Flow Measurements in the Divertor Region of DIII-D and Plasma Characterization using a Reciprocating Probe

Transcription

1 Flow Measurements in the DIII-D Divertor Flow Measurements in the Divertor Region of DIII-D and Plasma Characterization using a Reciprocating Probe J. Boedo, R. Lehmer, R. Moyer, J. Watkins, D. Hill, T. Evans, A. Leonard, C. Lasnier, R. Maingi, M. Schaffer QTYUIOP

2 Motivation Flows in the divertor region are important because they are involved in: Power flow to the target plates. Impurity transport. Particle transport. Particle and heat removal from the divertor area. Researching the private region and its boundary with the divertor leg and the X-point is important because: It affects divertor design, baffled and non-baffled. The interaction of the core/divertor leg/private region affects all of the above zones but a lot of the underlying physics is not yet clearly understood.

3 Results Measurements show that a the plasma flows at Mach 1 over most of the outer leg in a radiative divertor, these flows carry most of the heat to the plates by convection. Reverse flow has been observed at the separatrix between the private region and the divertor leg, consistent with measured radial electric field. Convected power deposition at the plate for attached a detached divertor is compared to that measured with an infrared camera. Steep gradients in plasma potential have been observed at the transition between the private region and the inner and outer legs. These electric fields should drive poloidal flows.

4 Attached vs Detached Plasmas

5 Geometry of the measurement Outer Leg Probe The divertor has been explored by moving the plasma so that the various regions of interest can be probed.

6 Flow Measurements in the DIII-D Divertor Mach Number Mach Number vs Height from Floor Discharge Attached Discharge Dettached Outer Leg A radiating divertor is able to generate high Mach # flows over a large section of the divertor Z(cm) These flows can convect heat in situations when conduction is not efficient.

7 Density Profiles Density vs Height from Divertor Floor Discharge Attached Discharge Dettached Outer Leg N e x10 12 (cm -3 ) The dettached plasma features high density (1E14 cm-3 vs 2E13 cm-3 ) which is fairly constant along the leg Z (cm)

8 Temperature profiles Temperature vs Height from the Divertor Floor Discharge Attached Discharge Dettached Outer Leg The temperature drops due to the enhanced radiation losses. T e (ev) Z (cm)

9 e q = κt 5 / nv [ ( T + T ) + mv + I ] II Parallel energy transport Parallel energy transport is classically given by: e Conduction dt ds II 2 e i 2 II Convection In detached plasmas conduction is poor due to flat temperature profiles. Convection (Terms 1, 2, 3) is dominant. In detached plasmas, the parallel velocity is high 2-4E6 cm/s and the density is very high (1E14 cm-3), thereby heat removal is performed by term 1, UNLESS Te is very low so the ionization energy I 0 (~13.6eV) becomes significant and term 3 dominates. 0

10 Convected heat flux measurement Heat Flux to Divertor Floor from IR camera Heat Flux (W/cm 2 ) Private Region Discharge Dettached Discharge Attached Separatrix from EFIT Divertor Leg Convected heat flux is 80% of the total for dettached plasmas. Conected heat flux is 30% of the total for attached plasmas Radius (cm) Probe

11 UEDGE calculations have shown that flow reversal is possible at the outer strike point and along the separatrix towards the X point. We have observed this behaviour experimentally.

12 Geometry of the measurement Private Region to Outer Leg In this configuration, the private region is sampled first and the separatrix is crossed at 5 cm

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14 Radial electric field is measured across the separatrix Vfloat (V) Floating Potential (V) Private Region Discharge t= 3440ms Z (cm) Plasma Potential (V) Outer Leg from EFIT Outer Leg Private Region to Outer Leg Vplasma (V) A radial electric Field of 20 V/cm is observed at the boundary between the private region and the divertor leg. The electric field induces poloidal flows of 2.0E5 cm/s towards the X point.

15 Reversed flow is measured along the separatrix Discharge t=3440ms Mach Number Flow Velocity (cm/s) Outer Leg from EFIT Private Region to Outer Leg Mach Vflow(cm/s) The Mach probe shows that flow reverses at the boundary between the private region and the outer leg Z(cm)

16 Pressure increases across the separatrix 350 Discharge t=3440 ms Density Temperature Outer Leg from EFIT 50 Private Region to Outer Leg 300 n e x10 12 (cm -3 ) T e (ev) Pressure increases towards the boundary between the private region and the outer leg and then stays constant Z(cm)

17 Plasma flow drags the impurities (carbon) towards the X-point. Thereby, understanding the flows has implications for impurity transport and core contamination. Reverse flow is needed to explain the measured carbon radiation distribution at the divertor (T. Evans)

18 Attached divertor plasma

19 Geometry for inner leg and private region measurements

20 Mach number at inner leg towards the floor Mach # Mach # and Flow Velocity vs Height from Floor Discharge Private Region Mach Vflow(cm/s) Separatrix from EFIT Inner Divertor Leg Z (cm) Vflow (cm/s) The plasma flows towards the floor as expected and no flow reversal (but a flow reduction) is seen at the inner separatrix for this discharge

21 Temperature and Density profiles T e and N e vs Height from the Floor N e x (cm -3 ) Private Region Density Discharge Inner Divertor Leg Temperature Te (ev) The temperature and density gradients in the boundary between the private region and the inner leg are very large Z (cm)

22 Plasma potential shows transition from private region to inner leg V float (V) Floating and Plasma Potential vs Height from the Floor Discharge V V float plasma Vplasma (V) Large gradient in the plasma potential marks the transition from private region to the inner leg. Poloidal flow towards the floor is driven Z (cm)

23 A basic treatment and equations for the interpretation of Mach probe data are found in K.S. Chung and Hutchinson Phys Rev A Vol 38, No 9 (1988). The sound speed is given by: c s = (T e + T is )/m i For normalized drift velocities, defined as; V s = ZT e m u = v V s The ratio of upstream and downstream current densities is: R J up J down = e Ku d where K=1.9, 1.7, 1.3 for Ti=0.2Te, 1.0Te, 2.0Te. In our case we will consider Ti=Te and therefore K=1.7.

24 3D carbon fiber center support rod Mach pin 1D carbon fiber BN insert pyrolytic washer stack Tip Areas: Mach Up: cm2 Mach Do: cm2 Double: cm2 pyrolytic graphite washer stack with BN interlayers inside 3 B t divertor floor 6 stack compression spring Boron Nitride spring cover Insulating mechanical interface fast drive tube outside height = Double Probe #1 Double Probe #2 Vf pin 3 pin 7 pin 6 pin height end radius /60 rod radius / / /18/95

25 There are many discharges without useful upstream Mach pin data, so we can use the envelope of the double probe current signal, which is equivalent to a full pin Isat. The total current collected by a fully exposed pin can be written as a sum of the downstream and upstream current. I envelope double = I up + I down The downstream current can be writen in terms of the Mach tip signal as: I down = I Mach A d A Mach = J Mach A Mach A d A Mach thus the current densities are: = J Mach A dp J double 2 A d = J up A d + J Mach A d Finally we obtain: J up = 2 J double J down which implies R 2 J double J down J down = e Ku d and the Mach number can be writen as: M = (1 / K )ln( 2 J double J down J down )

26 The area of the top of the probe is (round with 1mm radius) 3.1E-6 m2. One can estimate the radial fluxes using Γ r = D r r n For a D of 1 m2/sec (for L mode) and m2/sec (H-mode) and density gradients of 1E19 and 1E20 m-4 respectively we get a current of: ma Another alternative is to use the turbulent fluxes directly. The turbulent flux is of the order of 15E20 10^20 /m2/sec for L-mode (worst case), producing a current of: I = q Ä Γ r A r = q Ä Γ r πr 2 or ma, to be compared to a minimum of 40mA to 4 A measured currents. We therefore conclude that the effect of the radial currents on the pin caps are negligible

27 Does the probe perturb the plasma? hp=0.6 mm h=1.77 mm Γ Γ Γ = D n3dl II Γ II Γ II = 0.5nc s d 2 Γ L II = c s d2 6D For DIII-D Lpar=1 cm!!! As long as Lfloor>Lpar, life is good!

28 Conclusions For detached or semi-detached divertor Plasma flows at Mach 1 over an extended region in a radiative divertor. Heat removal is then predominantly done by convection over a large volume. The detached divertor features a continuosly accelerating plasma towards the plate, in agreement with the classical model. For attached divertor Flow reversal at the separatrix between the private region and the divertor leg has been observed in highly radiating plasmas