J. Plasma Fusion Res. SERIES, Vol.1 (1998) 99-l1z

Core Fluctuations and Non-Thermal Electron

Distributions at W7-AS
UASE Marcus, PERNREITER Walter and HARTFUSS Hans*
Max-Planck-Institut fiir Plasrnaphysik, EURATOM Ass., 85748 Garching, Germany(Received:30 September L997/Accepted: L2 January 1998)

Abstract
Two special applications of ECE radiometry at W7-AS, for measurement of temperature fluctuations and non-thermal electron distributions, are presented. In the first case correlation-radiometry isapplied. The fluctuation spectra are found to be composed of two components, a diffusive and a turbulent one, with relative levels of 0.L-2o/". The latter one is discussed in the frame of a turbulent mixing
process. To investigate non-thermal electron energy distributions a vertical ECE observation geometry

allowing for separation of both thermal and suprathermal populations and their time behaviour is used.
Experimental results by variation of plasma parameters are shown and compared with simulated spectra.

Keywords:
ECE diagnostic, temperature fluctuations, turbulence, non-thermal electron distributions,suprathermal emission

1. Core Temperature Fluctuations

are separated by a minimum around 15 kHz [a].

1.1 Characterization
Temperature fluctuation measurements are carried

1.1.1 Radial properties
The integrated relative power of the fluctuation
component below 15 kHz is about i/T:2% (150
Hz-I5 kHz). Its radial amplitude profile and radial
propagation properties (Fig. 1), determined from thecorrelation of different radial channels, point to a diffu-out in the context of turbulent transport, which is assumed to be driven at least in parts by fluctuations [9].

The sensitivify limit set by the inherent noise of an
ECE-radiometer can be overcome by correlation of two
separate radiometers focusing a common plasma volume [8]. Due to the demands for stationarity of the discharge during the correlation analysis only ECR heatedplasmas are investigated. Typical parameters are: 2.5 T
on-axis magnetic field, rotational transform t:t/3,
P."*r:300-800 kW (on-axis), peaked femperature
profile (7."* : 2.5 keV) and flat density profile

sive propagating temperature disturbance. Its origin is
located outside the observed radial range and it is propagating radially inward according to a heat diffusivitya:0.3-0.6) are observed simultaneously. The relativeof 1,n":0.8 mzls. A radial decay length of 2-3 cm (at45O Hz) and a radial coherence length > 20 cm (i.e. infinite with respect to the radial range diagnosed) confirm its diffusive character. This is in perfect agreementwith active temperature perturbation experiments
where ECRH is modulated. The radial propagation offluctuation spectra extend up to 50-150 kHz, withthe fluctuation component around 40 kHz is completely

radially increasing fluctuation power. Two components
which reveal completeley different physical behaviour

different. Its phase velocity 2n f/l<, is directed outward(q^,:2-8xL01e m-3). Six radial positions on the highfield side of the core plasma (r"o:5-10 cm, i.e. r/*01998 by The Japan Society of Plasma
C orres p ondin g author's e - mail : hartfus s@ ipp. mp g. de

Science and Nuclear Fusion Research99

HiseM. et al., Core Fluctuations and Non-Thermal Electron Distributions at W7-AS

1.51.0?

around 40 kHz which is believed to be the one relevant
to turbulent driven transport, is about 0.67o on- the7.2cmx 8.3 cm (0182)

high-field side. A clear coherence of 10-60% with aMirnov-coil signal indicates corresponding magnetic
field variations which are estimated to be in the orderof some T,/s. However, no mode-like spatio-temporal
structure can be identified [1]. Conelation of density
and temperature fluctuations have been found in a
combined reflectometry-ECE experiment [3] and will
be discussed in detail in a forthcoming paper.

'.It-0.5'11ioo.o-g-0.5-1.0

1.1 .3 Vanishing temperature gradient

1f [Hz]As expected in a turbulent, convective picture, thetemperature fluctuation component above 15 kHz disappears completely in the flat region of the temperature
profile. Measurements with reflectometry show significant density fluctuations even in the Vf:0 region, indicating persistent plasma turbulence. According to theFig. 1 Spectrum of the radial Wavenumber (.

(Fig. 1) with a dispersion relation which cannot be explained diffusively. Its coherence length (1.5-3 cm) issmaller than the observed radial region, pointing to aradial extended turbulence-like feature. Comparison ofhigh-field-side and low-field-side measurements show

convective picture, the VFdependence is separated
and the "turbulence" ) : it a,f is investigated. The
fluctuation component below 15 kHz is found in theV7:0 region with a similar amplitude like in the finite-.
gradient region, as expected due to its diffusive nature.

an about 2 times larger fluctuation level on the lowfield-side whereas the profile of the diffusive component is identical on both sides. i.e. it is constant on aflux-surfaces.

1.1 .4 Parameter scans

The dependence of the integral turbulence I ofthe component above L5 kHz on parameters like ECR

1.1.2 Correlation with Mirnov-coil signals
The integrated relative power of the component

o30-100kHz (0180ft)
PEcu lkwl

f-b\. o 830\a .'oo 830
a 410\\f "o z{:3:EE\ o'.\\1,-10 0'|

2010k [cm]

F0500

1000

1500

Tr". [eVtFig.2 ECRH power scan.

(0140 ff)/\n/oIoFI\^\12rt niem3t

tov

ni-11o,/t,
1 19 b:g
rr -fl^\1-..\r\911,li,m\.ip ?:3 q:+,=0.10

30-l00kHz (0140|f)n,o [l orom{lEa 8.7
L 7.9
4.7
2.3o

1.3tt/ffi- \'\rr\0.01

0500 1000 1500 2000 2500
Tece [eVlFig. 3 Density scan.

100

Hise M. et al., Core Fluctuations and Non-Thermal Electron Distributions at W7-AS

heating power, plasma density, rotational transform t
and plasma current has been investigated. Earlier results [5] are correspondingly found looking at ,i, .ln ther-scans discharges with poorest confinement show highest fluctuation or turbulence levels. An empirical attempt to assign local plasma parameters to the i profiles points to an universal decrease of ) n 7" withincreasing temperature. In the case of the ECRHpowerscan (Fig. 2\, an exponent a=-1.5 has beenfound, whereas a similar analysis for the density scanyields different exponents (a:-2...-3.5) for different

Io2.5

EE 2.OoEo 1.5
co1.0(6=(t0.50.0120

densities (Fig. 3).

130140frequency [GHzl150t60

Fig. 4 Measurements vs. simulation.

2. Non-Thermal Electron Distribution
2.1 Introduction
During ECRH heating at high power density, dis-radiation emitted by a region close to the plasma

tortion of the Maxwellian electron distribution function
is expected near the resonance zone with the creationof a suprathermal tail. In the simplest model a biMaxwellian distribution can be assumed consisting of athermal and a suprathermal part. Whereas low fieldside ECE observation does not allow for unique interpretation (ambiguity between position and energy), by
vertical observation the thermal emission is reduced toa relatively narrow frequency range because of thenearly constant magnetic field along the line of sight.
The down-shifted radiation emitted by the suprather-

centre. Decreasing the frequency the thermal resonancemoves to the plasma edge where the temperature ismuch lower. At frequencies below the minimum at138-139 GHz the thermal resonance is shifted outside
the plasma and only down-shifted emission ismeasured. Assuming a biMaxwellian distribution agood fit of the spectrum is possible (dotted line in Fig.
4). Spectra obtained by variation of plasma parameters
are given in Fig. 5. Doubling the density leads to astrong decrease of the detected radiation temperature,
although the central plasma temperature remains nearly
unaffected. If the ECRH power is changed from 400
kW to 800 kW the low frequency peak increases andshifts to lower frequencies, as expected because ofhigher central temperature. By varying the radial position of the vertically viewing antenna it was found outthat the radial extent of the spatial distribution of themal electrons is much less reabsorbed along the vertical

path with a vertically viewing antenna system with
Gaussian beam optics. It is arranged in a toroidal planeclose (/<D- 5') to that where the ECRH is deposited. It
is movable in radial direction. so it allows to localize thesuprathermal electrons as the only diagnostic in theECRH deposition plane. The non-thermal part of theemitted spectrum can be identified by comparison withnumerical calculations and by investigation of the different time behaviour of the two components.

oJ2.2 ECE spectra and numerical simulations

IIaEfELow density plasmas heated by high power ECRH
(400 kW and 800 kW, respectively, X-mode polarization) have been investigated. Fig. 4 shows a measured
spectrum for the case of P:800 kW, n"o:1X101e m-3
and vertical observation (circles). A ray-tracing code isused to calculate ECE spectra including a suprathermalcomponent. The ECE-spectrum is obtained by solving
the radiative transfer equation along the ray path. Dueto the presence of a local maximum of B along theviewing chord (close to the plasma centre) two peaksappear in the calculated ECE-spectrum (ftrll line in Fig.
) [2]. The maximum at higher frequencies comes fromoqco

2I*'INi:J iel.9t6

1Ei\r'iixxI..d,l\T0125 130 135 140 145 150

155

frequency [GHz]Fig.5 ECE spectra measured with the vertically viewing
antenna. (x) neo - 1 x 101s m'3, P= E00 kW; (O)neo=2x101s m-3, P=800 kW; (tr) ,eo=1x101e m-3,
P=400 kw.

101

Hdse M. et al., Core Fluctuations and Non-Thermal Electron Distributions at W7-AS4oooo,45o ,no.s
Zo0'07oFig.6 Time behaviour of the ECE-spectrum after ECRH power switch-off. Left: "minimum B"; Right: "maximum B". No difference can be determined in the decay of the suprathermal radiation within the frame of measurement accuracy.

suprathermals is only about 2-3 cm, coinciding withthe power deposition zone [6]. Calculations result in astrong dependence of the central density of thesuprathermals z$th on n"o and PECRH, 0.5o/o z"o ... n&th... 3o/o n.s, whereas there is only a weak variation ofTlth between 6 keV and 8.5 keV. Switch-off experiments give a further possibility to distinguish between
the thermal and the suprathermal component of theBesides Fig. 6 shows higher energies of the suprather-

mal in the "minimum B" configuration.

References[1] M. Anton et a/., in these Proceedings, p.259(1ee8).

l2l U.Gasparino et al., Proc. of the l9th EPS Conf.
on Contr. Fusion and Plasma Pftys. Innsbruck(L992) Vol II 1001.

spectrum [7]. The suprathermal radiation disappears ona time scale of about 1 ms which is attributed to theloss mechanism or at least to the collisional slowing-[3] M. Hiise, H.-J. HartfuaB and M. Hirsch, Correla-tion of Temperature and Density Fluctuations ondown. That shows the direct connection between
ECRH power deposition and creation of suprathermal

W7-AS, 10th Joint Workshop on Electron Cyclotron Emission and Electron Cyclotron ResonanceHeating, Ameland, The Netherlands (1997) (Singapore: World Scientific to be published).
[4] H.J. HartfuB, M. Hdse, C. Watts, M. Hirsch, T.
Geist and the W7-AS Team, Plasma Phys. Con-electrons.

2.3 Different magnetic configurations

At W7-AS the depth of the toroidal mirror of thetrol. Fusion 38,4227 (1996).

magnetic field configuration can be changed. Measurements have been made in the so-called "minimum B"
and "maximum B" configurations. In the latter a signi-[5] H. J. HartfuB, S. Sattler, M. Hdse, T. Geist andW7-AS Team Fusion Engineering and Design 34-

ficantly smaller number of electrons in the ECRH

3s,81 (1ee7).

launching plane is trapped. Fig. 6 shows the response ofthe ECE-emission after the switch-off of the ECRH for[6] W. Pernreiter et al., Non-thermal Electron Energy
Distribution from Yertical ECE-Measurements atW7-AS Stellarator, Proc. 10th Joint Workshop onElectron Cyclotron Emission and Electon Cyclotron Resonance Heating, Ameland, The Netherlands (1997) (Singapore: World Scientific to be

the two configurations. It is not possible to decide
exactly the maximal radiation temperature in case of"minimum B" configuration because of the technicallydetermined measurement gap between 138 and 141
GHz. But the "minimum B" configuration shows alarger amount of suprathermal electrons under other-published).

[7] M. Rom6 er a/., Plasma Phys. Control. Fusion 39,

wise same conditions (integration of suprathermal

rr7 (ree7).

[8] S. Sattler, H.J. HartfuB and W7-AS Team, Phys.

emission over the whole frequency range). This gives ahint that suprathermals are trapped in the local mirror,
which leads to a higher particle density in the dynamic

Rev. Lett. 72,653 (L994).

[9] C. Watts et al., Phys. Plasmas 3,2Ot3 (1996).

equilibrium of production, slowing down and drift.

LOz