The evolution of plasma parameters as governed by edge phenomena during Ion Bernstein Wave (IBW) heating

Journal

of Nuclear

Materials

616

145-147

(19X7) 61h- 620

North-Holland,

THE EVOLUTION OF PLASMA PARAME ‘ERS AS GOVERNED DURING ION BERNSTEIN WAVE (IBW) I EATING * J.R. WILSON, G.J. GREENE, S. SUCKEWER Pimma

R. BELL, A. CAVALLO, P.O. COLESTOCK. R. KAITA, D. McNEILL, E. MAZZUCATO, and A.W. WOUTERS

Physics Lahorulo~.

Key words:

heating

Princeton

Uniuersrty. Princeton.

Amsterdam

BY EDGE PHENOMENA

S.A. COHEN, J. HOSEA, M. ONO, K. SAT0 **T

NJ 08540, USA

RF waves, ICRF

IBW heating has been investigated in the PLT tokamak and compared to conventional fast wave ICRF heating. While central temperature rises of comparable magnitude are observed for both methods, several differences in plasma behavior, particularly in the plasma periphery, are observed. An increase in plasma density is observed in both ICRF and IBW heating. This increase, however, is larger for IBW heating at comparable power and is accompanied by a decrease in recycling as opposed to the increase observed during fast wave heating. This phenomenon is interpreted as an increased particle confinement time. A decrease in carbon VI line intensities is observed only during IBW heating. This decrease can be partially due to direct acceleration by the RF waves and partially to neutral attributed to the reduction of C5+ ion concentration density reduction. The increase in heavy metal impurities during fast wave heating is consistent with the increased sputtering expected from the enhanced recycling, however, this explanation cannot be used for the increase during IBW heating since recycling is reduced. Instead, impurity accumulation due to the enhanced particle confinement may be responsible.

1. Introduction Heating of fusion plasmas via the external launching of Ion Bernstein Waves (IBW) has been suggested as an attractive alternative to conventional fast wave ICRF [1,2]. Several differences in the wave physics of the two methods could lead to different properties of the heating. The short wavelength of the IBW (k I pi - 1) results in heating of the bulk ion population and does not produce ion tails. Higher harmonic heating is also possible allowing the use of vacuum waveguides of reasonable dimensions. In this report a comparison of plasma properties during fast wave ICRF and IBW heating under identical conditions in the PLT tokamak is given. Differences in the behavior of the recycling, density rise, impurity behavior and ion tail formation are observed indicating that edge phenomena in particular are not the same for the two methods.

tric field. With the inhomogeneous plasma density and magnetic field structure characteristic of the tokamak. a coupling between the two wave polarizations is expected [3]. RF magnetic problem measurements in the plasma periphery indicate that this cross contamination is - 10% in these experiments. The design of the antennas are identical in terms of materials and distance from the plasma so that differences in surface effects

‘Oa-l

2. General experimental results IBW waves are launched into the PLT tokamak using two parallel loop antennas, displaced poloidally, which carry RF currents in the toroidal direction. The current loops are covered with a stainless steel Faraday shield which is aligned to cancel the poloidal component of the RF electric field. This is the orthogonal polarization to the conventional fast wave ICRF antenna, which has a poloidal RF current loop and a Faraday shield designed to cancel the toroidal RF elec-

DI (IBWI TD= I.1 keV

251 0

\

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I

20

30

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IO

ENERGY

* This work was performed

under

AC02-76-CHO-3073. ** Permanent Address, Japan.

Institute

0022-3115/87/$03.50

0 Elsevier

(North-Holland

Physics

DOE Contract

of Plasma

Publishing

Science

Physics,

No. DENagoya,

Publishers

Division)

40

i keV)

B.V.

Fig. 1. Spectrum of charge exchange neutrals heating with IBW and fast wave. _1RF=160kW, P cm

for H-minority 13 n,=1.5XlO

617

J.R. Wilson et al. / Ion Bernstein waue heating can be attributed to the difference in antenna polarization and wave physics. For purposes of direct comparison with fast wave ICRF the experiments have been performed at the same frequency, 30 MHz, and in the same heating regimes, fundamental hydrogen or helium3 minority in deuterium plasmas and three-halves harmonic heating in deuterium. The central deuterium bulk temperature increases for the two methods are identical when the plasma density evolution is programmed to be the same. In order to achieve this last condition gas puffing during the ICRF heating to match the density rise that occurs during IBW heating. While the increase in the deuterium temperature is the same, the behavior of the hydrogen minority is completely different as can be seen in fig. 1. For these fast wave ICRF experiments the RF wave energy is first deposited in the hydrogen minority and then distributed to the bulk ions and electrons through collisions. In IBW heating the RF wave energy is expected to be absorbed directly by the bulk ions with little absorption by the hydrogen [4]. The high energy tail observed by charge exchange during fast wave heating is not present during IBW confirming this picture. The small remaining nonthermal component is consistent with the 10% contamination of polarization mentioned above. 3. Density

rise behavior

The time evolution of the plasma parameters is shown in fig. 2 for 330 kW of IBW heating. A large density rise from E, = 1.9 x lOI cmm3 to 2.9 X 1013 cmm3 is observed. The dependence of this density rise on RF power is shown in fig. 3. The density rise is linear with power for powers up to - 250 kW and then saturates at 66, = 9 X 1012 cme3. Fast wave ICRF produces a denI

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Density

ii, (P,f=O)=2.1

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200 P,f (kW)

100

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I

300

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400

Fig. 3. Density rise versus power for IBW heating in 3He minority mode. sity rise of - 2 x 1012 cmm3 at P,, = 500 kW under identical conditions. It is curious that a saturation in density rise for fast wave heating is observed at P,, - 2 MW [5]. THis leads to the speculation that the density rise during ICRF is due to the > 10% IBW contamination of the wave polarization. However, this convenient explanation is contradicted by the different behavior of other parameters associated with the density rise. The time behavior of the H, emission for the two methods is shown in fig. 4. The density rises in both cases are identical. The IBW rise is seen to be accompanied by a decrease in the H, emission while that for ICRF is accompanied by an increase in H, emission. Gas puffing was required during the ICRF to match the IBW density increases; however, even in the absence of gas puffing a decrease in H, is never seen during fast wave heating. H, light is monitored at several toroidal locations including both the limiter and antenna. The drop in H, light is seen everywhere although it is not as pronounced when viewing from behind ther antennas. The behavior of the low energy (< 1 keV) neutral particle efflux is shown in fig. ’ This flux is measured utilizing a time of flight low

R------l

400

Fig. 2. Time evolution of electron and ion temperature and electron density for P,, = 330 kW.

600 TIME(msec)

800

Fig. 4. A typical time evolution of the H emission during fast wave and IBW heating.

1000

intensity

618

J. R. Wdsm7 et ui. / Ion Bernster,7 wu~ hecmp

recycling), is then apparently due to an increase in particle confinement time. Thomson scattering measurements of the density profile (fig. 6) show that the density increase is centrally peaked. A similar improvement in confinement time is observed during lower hybrid experiments on Versator II [7]. 4. Impurity behavior

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I

, ti5%w~4

I

200

400

I

I

1

600

600

TIME (msec) Fig. 5. Low energy ( < 1 keV) neutral flux for IBW and fast wave heating.

energy neutral spectrometer which is located well away toroidally from both limiter and antenna [6]. A large decrease in the neutral efflux during IBW is seen while no reduction is observed for ICRF. This decrease in flux is accompanied by an increase in the average energy of the neutrals. In addition, a reduction in the level of charge exchange signals and spectroscopic lines whose amplitude is sensitive to neutral density support the conclusion that the neutral particle source is reduced during IBW in spite of the density increase observed. This density increase, unlike that observed during ICRF (which can be explained by enhanced I

I

I

I

I

I

During IBW heating, plasma impurity behavior has been monitored by various spectroscopic instruments. One of the quantities which characterizes the plasma impurity level is the effective Z. This quantity can be readily monitored by the so-called Z-Meter [8] which computes Z,,, in the central part of the plasma through the measurement of the Bremsstrahlung radiation intensity, plasma density, and electron temperature. This relatively simple measuring technique allows continuous monitoring of the plasma Z,,, during the heating experiment. The values obtained generally fall between Spitzer and neoclassical Z,,,. During the injection of IBW power up to 400 kW (the maximum power thus far injected), the Z,,, remains about the same level as in the ohmic discharge before the RF pulse. Over this range of density Z,,, is usually constant in ohmic discharges in PLT. However, there is an interesting variation in Z,,, observed during IBW heating when the toroidal magnetic field is varied. The change in the toroidal magnetic

200

I

P

I

,

I -20

I

I 0 RADIUS

I 20

205

I 40

(cm)

Fig. 6. Density profile before and during IBW. P,, ‘He minority IBW heating.

= 330 kW.

2 I7

228

24

251

rf

2 62

= 200

kW

2 74

BT’ T 1 Fig. 7. Z-effective versus Br. Z-effective being normalized to the pre-RF ohmic Z values.

J. R. Wilson et al. / Ion Bernstein waue heating field causes a significant change in the IBW wave propagation physics. In fig. 7, the measured Z,,, during IBW heating, normalized to the pre-injection ohmic value is shown, as a function of the toroidal magnetic field. The Z,,, shows a relatively large change. In the insert shown above the graph, the relative ion cyclotron resonance positions for the respective field values are shown. In general, good IBW launching occurs when a cyclotron resonance of the majority species is located behind the antema (located in the low field region of the plasma) as shown in the cases for lower and higher fields. Z,,, in these cases shows very little change. Since in these cases the next cyclotron resonance occurs well inside the plasma, good ion heating also occurs. For the off-resonance case where the central cyclotron resonance has moved out toward the antenna and no resonance exists just behind the antenna (as shown by the intermediate field case) the rise in Z,,, is significant. In this regime ion heating is also relatively poor. We, therefore, find that good IBW launching conditions are important for impurity control as well as for a good ion heating. 5. Carbon impurity behavior The behavior of various impurity lines during IBW heating is of some interest. The low ionization states such as C II and C III show a general increase of

CARBON

IMPURITY

619

- 50% during IBW heating. This rise can be interpreted as a rise in the carbon influx into the discharge. However, higher ionization states, such as C V and C VI, show a very different behavior. As can be seen in fig. 8, the estimated ion density from C V and C VI radiation show a drop during IBWH. There are several possible explanations for the observed reduction in the carbon ions. Since the electron temperature profiles show little change during IBW heating, a simple temperature profile effect on the line radiation can be ruled out. One possibility is a selective enhancement of the carbon ion particle transport by the RF field. However, so-called RF pump out be resonant interaction of the IBW wave field with the carbon ion cyclotron resonance does not fully explain the observation since the reduction does not correlate well with the location of the carbon ion resonance. The large reduction in the neutral outflux during IBW heating observed by the LENS measurement as described above may be playing a significant role here [9]. The C VI line at 520 A (n = 4 + 3) which is neutral sensitive, shows a very large decrease in intensity. Although the 33 A (n = 2 -+ 1) line is insensitive to the neutrals, the large reduction in neutral density in the plasma may cause an actual reduction in the C5+ ion population since the population of this state from charge exchange of C 6+ is a strong process. Further detailed modeling work would be required to assess this possibility. The reduction in the C4+ ion population may be an indication that the reduction is not fully due to the neutral particle effect. 6. Metallic impurity behavior

IBWH 300

I 400 TIME

I 500

I 600

I 700

(MS)

Fig. 8. Time evolution of carbon spectroscopic emission intensity normalized to the plasma density for lines representative of various ionization states.

The metallic impurity level shows a general increase during IBW heating. The main metallic impurity is iron. The fractional increase of iron during IBW heating may be as large as 50-100 %. The obvious source of the iron is the Faraday shield which is made out of stainless steel. This is also evident from the increase in the neutral chromium line emission from the antenna surface during IBW experiments. A better Faraday shield design or elimination of the Faraday shield by using a waveguide launcher should be able to reduce the iron influx significantly. Other metallic impurities such as titanium show a smaller increase. Overall, the observed constant Z,,, during IBW heating may be a result of the reduction in carbon impurities compensating the increase in iron impurities. An additional unknown is the behavior of impurity ion concentrations due to the improved particle containment time, which might lead to accumulation of impurities as opposed to an increased source term. 7. Summary Significant differences in plasma behavior, density rise, H, emission, and impurity behavior are observed

when comparing fast wave ICRF and IBW heating under identical conditions. An increase in particle containment time is observed during IBW heating. A reduction in C 5 + ion population is also observed which may be due to RF pump out or due to neutral density reduction. The density rise during IBW experiments at low powers is significantly larger than for fast wave ICRF but saturates above 250 kW. References [l] S. Puri, Phys. Fluids 22 (1979) 1716. [2] M. Ono. Phys. Rev. Lett. 45 (1980) 1105

[3] F.N. Skiff, P.L. Colestock and M. One. Phy\. Fluids 2X (1985) 2453. [4] M. Ono, <;.A. Wurden and K.L. Wang. Phys. Rec. Lctt. 52 (19X4) ?7. [5] S.C. Luckhardt et al.. Phys. Fluids 29 (1986) 6. [6] E. Mazzucato et al.. 10th Conf. on Plasma Physics and Controlled Nuclear Fusion Research, London. Vol. I (19X4). p. 433. [7] D. Voss and S. Cohen Rev. Sci. Instr. 53 (19X2) 1696. [X] K. Kadota. M. Otsuka and J. Fujita. Nucl. Fusion 20 (19X0) 209. [9] P.G. Carolan and V.A. Piotrowicz. Plasma Phys. 25 (19X3) 1065.