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Impact of rotation and momentum input on the L-H transition. R Sartori Thank you to the people who contributed. Outline of the talk L-H threshold and: Changes in rotation by changing NBI torque (DIII-D) Changes in rotation due to changes in SOL flow (Alcator C-Mod)

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R sartori thank you to the people who contributed

Impact of rotation and momentum input on the L-H transition

R Sartori

Thank you to the people who contributed


Outline of the talk

L-H threshold and:

  • Changes in rotation by changing NBI torque (DIII-D)

  • Changes in rotation due to changes in SOL flow (Alcator C-Mod)

  • Changes in rotation due to TF ripple (JET)

  • Changes in rotation due to changes in momentum input: NBI vs no momentum input ICH and ECH (Asdex Upgrade and JET)

  • Changes in rotation due to RMP


DIII-D: L-H threshold depends on applied beam torque and toroidal rotation

  • The L-H threshold power is reduced by a factor of 3 going from prevalent co-injectionto prevalent counter in USN (ion grad-B drift away from X-point)

  • Smaller reduction of PLH in LSN  ~80%

  • No difference between USN and LSN for counter and balanced injection

  • Reduction in edge toroidal rotation is correlated with the observed reduction in L-H threshold power

P Gohil, H-mode workshop 2007


DIII-D: L-H threshold depends on applied beam torque and toroidal rotation

The L-H transition can be induced by reduction of applied torque at constant power


The experiment

DIII-D: L-H threshold dependence on torque/rotation is independent of ion mass

The experiment

P Gohil, NF, to be published

  • The increase of PLH with torque is seen also in Hydrogen plasmas

  • In Hydrogen a factor of 2 larger threshold observed in the range of torque explored


Description of the experiment

DIII-D: Changes in rotation/torque correlate with edge turbulence dynamics

Description of the Experiment

GR McKee, IAEA 2008

D J Schlossberg, Phys. Of Plasmas, 2009

  • Changes in fluctuation level and flow pattern between co-injection and balanced injection

  • Turbulence flow reversal in balanced injection  poloidal flow shear

  • Correlation between changes in injected torque and poloidal group velocity of turbulence near edge

  • spectra obtained in 100 ms before L-H. Phase shift of poloidally displaced channels Z=1.2cm.


Description of the experiment1

DIII-D: Increased Er shear at lower rotation turbulence dynamics

Description of the Experiment

  • Interplay between pressure gradient and v X B terms in Er:

  • lower contribution of toroidal rotation term  increase in Er shear at the edge de-correlation and suppression of turbulence .


Alcator C-Mod: Changes in L-H threshold correlated with changes in SOL flows/rotation

From H Hubbard, EPS 2004

  • Changes in core rotation correlated with changes of SOL flows (largest in inner SOL)

  • PLH two times higher in USN (ion B drift away from X-point )

  • Strong parallel flows are driven in the inner SOL, co-current for LSN, counter current for USN

  • At the L-H threshold flows and rotation near a constant value, independent of configuration  strong suggestion that SOL flows are causing differences in PLH with configuration


Comparison of density and temperature and toroidal rotation profiles

JET: Variation of toroidal rotation due to TF ripple changes changes in SOL flows/rotation

Comparison of density and temperature and toroidal rotation profiles

  • Lower rotation with increasing ripple. Edge rotation becomes negative at high ripple

  • ASCOT calculation  that dominant mechanism for reduction of rotation is banana orbit diffusion  induces a radial return current  JXB torque in counter current direction. For ripple of 0.5% this torque was found to be 20-30% of that supplied by the NB system

1.5% ripple: L-modes with low level of beam power

P De Vries et al, NF 48 2008


Comparison of power and torque density

JET: L-H threshold is not affected by large variation in toroidal rotation due to TF ripple

Comparison of power and torque density

  • 0.08% ripple  v in co-current direction

  • 1 % ripple  v in counter-current direction

  • Similar Te and Ti the transition

  • v unaffected by ripple

Y Andrew et al, PPCF 50 2008


Global and pedestal confinement is similar

Asdex Upgrade: L-H threshold is independent of heating method momentum input/rotation

Global and pedestal confinement is similar

  • No difference in L-H power threshold and in the density dependence of PLH between ICH and NBI, for both D and He plasmas

F Ryter et al, NF 49 2009


JET: L-H threshold is independent of heating method method momentum input/rotation

  • JET results shows that L-H access is independent of heating method, as seen previously in JET for all H isotopes

    • New data confirms the results and shows typical changes from ~6-8 kHz (NBI+ICH) down to ~2.5KHz (ICH) at 3T with similar threshold

Data from 1MA/1T to 3.8MA/3.8T

LH scaling from Righi et al NF 39, 1999

Andrew et al, PPCF 48 2006


Effect of RMPs on the L-H transition method

Summary from Carlos Hidalgo- H-mode workshop talk

JET

No effect observed (2009 experimental campaign)

MAST

On MAST there is observation of the delay of the L-H transition time if the n=1 coils are applied before the L-H transition but not with n=3.

DIII-D

In Helium plasmas PL-H increased by >50% in the presence of n=3 RMP perturbations.  

NSTX

PL-H increases from ~1.4 to 2.6 MW with higher n=3 current (~65% increase for Pheat/ne)


Summary and future work
Summary and future work method

NXTS and JET results on L-H threshold with RMP

  • PLH increases from ~1.4 to 2.6 MW with higher n=3 current

  • PLH unaffected by application of RMP if effect exists it depends on the strenght of the perturbation

  • 30% decrease in rotation


Summary method

  • Evidence from DIII-D of strong effect of beam torque on L-H threshold

    • The effect persist when mass (and X-point height) are changed

    • For counter-injection, it reduces differences between favourable and unfavourable grad-B direction

  • The effect is less strong in favourable grad-B direction (why?), therefore effect on threshold scaling not as large as largest reported variations

  • When rotation is changed by changing heating methods and ripple the L-H threshold is unchanged, with RMP it increases or is unvaried  suggest weaker link with rotation in DIII-D results


Comparison with same density and total power

JET data : LSN configuration and reversed B method t and Ip

Comparison with same density and total power

  • Reversed Bt direction ion B drift away from X-point and counter injection

  • Forward Bt direction ion B drift towards X-point and co injection

  • No difference between L-H power threshold with ion B drift direction in the JET data.


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