Experiments to test an intra-island scoop limiter on TEXT

Journal

of Nuclear

Matcnal\

812

145%147,1Yx7)

North-Holland.

EXPERIMENTS T.E. EVANS,

TO TEST AN INTRA-ISLAND

J.S. DeGRASSIE,

G.L. JACKSON,

XI:! -X1x

Arn~terdam

SCOOP LIMITER ON TEXT * N. OHYABU

GA Technologies Inc.. P.O. Box 85608. San Diego. CA 921.38. USA

A.J. WOOTTON, K.W. GENTLE, W.L. HODGE, S.C. McCOOL, T.L. RHODES, B. RICHARDS, Ch.P. RITZ, W.L. ROWAN The Universiv

of Texas. Fusion Reseurch Center, Austin,

F. KARGER

TX 78712. USA

and G. HAAS ftirPlasmuphysik,

Max-Planck-Institut

Key words:

P.E. PHILLIPS,

moveable

mushroom

Eurutom Association,

limiter, resonant

D-8046, Garching hei Miinchen,

helical divertor.

thermal

limiter

Fed. Rep. Germany

shielding,

ergodic

magnetic

limiter

An instrumented scoop limiter probe is being operated on TEXT to test the concept of limiter cooling and improved particle removal efficiencies using an externally-applied resonant magnetic field perturbation (the resonant helical divertor concept). Cooling of the limiter face has been demonstrated for limiter positions ranging from rl. = 29.0 cm inward to rL = 25.5 cm (the Text primary poloidal hoop limiter radius r, = 27.0 cm). Pressure rises in the limiter throat of approximately 40% are observed under optimized conditions. Interchangable limiter heads with thicknesses of 1.0 cm and 0.3 cm have been

used to examine particle ducting into the scoop aperture. Experimental results are discussed along with observations of the limiter floating potential, H, recycling emissions, pressure measurements, and edge density and temperature measurements.

1. Introduction There are currently several active programs among the major tokamak laboratories to develop mechanical pump and scoop limiters and to investigate the effects and performance of these devices on plasma confinement and fueling [l-3]. A review of the basic concepts involved in mechanically controlling particles and heat near the edge of a tokamak can be found in the paper by Mioduszewski [4]. Conceptually these devices represent a simple approach for managing the plasma density, recycling rates, impurities, and heat flux at the reactor wall. The experiment discussed here is based on an idea proposed by Karger and Lackner [S] in which a limiter blade positioned in the center of a stationary magnetic island is protected from plasma damage while simultaneously increasing the particle removal efficiency. The island is formed by driving relatively small currents through an external coil set. It is expected that the “resonant helical divertor” configuration, as it was originally termed, will force particles and heat along the island separatrix into the limiter aperture where particles can be neutralized and collected. We are using a relatively simple set of ergodic magnetic limiter (EML) coils [6] to examine this concept for cases in which the ratio of poloidal to toroidal mode numbers, ma/no, of the primary resonant islands are either 3 or $. Due to tk;e finite poloidal extent of the EML coil currents and * Work supported by the US Department of Energy Contract No. DE-AC03-84ER53158. 0022-3115/87/$03.50

(North-Holland

0 Elsevier

Physics Publishing

Science Publishers Division)

B.V.

toroidal coupling, spatial sidebands are generated, primarily at m = m0 f 1, creating multiple resonant radii. With sufficient perturbation strength, a quasi-ergodic layer is produced. However, for our purpose here, a non-ergodic single-island mode is desired. 2. Experimental background The scoop limiter module is located approximately 35 o toroidally (clockwise) from the primary TEXT hoop limiter on a top port box flange. The plasma current is clockwise for ma/no = : operation and counterclockwise for s operation. The toroidal magnetic field is counterclockwise. The machine parameters used here are R = 1.0 m, B, = 1.5-2.0 T, Z, = 1.5-5 x 10” cmm3, or {. The scoop limiter module layout is and q,-: shown in fig. 1 as it is mounted on the machine. Langmuir probe and fast pressure gauge feedthroughs as well as the slow pressure ion gauge head are located at the top of the moveable limiter tube. These are electrically floating along with the limiter head, sleeve, and tube. Electrical isolation and vacuum integrity are maintained with a Vespel seal located at the top end of the ASDEX bellows drive unit. This allows limiter floating potential measurements or electrical biasing of the limiter head with respect to the vacuum vessel [7] (the primary limiter is at vessel potential). The limiter head is mushroom shaped and interchangable to allow variations in face contour and blade thickness. The ASDEX drive unit has an 80.0 cm stroke allowing access to a 35.0 cm segment of a vertical chord through the center of the vacuum vessel with a primary

813

T. E. Evans et al. / Intra-island scoop limiter on TEXT LANGMUIR PROBE AN0 FAST PRESSURE GAUGE ELECTRICAL FEEOTHROUGHS

SLOW PRESSURE ION GAUGE

/

OOUBLE LANGMUIR PROBE ELECTRICAL NNECTION THROUGH

STAINLESSSTEEL SLEEVE RETAINER

SITIONING

SHAFT

VESPEL ELECTRICAL BREAK AND VACUUM SEAL ASDEX BELLOWS AN0 DRIVE UNIT

L

CONDUCTANCE APERTURES 135 cm

FAST PRESSURE GAUGE APERTURE

3 mm GRAPHITE APERTUREALSO

DOUBLE LANGMUIR PROBE TIPS (2 SIDES

INSULATING GUIDE FOR CENTER SHAFT, ISOLATION EELLOWS-

SCOOPAPERTURE

I( 15cm

SCOOP HEAD AND SLEEVE AREA WITH FAST PRESSUF GAUGE -

44.6 cm

TEXT RING LIMITER RADIUS

Fig. 2. Detailed

Fig. 1. Moveable limiter module mounted on TEXT top port box number P-10.

hoop limiter radius of r, = 27.0 cm. A detailed view of the moveable limiter head assembly is shown in fig. 2. A scoop aperture is formed between the back surface of the mushroom head and the limiter sleeve. The head and sleeve are POCO-graphite. The thick head is 10.2 cm in diameter with a 10 mm blade thickness and is coated with a thin layer of C + Sic developed at GA [S]. The thin head is uncoated with a slightly smaller diameter and has a 3 mm thick blade. The limiter sleeve may be adjusted on the tube allowing variable scoop apertures. The sleeve is equipped with a pair of double Langmuir probes, one on the electron drift side and the other on the ion drift side. The head, sleeve, and aperture designs were selected because of their flexibility and ruggedness. They also provide a simple geometry for testing the resonant helical divertor concept. The contours and dimensions were specified to optimize the insertion of the mushroom head into an $ magnetic island o-point (m = m0 + 1) since the width of this island is the smallest of the candidate resonant islands to be used.

STANDARD 10 mm POCO GRAPHITE HEAD SIC COATED

L..

I TEXT TORDUE FRAME

IR CAMERA VIEW

view of the mushroom sleeve.

-

GT

head and cylindrical

Limiter specific pressure diagnostics include: a slow ion gauge for measuring the neutral gas pressure in the collection tube (with a volume of 4.3 X 10’ cm3), having a response time approximately 100 ms, and a fast neutral density gauge, designed by the the ASDEX group [9], built into the stem of the mushroom head and having a response time on the order of 1 ms. The front face of the moveable mushroom limiter (MML) is viewed through a CaFJ window mounted on the bottom of the MML port box. Two H, monitors and an infrared camera share this window allowing simultaneous measurements of the hydrogen recycling rates on the electron and ion drift sides and full-face MML heat load imaging, respectively. A visual television system with an H, filter is used for tangentially viewing the hydrogen recycling near the MML head and at the primary hoop limiter. Several island resonances have been used to investigate MML cooling, floating potential variations, local recycling, and pressure responses in the scoop aperture. Fig. 3 shows a typical experimental configuration in ‘. which the 4 island resonance is aligned wrth an o-point at the limiter head radius. The simplest case (fig. 3(a)) is one in which the axis of the MML is coincident with a single $ island o-point just inside of the plasma boundary. The intent is to divert particles flowing along the B field lines normal to the plane of the figure into the MML scoop and to neutralize them on the stem of the mushroom. With large radial island widths, as in the

x14

POLOIDAL

I -27.0

I

I

I 0

-13.5

13.5

VERTICALPOSITION

I 27.0

(cm)

Fig. 3. (a) Simple IH,,,‘H,~= z intra-island limiter picture on TEXT. (b) Realistic oricntationa of the I)) = PI,, 2 I resonant islands at port P-10 a\ calculated by a simple Martin-Taylor type mapping code.

case of $, the MML face is also protected from heat and particle fluxes flowing on the inner island separatrix. On the other hand, fig. 3(b) is a realistic representation of the island topology for the present MML location on TEXT. Here, we see that a 2 island o-point is reasonably well aligned with the head of the MMI, but that the m = m, + 1 sidebands limit the the radial width of the island and create a more complex experimental geometry for purposes of data analysis. We also find that the 4 island o-point, which may be driven with a modest reconfiguration of the EML coils, is well aligned with the MML in its present location. By reversing the direction of I,, the current in the EML coils (i.e., with 1, -+ 1;1), the island o-points are interchanged x-points. The position, size, and structure of the resonant islands at any toroidal cross section are computed

using a magnetic field line tracing code developed at GA. This program is typically run in conjunction with another program being developed at TEXT and GA which is based on the Martin-Taylor [lo] mapping technique. The magnetic field structure shown in fig. 3 was generated using this mapping program [Ill. 3. Experimental observations and discussion Initial observations of the MML indicate that the present EML field on TEXT is not sufficiently coherent to produce particle ducting as expected from the intuitive model. Heat channeling to the TEXT primary limiter has been reported [12]. However, with the localized MML, the effective parallel connection length, L ,, , is greatly increased. Thus, a field line must circulate many

T. E. Ewns et al. / Intro-islund scoop linnter on TEXT

more times toroidally between MML intersections. the weak resonant perturbations, Nevertheless, 6B,/B - 10m3, produce significant changes in the plasma/MML interaction and in the edge plasma parameters. The EML field is pulsed 100-300 ms during the steady state portion of a TEXT discharge [6.12]. Comparison is made between identical discharge conditions with and without the EML applied. We have observed interesting EML effects on the MML ranging from increasing pressure to decreasing pressure and heat flux. Additionally, complex variations in the floating potential and particle flux as well as increases and decreases in H, have been observed. 3.1. Heat flux measurements The temperature imaged throughout

on the MML face is continuously the plasma discharge using an IR

815

camera with a resolution time of 16 ms/frame. In general, when the EML field is pulsed, there is a sizable drop in the global MML face temperature as well as a significant change in its distribution. Typically, we see a preferential temperature drop on one side of the limiter face (e.g., the electron drift side) as opposed to the other and find that the details of this change in the thermal distribution are strongly dependent on qr (the safety factor at the MML radius), rL, 1I, 1, and the direction of I, for each m/n. Part of the thermal data analysis involves computing the heat flux and total energy incident on the MML face using a one-dimensional thermal diffusion code [13]. At present, only the 3 case has been analyzed with this code. In this case, with T,_= 25.6 cm and I;i = 4 kA we find that the total heat load on the limiter is reduced. At the hottest points on the electron and ion drift side the incident heat flux is reduced from 612 to

TEXTSHOT

6 DEC.85 13:00

(b)

TIME (MS) Fig. 4. (a) P,. the pressure at the MML neutralizer plate. as a function of time into the discharge with ~I~~/u,~ = :. I& = 5 kA. and rL = 27.0 cm. A 40% increase in P, is observed during the EML pulse. (b) TEXT plasma current. I,, and density. n,. during the same shot as in (a).

313 W/cm’ and from 536 to 389 W/cm’, respectively. (The plus sign in I, + indicates a helical EML current direction resulting in an overlap of the island x-point with the MML vertical axis.) This heat flux reduction is consistent with the idea of ergodic fields acting as “thermal diffusers” resulting in the spreading of the edge plasma thermal energy uniformly around the vessel wall and reducing the heat flux at any point inside the ergodic boundary plasma layer. On the other hand, the detailed structure of the thermal distributions are suggestive of coherent islands, as revealed by their dependence on the particular mode selected and the direction of I,. 3.2. MML pressure and Thomson scattering temperature measurements Using the slow ion gauge with several MML aperture widths, w, we find that the collection chamber neutral pressure, P,, increases by approximately a factor of 3 between w = 20 mm and w = 3 mm. Thus, the collection efficiency scales as (YW-‘.~, where OL is a constant which is is presumed to be proportional to the length of the aperture [2]. The fast pressure gauge is used to measure AP,, the change in pressure at the MML neutralizer plate, when the EML is pulsed. In a general, A P, is negative at most rL’s indicating pressure drop consistent with an ergodic field topology.

Positive AP, has been observed for both i and 1 resonance; but, in each case, this occurs for the MML near the primary limiter radius. These pressure rises can be quite substantial (fig. 4(a)). Under optimal conditions with a proper choice of rL and 1,; , we have observed a + 40% A P, when w = 3 mm. Fig. 4 shows the parameters used for the largest +A P, response with ma/no = i. Note that this occurs with r: indicating that an island .x-point is coincident with the MML scoop aperture. This is also the case for + A P,, with the 5 mode. In order to test the 9 sensitivity, we have done experiments in which Ip was ramped at various rates during the EML pulse. We have observed resonances in P,. the MML H,, and the MML Langmuir probes signals. Typically, these occur at all rL’s when q,_ = 7, !, i, and 4. We also find that the MML H, follows P, quite closely. An example of the H, resonances are shown in fig. 5 for rL = 25.5 cm and rL = 24.5 cm. Note the resonant dips in the H, at each of these radii correspond closely to the T, $, and 3 qL crossings as indicated by the circles, squares, and triangles, respectively. The e resonance occurs regardless of the EML pulse condition, which may be indicative of a plasmagenerated tearing mode on the 9 = 3 rational mode surface. As mentioned above, we observe the +A P, at an island x-point and -APL at an island o-point. This is

4.0

1.0

_ --qL=2.9_--

0

100

200

300

400

500

TIME (MS)

Fig. 5. H, radiation from the MML face during a qr scan for two different MML positions. Resonant dips designated with circles, squares, and triangle are relatively well correlated with 4,. crossing of the f , !. and 5 mode rational surfaces, respectively.

817

T. E. Evans et al. / Intra-island scoop limiter on TEXT

400

100

-28

-26

-24

-22

VERTICAL

Fig. 6. Change

POSITION

-20

-18

-16

(cm )

in the edge Thomson scattering profile with a i, I& = 7 kA EML pulse and the MML removed. The primary limiter, indicated by the crosshatched area, is located = 180” toroidally from the Thomson system,

consistent with measurements of ne using the moveable TEXT Langmuir probes. These measurements show + A n, at the island x-point and -An, at the island o-point. In addition, we see a general drop in n, and T, in the scrape off layer during the EML pulse. The primary limiter exponential density fall of length, X,, is typically increased from > 1.6 to 2.5 cm and the energy fall off length, Xr, is increased slightly during the EML pulse. Typical ambient values for n,, c, and $, at the limiter radius without the EML are 1.9 X 1012cmm3, 20

eV, and + 80 V, respectively. These change slightly as the MML is inserted beyond the primary limiter radius. The Langmuir probe temperature measurements are connected to the edge Thomson scattering temperature measurements at r = 26.0 cm. The edge temperature profile is measured with Thomson scattering and shows a strong dependence on the EML pulse conditions. While there is essentially no change in T, inside, a region defined by the ergodic field structure, the edge temperature is significantly

i

o 2

0

WITHOUT

22

23

EML

24

25 MML POSITION

Fig. 7. MML floating

potential

variation

TEXT

26

27

(CM)

with and without

the EML fields.

28

reduced. Fig. 6 shows a well defined ergodic region over which the Tc profile is approximately flat. Under some conditions, we have also observed structure on this profile which seems to be related to the presence of coherent islands, 3.3. MML

flouting

potential

n~easurtvnents

A discussion of the variation in the MML floating potential as well as a simple model for this variation has been presented elsewhere [14]. We do observe evidence of magnetic field line mixing during the EML pulse. Fig. 7 shows the MML floating potential both with and without the EML pulse. Note that the potential becomes uniformly distributed across the narrow ergodic boundary layer and is approximately zero during the EML pulse. There is also a suggestion of some structure due to coherent islands between rl. = 25.0 cm and Y,_= 26.0 cm. This data was taken during { operation for I;I =4kA. 4. Summary and conclusions Coherent island structure has been observed on the MML H, monitors, the MML Langmuir probes, the TEXT Langmuir probes, as well as on other TEXT diagnostics not reported here. In addition to the small coherent islands, we have observed EML-induced stochastic field structure in a narrow-edge region using the edge Thomson scattering measurements, and the MML floating potential. Pressure changes in the MML collection chamber have been observed. Density increases and decreases near island x-points and o-points have also been observed. We have found that pressure increases occur only at island x-points and only in a small region (i.e., of the order of 1.5-2.0 cm) centered on the primary TEXT limiter radius for this MML toroidal location. The reasons for this unexpected behavior are

currently being investigated but the preliminary conclusion seems to be that particles are not confined well enough to the field lines to make the many toroidal circulations required to be collected by the MML. A clear test of this conjecture would be to use larger nz,,/n,, = 3/l islands and a limiter with a much greater poloidal collection area. In general, these results indicate that larger, more coherent islands with a greater poloidal extent are needed and that large poloidally-cxtended limiters will be required to fully evaluate the concept of a resonant helical divertor. Nevertheless, the data presented here show that small resonant magnetic perturbations can be used to effect the heat and particle interaction of the plasma with a limiter. The authors greatfully acknowledge the advice and efforts of N. Brooks, T. Taylor, R. Larsen, T. McKelvey, X. Yu, S. Zheng, J. Jagger and T. Herman during the design and installation of the MML module. References [11 PI [31 [41 [51 [61 [71 [Xl [91 PO1 Pll [=I P31 [I41

P. Mioduszewski et al., J. Nucl. Mater. 121 (19X4) 2X5. R. Budny et al.. J. Nucl. Mater. 121 (19X4) 294. A.E. Pontau et al., J. Nucl. Mater. 121 (19X4) 304. P. Mioduszewski, J. Nucl. Mater. 111 & 112 (19X2) 253. F. Karger and K. Lackner, Phys. Lett. 61A (1977) 3X5. N. Ohyabu et al., J. Nucl. Mater 12X & 129 (19X4) 266. P.E. Phillips ct al.. these Proc. (PSI-VII), J. Nucl. Mater. 145-147 (19X7). G.R. Hopkins, P.W. Trester and J.L. Kaae. J. Nucl. Mater. 12X & 129 (19X4) 802. G. Haas et al.. J. Nucl. Mater. 121 (19X4) 151. T.J. Martin and J.B. Taylor. Plasma Phys. Controlled Nucl. Fusion 26 (19X4)1 321. X.S. Yu, private communication. N. Ohyabu et al.. Nucl. Fusion 25 (1985) 16X4. T. Taylor. private communication T.E. Evans et al.. Bull. Am. Phys. Sot. 30 (19X5) 1569.