Impact of Nonlocal Electron Heat Transport on the High Temperature Plasmas of LHD

Impact of Nonlocal Electron Heat Transport on the High

Temperature Plasmas of LHDN. Tamura, S. Inagaki, T. Tokuzawa, K. Tanaka, C. Michael,

T. Shimozuma,S. Kubo, K. Ida, K. Itoh, D. Kalinina, S. Sudo,Y. Nagayama, K. Kawahata, A. Komori and LHD team

NIFS, National Institutes of Natural Sciences, JAPAN

21st IAEA Fusion Energy ConferenceChengdu, China

16-21 October 2006

2/10N. Tamura, FEC2006, EX/5-6, Oct. 19, 2006

Full understanding of electron heat transport is necessaryfor achieving a good predictive capability

Experiments on toroidal plasmas shows nonlocality qe(1) = f(Te(1), Te(1-), Te(1+), …, Te(1), Te(1-), …)in electron heat transport

Phase inversion ofcold pulse polarityProfile resilience Fast plasma response

(non-diffusive, ballistic)

Possible theoretical interpretation is “nonlocality” in turbulence (e.g. turbulence spreading)

Observations in LHD heliotron give new insight into nonlocal transport Because LHD has

– different magnetic configuration (normally negative magnetic shear)

– no tokamak-like stiffness in Te profile

Phase inversion ofcold pulse polarity

Recently observedin LHD

Electron heat transport can be explained only by local transport? local transport assumption:

qe(1) = f(Te(1), Te(1), ne(1), ne(1), …)


Significant rise of core Te in response to edge cooling in LHD

Edge cooling experiment of LHD showsa significant rise of core Te

No change in low-m MHD modesNo density peaking like PEP, RI-

mode

Electron heating dominates (Te/Ti > 1)

Difference between Te measured and that simulated based on simple diffusion model is pronounced in the core ( < 0.6)little at the edge ( > 0.6)


Characteristics of Nonlocal Te rise in LHD plasmas1) Condition for nonlocal Te rise

Inverse relationship between increment of core Te due to nonlocal effect and ne observedSame as in tokamaksIn LHD, no differences among

heating methods(but always electron heating)

In the LHD, the nonlocal Te rise observed in various plasmas:ECH plasma (i.e. net-current free plasma)

Toroidal plasma current and high-energy ion are not a factorNBI plasma (still Te/Ti > 1)

High-energy electron is also not a factor


Characteristics of Nonlocal Te rise in LHD plasmas2) Variety of time response

Larger dTe/dt Stronger edge cooling

simultaneity

Te rise delayed

ne increases


Characteristics of Nonlocal Te rise in LHD plasmas3) Dependence of delay time Favorable condition for delay of nonlocal Te rise

Higher collisionality b* in the coreShorter Te gradient scale length at the edge


Transient response analysis reveals complex relationshipbetween heat flux and Te gradient

Reduction of qe/ne is not accompanied by changes in local TeEvidence against “standard transport theory” (local & diffusive)

Turn-back of qe/ne is also independent of local Te

r

eee d

t

trTn

rtrq

0

),(

2

31 ),( Heat flux perturbation

nonlocal

local nonlocal


Reduction of normalized heat flux due to nonlocal effect takes place in a wider region

Region where reduction of qe/ne prominently appear is far from rapidly cooled region and strongly heated region


How can we understand nonlocal Te rise in LHD?

Clue from LHD experiment

Physical mechanism of long-range correlation is unclear It should have characteristics as follows:

Response delayed with higher b* & shorter Te gradient scale length

Radial extent close to a

1. Nonlocality in e-transport revealed by edge cooling

nonlocal

2. Transitions between “nonlocal” and “local” in e-transport also revealed

local nonlocal

Long-range

Short-range

Long-range

Short-range

Long-range

Short-range

rati

o

On a basis of radial correlation


Summary

Nonlocal Te rise invoked by edge cooling observed in low density and high temperature plasmas of LHD as well as tokamaks

New aspects of nonlocal Te rise from LHDObservation in net-current free plasmaTime response of nonlocal Te rise is variable

Time response of core Te rise is quicken by larger edge perturbation (larger Te gradient scale length?)

Delay of nonlocal Te rise increased with…increase in collisionality in the coredecrease in Te gradient scale length at the edge

Transient response analysis suggestscomplex relationship between flux and gradienttransitions between “nonlocal” and “local”


END


No relationship between density fluctuations and

nonlocal Te rise is observed in LHD


No relationship between density fluctuations andnonlocal Te rise observed in LHD

No significant change in core density fluctuation (measured by X-mode Reflectometer) observed after the onset of core Te rise


Comparison with a linear gyro-kinetic

calculation


Comparison with a linear gyro-kinetic calculation

To esimate growth rate ITG and real frequency r(ITG) of ITG/TEM modes, GOBLIN (GyrOkinetic Ballooning LINnear equation solver) code usedComparison is done for two time slices

1.38s (Before TESPEL injection) 1.40s (Just before nonlocal Te rise)

Profiles of ne, Te, Ti as input parametersne: increase outside ~ 0.6Te: almost not changedTi: Profile shape of Ti is assumed to be sam

e as that of Te (only Ti0 is measured)

In the code, collisionless approximation is usedStabilization of TEM modes due to the incre

ase of collisionality* cannot be expressed by the GOBLIN code*F. Ryter et al., PRL 95, 085001(2005)


Linear gyro-kinetic calculation shows TEM turbulencedominant at the periphery of the plasma with nonlocal Te rise

ki is fixed at 0.5

Calculation resultsITG/TEM is unstable

outside ~ 0.2TEM-driven component

dominant outside ~ 0.5

Experimental resultsMaximum change of normalized

heat flux takes place around ~ 0.4


Characteristics ofe-transport in LHDw/o nonlocal Te rise


A gyro-Bohm like dependence can explain low-power NBIheated plasmas in LHD

Critical gradient scale length is unclear A gyro-Bohm like dependence of e on Te observed tr PB observed e weakly depends on Te


Difference between Nonlocal Te rise

& CERC


Nonlocal Te rise and CERC coexist

Nonlocal Te rise in CERC plasma Feature of CERC

Strongly peaked Te profile

Associated withEr bifurcation

Antithetical feature ofnonlocal Te riseHeat flux jump takes

placein a wider region of plasma

NOT associated withEr bifurcation


Transient response analysis for

stronger edge cooling


Transient response analysis suggests heat flux jump rateincreased with stronger edge cooling

Larger dTe/dt Stronger edge cooling

Heat flux jump rate increased


Experimentally observed Interaction

betweenthe core and the edge


Transient response analysis indicates existence ofinteraction between the core and the edge

qe/ne with > 0.4 decreases prior to reduction of that with < 0.4 (A)

Termination of decrease in qe/ne seems to propagate from core to edge (B)Decrease in qe/ne with

> 0.2 seems to be overshot, and goes back to a metastable level

Turn-back of qe/ne to pre-injection level is started almost simultaneously, seems to propagate from edge to core (C)

Documents

Impact of Nonlocal Electron Heat Transport on the High Temperature Plasmas of LHD