The effect of a single blade limiter on energetic neutral beam particles in doublet III

487

Journal of Nuclear Materials 128 8t 129 (1984) 487-492

THE EFFECT OF A SINGLE BLADE LIMITER ON ENERGETIC NEUTRAL BEAM PARTICLES IN DOUBLET III T.W. PETRIE, C. ARMENTROUT, K.H. BURRELL, T. HINO, C. KAHN, J. KIM, J. LOHR, L. ROTTLER, D. SCHISSEL, H. ST. JOHN and the Doublet III Physics Group GA Technologies,

Inc., San Diego, California 92138, USA

Key words: single blade limiter, Doublet III, Monte Carlo simulation,

power loss

Energetic beam ion collisions with the main limiter can be a significant power loss process under certain operating conditions in Doublet III. Furthermore, these collisions may cause measurable damage to the limiter itself. Under low current and low toroidal field conditions (e.g., I, = 290 kA and B, = 6.3 kG), 20-38% of the inferred absorbed beam power may be deposited directly on the ion drift side of the limiter by the beam ions. However,for higher plasma current and toroidal fields (e.g., I, = 480 kA and B, = 15 kG), the fraction of inferred absorbed beam power deposited on the limiter is reduced to < 10%. Monte Carlo code simulations show that this loss of beam power is primarily a result of the large poloidal and toroidal gyro-orbits of the energetic beam ions. Other factors which may enhance beam ion losses to the limiter are (1) large separation distances between the primary limiter and the (outboard) vacuum vessel wall, and (2) plasma density buildup near the plasma edge during high gas puff operation. In addition, our data suggests enhanced plasma density and recycling near the limiter. This localized density can cause appreciable premature ionizations of the incoming beam neutrals and thus reduce the effective plasma heating of the beamline which is immediately upcurrent of the limiter. The prematurely-ionized beam particles from this adjacent beamline are responsible for much of the damage to the ion drift side of the limiter. We have found that under certain operating conditions (1) the direct beam heating of the limiter is 50% greater and (2) the stored plasma energy is 10% less when the beamline immediately upcurrent of the limiter heats the plasma. Thus, the relative positions of the limiters to the beamlines are important in designing future tokamaks.

1. Introduction We have measured the heat flux to a single blade limiter in beam-heated, dee-shaped plasmas, and have found that the heat flux is much stronger on the ion drift side than on the electron drift side [l]. Analysis of low toroidal field plasmas (B, = 6-8 kG) suggests that this asymmetric heat load on the limiter is due to energetic (co-injected) beam particles striking the limiter on the ion drift side [2]. A more recent analysis, which includes charge-exchange data and Monte Carlo code simulations, supports this beam ion loss model [3]. Many of the beam ions born on the outboard side of the plasma lie on trapped (“banana”) orbits. Some of these trapped particle orbits carry beam ions directly into the limiter. However, many trapped beam ions, which have large toroidal gyro-radii, can also collide with the limiter, even though their guiding-center orbits do not intersect the limiter. (The “cross-section” for the beam particle/limiter collision effectively increases, as B, decreases.) In addition to low-B,, other factors which might degrade beam ion confinement are: (1) low plasma current (i.e., large radial particle drifts), (2) strong gas puffing at high density (i.e., fast ion birth profile skewed toward the plasma boundary), and (3) a large separation between the limiter and outboard vacuum vessel wall (i.e., premature ionization of beam neutrals in the shadow of the limiter during high density operation). 0022-3115/84/$03.00 Q Elsevier Science Publishers (North-Holland Physics Publishing Division)

B.V.

In this paper we examine how much of an effect these factors have on beam ion confinement in Doublet III, and discuss the impact that these beam ion losses make on both beam heating efficiency and limiter preservation. Section 2 briefly describes the relevant experimental arrangement in D-III. In section 3 we assess the beam ion losses on the primary limiter for five distinct power scans (Pinput < 6.5 MW). In section 3 we also note the effect on beam heating of locating a neutral beam injector near a source of high recycling. In section 4 we discuss our results.

2. Experimental

arrangement

Three neutral beam injectors, each with two Berkeley-type ion sources, are located at toroidal angles of 90 ‘, 210 ’ and 330 O. They inject hydrogen beams at a full energy of 67-75 keV in the co-current direction with a tangency radius of 63 cm. The beam power carried by full-, half-, and one-third energy beam particles is in the approximate ratio of 32 : 38 : 28; 2% of the injected power is in the form of impurities. In cases (II-V) which are described in the following section, the dee-shaped, deuterium plasmas are limited on a single TiC blade, which is located at a toroidal angle of 300”. The deuterium plasmas considered in case (I), however are limited by three tile arrays attached to the inside vacuum vessel wall near 60 O, 180 ‘, 6. LIMITERS;

EXPERIMENT

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W. Perrie et al. /

A

single blade

iimiter

nearly-identical shots, We do this to avoid excessive cluttering of data points on our graphs. 3. I. Limiter positioning with respect to the plasma

Fig. 1. Flux plots for the five cases described in table 1 and discussed in the text: (a) case (I), (b) case (II), (c) case (III), (d) case (IV), and (e) case (V).

and 300”. (The tile arrays at 180’ and 300 o are Tic-coated, while the tile array at 60° is Sic-coated.) Fig. 1 shows the representative flux plots for each of the five cases. Note that in case (I) the separation between the outermost flux surface and the front face of the primary limiter is 8-9 cm. Heat flux to the blade limiter (referred to as the “primary” limiter) is determined by infrared detection of the surface temperature. A scanning infrared camera views all of the electron drift side and most of the ion drift side of the primary limiter. A more complete description of the diagnostic arrangement is given in ref. [2].

3. Results The operating parameters for the five cases we will examine are summarized in table 1. We have included information on the poloidal gyro-orbit (A) and toroidal gyro-radius (or) for a 70 keV beam ion born on the outboard plasma edge. The value of A is the maximum inward drift of the guiding center orbit from its birthpoint; pT is based on the value of B, at the ion’s birthpoint. We note that the value of A in all five cases is roughly the same. The distance from the front face of the primary limiter to the vacuum vessel wall (sep) is also shown in table 1. Each data “point” we will refer to in the following sections represents an ensemble average of several

Fig. l(a) shows a representative flux plot for case (I) plasmas. Because the separation between the front face of the primary limiter and the outermost flux surface is large (greater than twice the toroidal gyro-radius), we expect few energetic beam ions to strike the primary limiter. Since we, in fact, observe no asymmetric heating of the primary limiter even at the highest injection power (Pam,, - 6 MW), we conclude that beam ion collisions with the limiter are negligible. A representative flux plot for case (II) plasmas is shown in fig. l(b). Except for the obvious difference in positioning, case (I) and case (II) plasmas are quite similar. For example, although the line-average electron density is about 10% higher in case (I), ti, in both cases is nearly constant during beam injection [fig. 2(a)]. The gas puff rate and the Ti,-profile are also nearly constant in both cases during beam injection. In contrast to case (I), we detect heat loading on the limiter. We observe significantly more heat flux to the ion-drift side of the primary limiter than to the electron-drift side and interpret this asymmetric com~nent of the heat flux observed on the ion drift side as primarily due to energetic beam ions [2,3]. We can be more quantitative in discussing this asymmetric heating if we define: ,a ~ PLY - pLEL BEAM

whert p ION pLEL

power flux to the ion drift side, =power flux to the electron drift side, = beam power which is absorbed in the plasma, p:,, (i.e., s~nethrou~ is taken into account). For Case (II), fa is 0.07-0.08. This measured value of fa is consistent with beam power losses predicted from MCGO Monte Carlo modeling [4]. We also note that fa =

TABLE I PARAMETER Case

BT(kG)

I,(kA)

SUMMARY

A.(10’3

cmm3)

FOR CASES I-V

a(cm)

b/a

Ncm)

&cm)

sep (cm)

I

13.0

465

4.8 - 5.4

39 - 40

1.35 - 1.40

-23

3.6

7

II

15.2

480

4.4 - 4.8

39 - 40

1.35 - 1.40

-23

3.1

7

III

8.4

480

3.6 - 4.9

40 - 41

1.50 - 1.53

-28

5.5

7

IV

8.0

420

3.8 - 6.3

40 - 41

1.50 - 1.55

-20

5.8

12

V

6.3

290

4.6 - 5.8

38 - 39

1.38 - 1.44

-24

7.4

12

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T. W. Petrie et al. / A single blade limiter

(a)

8

3.3. Gas

./”

6

m‘E m

.&$2dE,

o4

-0 t

aI

. .. .. ... CASE

11

----CASE

III

--CASE IV -CASE V

IC

0

‘0

(b)

0.3 II Ill

,m 0.2 1 0.1

“0

B.*CASE

IV

+*+--SE

v

0 . .. .. ... + 2

. ........0

4

6

8

PlNPUT (MW) Fig. 2. (a) Line-average electron density E, is plotted against the corresponding total input power P,,,,, for each of the five cases; (b) the fraction of the inferred absorbed neutral beam power which strikes the primary limiter fe is plotted against P,NPUTfor cases (II-V); fb is negligible in case (I) plasmas.

is apparently independent P INPUT;SW fig. 2(b).

of the

total

input

power

3.2. Toroidal field effect The characteristics of case (II) and case (III) [fig. l(c)] plasmas are similar, except that case (III) has about one-half the toroidal field of case (II). It is instructive then to compare these two cases to evaluate the toroidal gyro-radius effect at moderate density (i.e., Ti, = 4-5 x 10” cme3). For the steady state discharges of case (III) in which P,,,,, = 3.5-4.5 MW, we find that fe = 14-158. This is a little higher than our Monte Carlo simulation, which predicts that = lo-11% of the beam ion power will strike the limiter. In comparing the experimental values of fa from case (II) and case (III), we find that reducing B, by a factor of 2 increases fe by 0.06 + 0.04. Even at the high end of this estimate, the increase in fe is still modest.

puff and

limiter/ wall separation effect

A high rate of gas puffing usually generates both a flat electron density profile and measurable plasma density beyond the limiter-defined flux surface (i.e., between the edge of the plasma and the outboard-side vacuum vessel wall) [2,5]. Under these operating conditions a sizable fraction of the neutral beam particles may be ionized at (or outside of) the plasma edge. Beam ions born here are likely to collide with the primary limiter prior to thermalization. Moreover, if this separation between the plasma edge and the vacuum vessel wall is increased, the incident neutral beam particles must traverse a greater distance through the boundary plasma, and stand a greater chance of prematurely ionizing. A comparison of case (III) with case (IV) [fig. l(d)] allows us to gauge the combined importance of the above two processes. In case (IV) the front face of the primary limiter is - 12 cm from the outboard vacuum vessel wall. This distance is about 5 cm more than that of case (III). The gas puff rate for case (IV) is - 350 Torr l/s, which is about twice that of case (III). All other parameters are approximately the same for both cases (see table 1). To make the comparison of fe more direct, we select data which has a common electron density. From fig. 2(a) this value of fi, is = 4.9 X 1013 cmm3. This corre= sponds to PINPUT= 4.5 MW in case (III) and P,,,,, 2.8 MW in case (IV). From fig. 2(b) the corresponding values of fe are 0.14 and 0.20 for cases (III) and (IV), respectively. Thus, doubling the gas puff rate and (almost) doubling the plasma/wall separation again produces a modest (i.e., 0.06 f 0.04) enhancement in beam ion losses to the limiter. We also note in fig. 2(b) that fe in case (IV) increases This increase in f B has more to do with the with P,,,,,. plasma density, which also increases with PIN,,,. When density and gas puff rate are constant during beam heating, as they are for case (II), we find no evidence of f B increasing with P,,,,,, even at the highest injection power. In case (IV), on the other hand, plasma density is not held constant, but increases continuously under strong gas puffing. We have measured the line-average electron density along a path tangent to the outboard edge of the limiter-defined flux surface (fii,BDY), (i.e., the chordal path essentially lies between the wall and limiter). Under case (IV) conditions, we find that TiEDY/fi, = 0.25-0.35, and that TizDY increases as ii, increases. Although we cannot determine the density distribution along this chord, bolometer channels, which look lo-12 cm outside the limiter-defined flux surface, detect substantial radiation. Both iifDY and bolometer data suggests then that the plasma may extend well outside the limiter-defined flux surface. This edge and boundary density can seriously impede neutral beam particle 6. LIMITERS:

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penetration of the plasma boundary, and result in a greater fraction of the neutral beam being ionized (and promptly lost) there. [MCGO simulations, based on iiBDY chordal information in modelling the boundary, e are in good agreement with measured fa values in case

ml. 3.4. Large pT and limiter/bean&e

effects

In case (V) [fig. l(e)] we examine the effect that large values of pT have on beam confinement. The toroidal gyro-radius (- 7.5 cm at the primary limiter) is about 20% of the minor radius of the plasma. This means that beam ions born at r/u > 0.8 stand a very real chance of striking the limiter at some time prior to thermalization. The results for case (V) shown in fig. 2(b) support this. The values of fa range from 0.20 to 0.38. One peculiar feature we find in case (V) is the bimodal values of fa for Pn.,ru, = 2.3 MW. The A symbols represent beam heating from the 210 o injector only, while the v symbols represents beam heating from the 330 O beamline only. The plasma shots used in this comparison have similar plasma properties (e.g., beam injection, global and edge plasma density). The measured difference in fa is 0.10 + 0.04.

4. Discussion Our results indicate that a variety of factors contribute to energetic beam ion losses on the primary limiter. These factors include low B, and strong gas puffing in conjunction with a large separation between the primary limiter and vacuum vessel wall. Separating the plasma from the primary limiter, on the other hand, mitigates the fast losses to the limiter. (This suggests that diverted plasma should have less beam ion loss as well.) Since our data appears to be consistent with predictions based on guiding center orbit theory in which toroidal gyroradius effects are included, we expect that a lower plasma current would also result in greater losses to the limiter. Unfortunately, our data at present does not allow us to test this idea. Recall in table 1 that the radial (inward) excursion distance of 70 keV beam ions born on the outer plasma edge is - 25 cm for the cases presented. Because 25 cm is well over half the distance from the edge to the plasma center, we expect that the loss of these “edge” beam ions on the limiter might well affect plasma heating. Figs. 3(a) and 3(b) seem to bear this out. Fig. 3(a) shows the change in [(wr/Ir X K~/~)] plotted versus the change in PI,,,, where W, is the total stored energy and K is the plasma elongation. The “change” is calculated with respect to the ohmic timeslice. Because rP and K are generally fixed during beam heating, we are actually plotting a number directly proportional to the change in the stored energy as a

(a)

0.25f

2 f 0.20” 3 :

0.15-

0

CASE I

0 CASE II * CASE Ill 0 CASE IV

t-5 3 IJ?O.lO7 0.05 -

1 0

Oo

CASE I

1

2

3

4

5

6

Fig. 3. (a) The change in ( WT/Zptc’/*) is plotted as a function of change in PINPUT; (b) the change in ( W,/Z,K’/~) is plotted as a function of the change in I’,,,,, less the beam ion power deposited on the primary limiter.

function of the change in P,,,,,; the _‘pand K factors allow us to normalize the data for direct comparison of the five cases [7f. From fig. 3(a), case (I) has the best heating efficiency. Case (II) is only slightly less efficient. In general, beam heating cases for which fn is large lie farther below the case (I) line; those beam-heated plasmas for which fa is small fall closer to the case (I) line. Of interest also is the “ bimodal” example we have discussed for case (V); the beam heating efficiency from the 210° beamline is significantly more than the beam heating efficiency from the 330 ’ beamline. Fig. 3(b) is similar to fig. 3(a), except that the beam ion component, which is lost to the primary limiter, is subtracted from the AP,,, of fig. 3(a). In a sense, the curves drawn in fig. 3(b) represent a “real” beam heating efficiency for the plasma. With the beam loss component subtracted out, the other four cases are much more aligned with case (I) results. We also note that the “bimodal” behavior found in case (V) vanishes, when the additional beam ion losses in the 330° injection is taken into consideration.

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T. W. Petrie et al. / A single blade limiter

Our interpretation of this bimodal behavior is based on an enhancement of plasma density around the limiter (330 beamline) due to recycling. The increased edge plasma density around the limiter in turn causes the beam particle deposition to be skewed more to the outboard side of the plasma for the 330° beams than for the 210“ beams. Thus, a higher fraction of beam particles launched from the 330’ beamline will pass in the vicinity of the limiter and they will be lost. We can, in fact, model this beam specific result both quantitatively and qualitatively, with a by Monte Carlo code analysis, if we assume that there is more density in front of the 330’ beamline than the 210 o beamline. Chordal measurements of plasma density adjacent to the 330 ’ beamline (and primary limiter) show substantial density between the limiter-defined flux surface and the vacuum vessel wall. In modeling the 330° beamline heating, we have assumed a constant density between the limiter-defined flux surface and the wall. The absolute value of this density is determined by a pathlength-corrected interferometer chord, which lies almost entirely in this boundary region. In modeling the 210 ’ beamline heating, we have assumed that the plasma density is insignificant 3 cm beyond the limiter-defined flux surface. Because the diagnostics on Doublet III cannot measure edge density at several toroidal locations, we have had to rely on indirect ways for determining the existence of an enhanced density near the primary limiter. Detailed power balance measurements which were done on ohmic plasmas have shown significant radiated power localized within 30 ’ (toroidally) of the limiter [5]. Both bolometer and infrared foil measurements have found that radiation fluxes near the limiter can be several times the radiation fluxes measured at locations far from the limiter [6]. This radiation asymmetry is more pronounced at high density. In addition we have found that at high plasma density achieved with strong gas puffing, power flow to the primary limiter falls sharply [5]. We believe that a dense, cold plasma dissipates incoming power either by recycling/impurity radiation or by charge-exchange with cold ions. A photograph of the primary limiter is shown in fig. 4. The ion drift side is to the right, while the electron drift side is to the left. Although configuration-dependent, most plasmas will interact with the limiter somewhere from about the middle of the third tile (from the top) to about the middle of the fifth tile (from the top). In this region of the limiter, the ion drift side shows considerably more wear than the electron drift side. This is not surprising, because the co-injected beam ions which collide with the limiter will preferentially strike the ion drift side. We have also observed the sudden appearance of strong local heating several centimeters below the point of plasma/limiter contact. This heating is associated with the turn-on of the 330° beamline. The location of

Fig. 4. The photograph shows the six tiles comprising the primary blade limiter. The ion drift side (to the right) shows much more wear than the electron drift side on the third, fourth and fifth tiles down from the top. This damage is believed due in part to energetic beam ion collisions with the limiter.

this heating is consistent with that predicted by guiding center orbit calculations made for the 330 ‘-injector beam ions born near the plasma edge. We suspect that much of the wear on the fourth and upper part of the fifth limiter tiles may be due to these 330 O-launched ions. The beam ion flux on the limiter can be substantial under certain conditions. For example, during the strong gas puffing (and high density) conditions of case (IV), we estimate an average beam ion heat flux on the ion drift side of the limiter of 3.5 kW/cm2. This measurement was made when both the 210 o and 330 o beamlines were injecting - 4.5 MW into the plasma. Because this heat flux was present for only about 100 ms, measurable damage to the limiter was apparently averted. However, if fluxes of this magnitude were to continue for several seconds, thermal stress and vaporization of the limiter would be problematic. Consequently, in preparing for the long pulse, high beam power experiments which are planned for next generation tokamaks, one should avoid operating regimes which generate significant beam-generated power loading on the limiter. One should also try to position the limiter as far away from the beam injection ports as possible.

Acknowledgement This work was supported by the US Department Energy, Contract DE-ATO3-84ER51044. 6. LIMITERS;

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The authors gratefully acknowhzdge the assistance of S. Ejima, E. Fairbanks, G. Johns, A. Bellman, D. Overskei, R. Stockdale, T. Taylor and E. Zawadski. The authors also thank C. Danielson and H. Lofstedt for help in preparing the manuscript.

References

111 T. Hino et al., to be published in J. Nucl. Mater. (1984). PI T. Petrie et al., to be published in J. Nucl. Mater. (1984). [31 T. Petrie et al., Bull. Am. Phys. Sot. 28 (1983) 1122. and G. 141 H. St. John, R. Harvey, F. Marcus, C. Armentrout Bramson, Bull. Am. Phys. Sot. 27 (1982) 1059. [51T. Petrie, C. Hsieh, W. Pfeiffer and R. Snider, Bull. Am. Phys. Sot. 27 (1982) 960. PI T. Petrie, M. Mahdavi and L. Rottler, to be presented at the 1984 IEEE Conf. on Plasma Science, St. Louis, MO, May 14-16. I71 K. Burrell, R. Stambaugh et al., Nucl. Fusion 23 (1983) 533.