Thermal load of a scoop limiter with changeable geometry

Journal of Nuclear Materials North-Holland, Amsterdam

145-147

(1987) 789-792 789

THERMAL LOAD OF A SCOOP LIMITER WITH CHANGEABLE GEOMETRY A.V. CHANKIN Ch. CRUNOW V.A. VERSHKOV

‘, V.M. CHICHEROV *, K. GijNTHER

‘, S.A. EFSTIGNEEV

2, J. LINGERTAT

’ and S.N. ZVONKOV

‘, S.A. GRASHIN

‘, H. GROTE

2,

2, N.A. KABASHOVA’,

’

’ I. V. Kurchatov Institute of Atomic Energy, Moscow, USSR ’ Zentralinstitut f6r Elektronenph_vsik, AdW der DDR, Hausvogteipl. 5- 7, 1086 Berlin, German Dem. Rep.

Key words:

tokamak,

scrape-off

plasma,

scoop limiter,

infrared

thermography

The thermal load of a T-10 scoop limiter with changeable geometry has been investigated by infrared radial dependence of the energy flux was found to be non-exponential. The limiter load decreases sharply density exceeds a critical value of ii, - 4.5 ~10’~ m-‘. It shows maxima for q(a) near 2 and 3.

1. Introduction During the past few years the thermal load of different T-10 limiters has been systematically studied by means of infrared thermography [l-6]. The objective of these studies was to extend our scientific understanding of plasma-limiter interactions and the structure of the scrape-off layer as well as to develop improved limiter concepts for future machines. During the last experimental campaign a scoop limiter with changeable geometry was used which simulates one of the eight limiters planned for T-15.

thermography. The if the mean electron

by the infrared camera has a resolution of 22 lines with 39 points per line and a time resolution of 40 ms. For the discharges investigated, the limiter was at a radius of 26.5 cm and 28.0 cm. The radius of the fixed ring limiter was 32.5 cm. Data evaluations have been restricted to stable, highly reproducible discharges with ohmic heating.

2. Experimental Fig. 1 shows schematically the limiter design. The geometry is characterized by the quantity h, which can be changed in the range of 0 mm < h < 20 mm. Thereby the mean angle ‘p of the central limiter plate versus the horizontal plane varies in the range of 0” < ‘p < 10”. The limiter has two symmetric channels with neutralizer plates at each end. All surfaces exposed to the plasma are covered with graphite tiles. Each channel is equipped with 3 Langmuir probes and 2 heat flux probes. In the uppermost graphite tiles 6 thermocouples were installed. The limiter was inserted through the bottom port of the torus. Its surface was observed with an AGA 750 infrared camera mounted on the top port of the limiter section. In this paper results mainly concerning the limiter thermal load are dealt with. Results on the scoop limiter plasma will be reported in a separate paper [7]. The infrared camera was calibrated by uniformly heating the limiter and comparing the camera signal to thernkcouple signals. The thermocouples were also used to monitor each discharge, allowing some comparison with the thermographic results. Due to the restricted angle of view only the central part of the limiter (25 cm X 15 cm) was accessible to observation (fig. 1). Each frame of this port produced 0022-3115/87/$03.50 0 Elsevier Science Publishers B.V. (North-Holland Physics Publishing Division)

Fig. 1. Schematic

of the scoop limiter (dimensions

in mm).

3. Results and discussion

ion

s/de

electron

s/de

..

~ . . *. ~ . . 1. . :

A typical surface temperature distribution is shown in fig. 2. There is essentially symmetry of the thermal load between the electron- and ion-drift sides of the limiter, although in a few cases probe measurements in the scoop channel show differences up to a factor of 8. In accordance with earlier findings [6], the symmetric situation on the limiter surface is typical for small limiter radii. The channels, however, are on a larger radius (a + 37 mm), where the wall can influence the energy transport towards the limiter. Poloidal shifts between the temperature maxima on both limiter sides are generally small. They are in the range to be expected from irregularities of the limiter surface on both sides. .Earlier it was found [6] that the poloidal asymmetry vanishes for a safety factor of q(a) - 2. The discharges evaluated in this respect (e.g. that in fig. 2) indeed have q(a) - 2. The phenomenon on the whole is under further investigation. The energy flux p, to the limiter surface has been evaluated by solving the equation

where AT is the measured surface temperature increase, K is the thermal conductivity and K is the thermal diffusivity. This has been done for each of the 39 points along that scanning line (toroidal direction x) which goes through the point of maximum temperature. This yields “hottest-line” energy flux profiles p,(x) which are shown in fig. 3 (dots). This profile is compared with a theoretical model based on the assumption of an exponential radial profile of the parallel energy flux p ,,, thus P,=P,~(O) sin #(x)

exd-v(x)/~l,

(2)

where y = r - u is the depth into the scrape-off and # is the angle between the magnetic field lines and the

4 2 0 -2 -4 -6 -6

-4

-2

0

2

4

6

Fig. 2. Typical field of isotherms on the limiter surface for shot No. 40986. Temperatures are given in OC. The ion-drift direction is from left to right, axes in cm. (a = 26.5 cm, q(o) - 2, EC = 3.5 x 1Ol9 mm3, h = 1 cm).

-7u

-5

0

ial

5

10

Fig. 3. Energy flux profiles p,(x) in the plateau phase of shot No. 40986 (dots) and model calculations (curves). x-axis in cm (h =1 cm). (a) X = 0.65 cm,(b) X = 0.35 cm, (c) X = 0.78 cm.

limiter surface. The functions v(x) and #(x) contain the geometry (toroidal profile) of the somewhat irregular limiter surface, which, for this purpose had to be determined to an accuracy of 5 pm. Moreover, the model takes into account the field ripple, a possible tilting of the limiter as a whole and the apparatus function of the infrared camera, but neglects finite Larmor radius effects. By adjusting p,,(O) and X the model was fitted to the experimental points (fig. 3a). Thereby we found that the model gives only rough fits for the experimental points and the optimal X changes with the limiter form: the smaller h the shorter X. Considering that the range of y monitored decreases with h, we arrived at the conclusion that the radial decay of p,, is non-exponential. Accordingly, fairly good fits using (2) with different decay lengths in different intervals of y have been obtained for the discharge on fig. 3. Analyzing in this way a complete experimental series with h-variation it turned out that with an accuracy of 10% all p,-profiles can be accounted for by a unique radial profile of

A. V. Chankin et al. / Thermal load

ofa scoop limiter

191

energy flux which may be written P,,(Y)

=P,,(O)F(Y),

Y = r-

=,

(3)

where F(y) is shown in fig. 4. An analysis of the thermocouple readings for the same series yields a mean decay length of the energy flux I- 0.8 cm, which is in line with fig. 4. Within the series of h-variation the value obtained for p,,(O) proved to be rather high (13.8 kW/cm’). Discharges of the type used (a = 26.5 cm, q(u) - 2, ii, = 3.5 X 1019 mm3) have been found earlier to cause a high limiter load [4]. Nevertheless, in the present experiment the maximum temperatures reached were relatively low in compliance with the flat limiter surface. This data is: 1030” C (h = 2 cm), 810°C (h = 1.5 cm), 620” C (h = 1 cm), 555 o C (h = 0.5 cm). They are to be compared with more than 2500°C obtained under similar conditions and a limiter with ‘p = 30 o [5,6]. The observed deviation from the exponential law of energy flux decay may be the result of a non-exponential 7”-profile in the scrape-off. Typically the electron temperature falls steeply near the limiter radius and remains constant further outside [8,9]. A behaviour of the energy flux similar to fig. 4 was reported from TEXTOR and discussed by the author in terms of an exponential decay [lo]. The total power absorbed by the limiter, P,;,, has been calculated by integrating (3) over the flux tubes in the limiter shadow. Figs. 5 and 6 show the dependence of the ratio a = P&( Pi, - Prad) on the mean electron density G, and on q(u). Here Pi, is the ohmic input uniform part of the power and Prad the toroidally bolometrically measured power loss. The results are similar to previous ones [4]. The arrows in fig. 5 indicate discharges which presumably are dominated by homogeneous runaway heating of the electron-drift side of the limiter. In these

/I;‘9 I

0

I

2

5

5, Fig.

5. a = Pli,/(

Pi, - Pra,& versus line averaged

density

6,

fora=28cm,q-2.5andh=lcm.

cases our power calculation underestimates the power lost to the limiter. As can be seen from fig. 5, a decreases sharply at a point, where the mean electron density ii, reaches the critical value iI, - 4.5 x lOi rne3. In the same density region a Langmuir probe located far away from the limiter showed a step-like increase in scrape-off layer density (fig. 7) and no change in electron temperature. A model which predicts the critical mean density Z, as the onset of a local state of limiter self-shielding was discussed in [4]. The now established simultaneous increase of the density distant from the limiter indicates that this state may be non-local, like a detached plasma [11,12]. The power ratio a, as a function of q(u), increases

I I

3

(r- a)/cm

q(a) Fig. 4. Experimentally determined function F( r - a) describing the radial decay of the energy flux in the scrape-off (dashed part by extrapolation).

Fig. 6. (I versus safety factor q(a) for a = 28 cm, iic - 3.5 x lOI mm3 and h -1 cm.

192 Variations of n, and q(a) significantly influence the fraction of power absorbed by the limiter. With Fe > 4.5 x 10” mm3 and q(u) - 2.5 the limiter load is low. This confirms the possibility of using pump limiters for large scale tokamaks under high-density conditions without overloading the leading edge. The authors thank the T-10 operating team and also the physics team of the Kurchatov Institute for valuable discussions.

References PI K. Guenther et al., Proc. 3rd Conf. on Engineering Prob-

‘.

I

2

3

4

5

Ac /101gm-3 Fig. 7. Density in the scrape-off nsoL versus line averaged density n,. nsoL was measured by a Langmuir probe located at a toroidal distance 7r/2 from the limiter on a small radius rs = 32.8 cm and a large radius R = 175 cm (R, =150 cm). Data points refer to the same series of discharges as in fig. 5.

as q(u) approaches 2 and shows a maximum near q(a) = 3. The obvious connection between integer values of q(a) and maximum 01 might reflect the occurrence of magnetic islands. In [13] the formation of stationary islands is predicted if the radius of a resonant magnetic surface comes near to the limiter radius. For all discharges investigated the total power absorbed by the limiter is lower by a factor of about 1.5 than in comparable discharges with a ‘p = 30”-limiter [4]. This difference may be explained by a enhanced energy reflection under conditions of grazing incidence. In the light of studies concerning the phenomenon of reflection and its conditions [14-171 this explanation seems to be quite reasonable. 4. Summary The thermal load of a T-10 scoop limiter with changeable geometry has been investigated by means of infrared thermography. Thereby the radial dependence of longitudinal energy flux was found to be non-exponential. The exact knowledge of the radial dependence is believed to be important for the design of high-load limiters.

lems of Nuclear Fusion, Leningrad 1984, Vol. 1 (Moscow, 1985) p. 86. PI J. Lingertat et al., Plasma Physics and Controlled Nuclear Fusion, 10th Int. Conf., London, 1984. Vol. 1 (IAEA, Vienna, 1985) p. 265. [31 K. Guenther et al.. Zh. Eksp. Teor. Fiz. 89 (1985) 861. [41 J. Lingertat et al., Proc. 12th European Conf. on Controlled Fusion and Plasma Physics, Budapest, 1985, Vol. 2 (1985) p. 559. [51 A.N. Vertiporokh et al., Proc. 12th European Conf. on Controlled Fusion and Plasma Physics, Budapest, 1985, Vol. 2 (1985) p. 563. [61 K. Guenther, et al., Proc. 12th European Conf. on Controlled Fusion and Plasma Physics, Budapest, 1985, Vol. 2 (1985) p. 567. [71 V.A. Vershkov. S.A. Grashin and A.V. Chankin, these Proc. (PSI-VII), J. Nucl. Mater. 145-147 (1987). PI P.C. Stangeby et al., Proc. 12th European Conf. on Controlled Fusion and Plasma Physics, Budapest 1985, Vol. 2 (1985) p. 579. [91 A.K. Prinja and R.W. Conn, J. Nucl. Mater. 128 & 129 (1984) 135. IlO1 U. Samm, Proc. 11th European Conf. on Controlled Fusion and Plasma Physics, Aachen 1983. Vol. 2 (1985) p. 413. [ill J. O’Rourke et al., Proc. 12th European Conf. on Controlled Fusion and Plasma Physics, Budapest 1985, Vol. 1 (1985) p. 155. VI J.D. Strachan et al., Proc. 12th European Conf. on Controlled Fusion and Plasma Physics, Budapest 1985, Vol. 1 (1985) p. 339. [I31 D.G. Baratov et al.. Proc. 12th European Conf. on Controlled Fusion and Plasma Physics, Budapest 1985. Vol. 2 (1985) p. 682. [14] W. Eckstein and H. Verbeek, IPP Garching Report 9/32 (1979). [15] R. Chodura, J. Nucl. Mater. 111 & 112 (1982) 420. [16] D. Hildebrandt et al., Proc. IAEA Tech. Committee on Divertors and Impurity Control (IPP Garching. 1981) p. 47. [17] S.A. Cohen et al., Nucl. Fusion 21 (1981) 233.