Heat Transport and Plasma Rotation during Cold Pulse Experiments in Ohmic L-mode Plasmas

Abstract

C.Gao1, J.E.Rice1, H.J. Sun2,3, M.L.Reinke1, N.T.Howard1, Y.A.Podpaly1,4, L.Delgado-Aparicio5, A.E.Hubbard1,

M.Chilenski1, J.W.Hughes1, J.Walk1, Y.Ma1, M.Greenwald1, N.Tsujii1, P.Ennever1, M.Porkolab1, W.Rowan1 and Alcator C-Mod1

1MIT Plasma Science and Fusion Center 2WCI Center for Fusion Theory, NFRI, Daejeon, Korea 3SWIP, Chengdu, China 4NIST EBIT facility 5Princeton Plasma Physics Laboratory

Cold pulse experiments

Thermal transport is studied by laser blow-off impurity (CaF2) injection.

The non-local effect, a cooling of the edge electron temperature with a

rapid rise of the central electron temperature, which contradicts the

“local” assumption of transport, was observed in low density linear

Ohmic confinement (LOC) regime plasmas. Simulation shows this

phenomenon can be explained by an inward heat convection term in

the heat flux equation. In high density saturated Ohmic confinement

(SOC) regime plasmas, the thermal transport becomes “local”: central

electron temperature drops in response to the edge cooling. The

transition density is very close to the rotation reversal critical density.

This indicates the possible correlation between thermal and momentum

transport, which is also linked to the trapped electron mode (TEM) to

ion temperature gradient mode (ITG) transition. TRANSP analysis

shows the heat transport in the electron channel is reduced and heat

transport in the ion channel is enhanced when the plasma switches

from the LOC to the SOC regime.

Phase of VΦ, P and P

Ti and rotation measurement on C-Mod

High Resolution X-ray spectrometer with Spatial Resolution (HiReX-Sr) [2] is used for ion temperature and rotation profile measurement at

Alcator C-Mod. The measurement uses spherically bent crystal to

provide spatial resolution of argon and calcium’s helium-like and

hydrogen-like spectra.

At Alcator C-Mod, it is shown that there is a strong connection between

the LOC-SOC transition and the abrupt direction switch of core rotation

(rotation reversal) [1]. The transition/reversal happens at a critical

density, above which the transport is dominated by ITG modes, and

below which the transport is dominated by TEMs. Recently cold pulse

experiments added the non-local transport effect to this connection.

(a) HiReX-Sr spectrometer

(b) Spectrometer’s optical components

(c) Helium-like argon spectrum

(d) Profiles of impurity emissivity, temperature and toroidal rotation calculated by THACO[3] .

(a)

(b)

(c)

(d)

Cold pulse modulation

Conclusions and future work

References

[1] J.E.Rice et al., 2011 Nucl. Fusion 51 083005.

[2] A.Ince-Cushman. PhD Thesis. (2008)

[3] M.L.Reinke et al,. Accepted by Rev. Sci. Instrum (2012)

[4] J.E.Rice et al., Submitted to Nuclear Fusion (2012)

[5] Jacchia A. and Mantica P. 1991 Phys. Fluids B 3 3033

Motivation

This work is supported by USDoE award DE-FC02-99ER54512

LOC-SOC, nonlocal-local and rotation transitions happens at similar

density.

LOC: Ip = 0.8MA, ne20 = 0.55 SOC: Ip = 0.8MA, ne20 = 1.3

Time traces of line averaged electron density (TCI), core and edge

electron temperatures (ECE), core and edge ion temperatures (HiReX-

Sr). Dotted vertical line indicates time of LBO injection.

l Confinement time, nonlocality and rotation reversal

Heat pinch in nonlocal transport

e e e e e eq r n T V r n T

For LOC plasma, the nonlocal effect can be explained by adding an

inward pinch term in the heat flux equation. Simulation result agrees

well with measured data.

• FFT is used to obtain the amplitude and phase profile.

• For LOC, fundamental frequency amplitude increases towards the

plasma center, which contradicts pure diffusive local transport.

• The inversion radius (where electron temperature begins to

increases) is near the q=1.5 flux surface.

• For SOC, the amplitudes decreases towards plasma center.

Transport is more diffusive like.

Cold pulse modulation experiments are performed at different densities.

LBO is operated at 10Hz.

• Ip=0.8MA, Bt=5.45T

• R0=0.68m , a=0.21m

• Te measured from ECE at r/a=[0.69, 0.72, 0.773, 0.797, 0.821, 0.844, 0.865]m.

• Line averaged density from TCI

• Line integrated plasma rotation from central channel of HiReX

Density ramping experiment clearly shows that the non-local effect

disappears when core plasma rotation reverses direction.

LOC: Core Te increases(nonlocal)

Co-current rotation

SOC: Core Te drops (local)

Counter-current rotation

Toroidal rotation(left) and it’s contribution to radial electric field(right) for

LOC(red) and SOC(blue) shots.

Confinement time: MHD

MHDOH aux

W

dWP P

dt

For SOC plasma, the pinch term is negligible.

Heat transport of LOC and SOC plasma

LOC -> SOC

• Normalized electron heat transport is reduced by ~ half.

• Normalized ion heat transport is enhanced by a factor ~ 2.

• Heat transport: electron channel -> ion channel (TEM->ITG?)

Linear Gyro-kinetic simulations will be performed to characterize the

turbulent transport in these experiments.

• Nonlocal effect has critical density, which is close to rotation reversal

and LOC-SOC transition density.

• Nonlocal effect can be explained by turning on an inward heat pinch

term.

• Modulation experiments also show the heat transport is not purely

diffusive in LOC/co-current rotation plasma, where non-local effect

exists.

• Transport analysis shows from LOC to SOC, normalized electron

heat transport is reduced, normalized ion heat transport is

enhanced.

Radius of rotation reversal

archor point and electron

temperature profile flex point

has strong dependence on

1/q95[4].

• Further work needs to be done to determine the diffusion and

convection coefficients [5].