effects on the egde momentum

Spolaore et al JNM 2001

Different flow profileat the edge. With He less shear at the edge.


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)

Fast ion confinement in RFP (MST)

Possibility of exploiting neutron diagnostics in D plasmas Even with small D2 doping Diagnostic NBI would help.

1ms

20 ms

Isot

opic

effec

ts??


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

RFP’s Mass effect on rotation


Fiksel et al PRL 103, 145002 (2009)

Ion heating in MST

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 perpendicularheating.


QSH physics


H and He

RFX-mod QSH and plasma mass/charge No major differences in QSH persistency or duration in H and He plasmas


Selezionati F e Ip

H and HeNeither with a selection in current

RFX-mod QSH and plasma mass/charge


Selezionati F e n/ngw

H and HeNor with a selection in n/nG

RFX-mod QSH and plasma mass/charge

Is the QSH physics dominated by electron channel ??


LH Transition in Tokamak


IE in Tokamaks: L-H power theshold

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 H-neutral beam injected heated discharges, which are still higher. Gohil FEC 2010 Daejeon

DIII-D

Complex dependence of LH transition on main plasma ions , but in generalPTh_H> PTh_He> PTh_D and PTh has a minimum when plotted vs density


NSTX


NSTX the L-H threshold power for Heis 20 to 40% greater than that for D

Li wall conditioning can ease access to theH-mode significantly.

Potential complication with utilizing anELM suppression system which applies low-n magnetic perturbations to the plasma edge in apreventative mode prior to the transition, as this can result in at least a 50% increase in P-Th

Kaye FEC 2010 EXC/2-3Rb



JET


Density Limit


Greenwald PPCF 2002ASDEX, 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 notfound to be particularly important for the density limit.

Density limit in Tokamak


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.)


Plasma Wall Interaction


PWI: erosion yield

Very complex processes that dependon ion energies, experiment history Wall materials, deposited layers etc.

Deuterium produces larger physical Sputtering


PWI : H vs He

For low energies ( <200 eV) Strong Chemical erosion for H


PWI : H vs He

H induced erosion mainly chemical ; DEPENDS ON SURFACE TEMPERATURE

He induced erosion is physicalDOES NOT DEPEND ON SURFACE TEMP.

He plasmas more reproducible


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


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 influences of main gas choice on rfp plasmas rfx-mod programme...

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