Divertor Detachment on TCV R. A. Pitts, Association EURATOM-Confédération Suisse,, CH- LAUSANNE, Switzerland thanks to A. Loarte a, B. P. Duval, J.-M. Moret, J. A. Boedo b, L. Chousal b, D. Coster c, G. Gunner b, J. Horacek d, A. S. Kukushkin e, D. Reiter f and the TCV Team a EFDA-CSU, Max Planck Institut für Plasmaphysik, D-878 Garching, Germany b Fusion Energy Program, University of California, San Diego, CA 993-7, USA c Max-Plank-Insitut für Plasmaphysik, Boltzmannstr., D-878, Garching, Germany. d IPP, Academy of Sciences of the Czech Republic, Za Slovankou 3, POB 7, 8 Praha e ITER Joint Central Team, Garching Joint Working Site, Boltzmannstr., D-878, Garching, Germany. f Institut für Laser und Plasmaphysik, Heinrich-Heine-Universität Düsseldorf, Universitätsstr., D- Düsseldorf, Germany
Outline Introduction - TCV and divertor configurations Experiment - divertor detachment Simulation - results so far from B-Eirene High Langmuir probe T e s
TCV design parameters R =.88 m a =. m κ = 3 (max.) B T =. T (max.) I p =. MA (max.) Tokamak à Configuration Variable Designed to produce a large variety of plasma shapes. Achieved so far: κ =.-.8 δ = -.8-. I p =. MA
Variable Plasma Shapes
Plasma Shaping and Control Plasma Shaping: separately controlled poloidal field coils. Vertical Stabilisation: passive: conducting vessel wall (decay time of m= vessel currents ~8ms). active: feedback control with fast coils inside (response time <.ms) and slow coils side (response time~ms) the vacuum vessel. Highest elongation: κ =.8 (at I P =7kA). Highest current: I P = MA (at q 9 =.). [F. Hofmann, et al., Nucl. Fusion 38 (998) 399]
Electron Cyclotron Heating System.8 ECH at second harmonic (8.7GHz): gyrotrons, each kw. equatorial and upper lateral ports. Steerable mirrors for poloidal (during a shot) and toroidal (between shots) scanning. Cut-off density (X-mode): n e =.x 9 m -3 ECH at third harmonic (8GHz): gyrotrons, kw, so far - lateral launch. For, 3 gyrotrons, top launch. Cut-off density (X-mode): n e =.x 9 m -3.......8....8....8 Ip=8kA,q=./.8,k=./.,d=./.,li=.7 y.. An Example of The Toroidal Coverage of the X RF Beams Sector UL Sector UL Sector EQ Sector EQ Sector 7UL L3 L L.. Sector UL Sh..8.......8 x
Divertor Configurations for Detachment Study the influence of er divertor geometry on characteristics of detachment Note high poloidal depth of LFS divertor and short depth at HFS. Horizontal er target, vertical inner target. Bx B drift direction away from X-point. Ohmic discharges only. Midplane to er target connection length m Inner Target Probes #78 f exp =.8 7 cm Reciprocating Probe #783 f exp =. #783 f exp = 9.3 Outer Target Probes
Single Null Diverted Equilibria #,.8 s I p = 3 ka δ=.3, κ=.3 z Xpt = 7 cm L cm L cx in L cm f exp = m = m = 8 =.8 #3,.8 s I p = 3 ka δ=.3, κ=.3 z Xpt = 7 cm L cm L cx in L cm f exp = 3 m = m = m =. #7,.8 s I p = 3 ka δ=.3, κ=. z Xpt = 7 cm L cm L cx in L cm f exp = m = m = 3 m =.9 #8,.8 s I p = 38 ka δ=.3, κ=.3 z Xpt = 8 cm L cm L cx in L cm f exp = 9 m = m = m =. #,.8 s I p = ka δ=.33, κ=.83 z Xpt = 8 cm L cm L cx in L cm f exp = 3 m = m = 3 m = 7.7 #7,.8 s I p = 3 ka δ=.3, κ=. z Xpt = cm L cm L cx in L cm f exp = 39 m = 3 m = m =.
Typical Density Ramp Discharge Detachment studied so far only in ohmic L-mode density ramp discharges. Mostly at I p = 3 ka to avoid ohmic H-mode transition. Detachment observed in density ramp discharges: at max density: TOT P R /P Ω.8,.n GR. n e Density ramp starts already at high values (otherwise locked modes at divertor formation) er divertor already probably in high recycling. 8.8... 8 n e [ 9 m -3 ] P R [kw] P R TOT P Ω D α (div) [au] #78 Z eff (X-rays) P R DIV Inner divertor never detaches except near separatrix at highest. n e j sat [Acm - ] LP# LP#....8.
Divertor Langmuir Probes 3 probes in the central column tiles, in the flat part of the vessel floor. Cylindrical, Ø. mm, with spherical tip protruding. mm above the tile surface. Clearance to tile,. mm, material, polycrystalline graphite. Acquisition at khz for selected probes, khz for most. Probe voltage sweep typically at Hz using programmable waveform to give best resolution in the vicinity of V f. Floor Central Column
Visible CCD Camera Image Reconstructions.8..... Height...3... Reconstruction Grid..7.8. Toroidal fan of CCD chords Poloidal projection of pencil beam trajectories - camera position effectively divides the FOV.7.8.9. R Iterative non-negative least squares, y:data, T: transfer matrix, x: inverted data ( y Tx)
Reciprocating Probe Seen from inside torus Current orientation 3 B On loan from UCSD - Thank you!!! Fully integrated into TCV control system - Vsystem live database. Pins & 3 floating (k tip resistors). Pins,, operated as swept single Langmuir probes. Voltages applied w.r.t. vacuum vessel.
Reciprocating Probe - Example of Raw Data 8 Pin V f [V] Pin j [Acm - ] Pin V f [V] Fast Position [V] Pin j [Acm - ]....7.8.9.. Pin V [V].98.98.99.99......3 n e [ 9 m -3 ] 8....8.
Total Radiation Reconstructions - Density Ramp t =.s t =.s t =.7s t =.9s t =.98s t =.s t =.s Divertor very cold early on in the density ramp. Radiation quickly concentrated around the X-point. n e [ 9 m -3 ] 8 #783 Density Ramp....8. j sat [Acm - ] LP#....8.
Total Radiation Reconstructions - Low Density t =.s t =.9s t =.7s n e [ 9 m -3 ] P R, P Ω [kw] 3 #783 Low density reference case P Ω P R DIV P R TOT....8. j sat [Acm - ] Z eff (X-rays) 8.9.8.7. LP#.....8. Low density reference case - er divertor in low recycling - less radiation inside separatrix at the X-point.
Outer target Inner target Profiles of Target Ion Flux Outer target Inner target Outer target Inner target Total Current [ka] j sat [Acm - ] Separatrix distance at the target [cm] 3... #78 f exp =.8 7 8 9 n e [ 9 m -3 ] O Outer Target + Inner Target Separatrix distance at the target [cm] 3... #783 f exp =. 7 8 9 Separatrix distance at the target [cm] 3... #78 f exp = 9.3 7 8 9 n e [ 9 m -3 ] n e [ 9 m -3 ] Extent of partial detachment sensitive to extent of flux expansion. Clear signs of probe shadowing by neighbouring tiles as f exp increases (angle )
Degree of Detachment (DOD) DOD n e Isat( ) div I sat DOD n e div sep the probe ion saturation current to the divertor targets. Describes the extent to which the -point model scaling Γ n e is obeyed. Detachment at er separatrix more marked for low f exp. Integral DOD s increase with increasing f exp. Small (%) increase in P Ω has a noticeable effect. div I sat Degree of Detachment I p = 3 ka O Ι Ι in j (sep) j in (sep) #78 #783 I p = 38 ka #787 #789 7 8 9 #78 #783 7 8 9 3 n e [ 9 m -3 ] n e [ 9 m -3 ]
Visible CCD - D α ( nm) - #78 t =.8 s t =. s t =. s t =.8 s 3 n e [ 9 m -3 ] 8....8. j sat [Acm - ] LP#....8.
Visible CCD - D α ( nm) - #783 t =.8 s t =. s t =. s t =.8 s 3 3 n e [ 9 m -3 ] 8 j sat [Acm - ] LP#....8.....8.
Visible CCD - D α ( nm) - #78 t =.8 s t =. s t =. s t =.8 s 3 n e [ 9 m -3 ] 8 j sat [Acm - ] LP#....8.....8.
Visible CCD - CIII ( nm) - #783 t =. s t =.8 s t =. s t =.8 s 3 3 n e [ 9 m -3 ] 8....8. j sat [Acm - ] LP#....8.
B-Eirene Modelling Ion Flux onto Outer Divertor [s - ] 8... 3 3. n sep [ 9 m -3 ] 3 Recombination Sink [s - ] Recombination [%] 8... 3 3. n sep [ 9 m -3 ], #78, #783, #78 P SOL =.3 MW D, χ =.,.9 m s Y chem = 3.%, R = Z eff.8....3........ 3 3. n sep [ 9 m -3 ] Radiated Power [MW] Preliminary B-Eirene modelling by A. Loarte, EFDA-CSU Garching. Start at lowest density in the ramp. Fix P SOL, adjust Y chem to get carbon radiation consistent with the measured target ion fluxes. Choose D, χ for approx. match to upstream profiles. Unable to obtain anything like enough recombination via and 3-body processes with artifically increasing the rate coefficients by at least a factor.
B-Eirene and Experimental Profiles Compared Experimental upstream and downstream profiles in reasonable agreement at low density. B-Eirene predicts low T e even at lowest densities in the ramp target probes probably unreliable except for j sat. Clear profile broadening as detachment proceeds. Approx. optimisation for lowest density in ramp - needs more work - in progress at CRPP. n e T e [ 9 evm -3 ] T e [ev] n e [ 9 m -3 ] n e [ 9 m -3 ].. 3. 3. 3.....8. 3 #783 #783 RCP (Pin ) Outer Target Inner Target... n e [ 9 m -3 ] 3 8....8. B: midplane B: er target Midplane separatrix distance [cm] Wall radius at er midplane 3
B-Eirene: Degree of Detachment Ion Flux onto Divertor [m - s - ] 3.... x 3 B, n sep =.x 9 m -3 B, n sep =.7x 9 m -3 B, n sep = 3.7x 9 m -3.3x 9 m -3 7.9x 9 m -3.x 9 m -3.x 9 m -3 #78 n e Expt. Separatrix Distance at Outer Target [cm] Good quantitative agreement between code and experiment for DOD s except for er target separatrix detachment, even with factor increase in recombination rate coefficient. Target profile shapes not matched. Degree of Detachment #78 #783 #78 Ι Ι in j (sep) j in (sep) 7 8 9 n e [ 9 m -3 ] 3 n sep [ 9 m -3 ]
B-Eirene: Radiation Distributions- CIII n sep =.8x 9 m -3 n sep =.9x 9 m -3 n sep =.x 9 m -3 n sep =.x 9 m -3 n sep =3.x 9 m -3 n sep =3.7x 9 m -3 Photons m -3 sr - s - CIII TCV-78- CIII TCV-78- CIII TCV-78-3 CIII TCV-78- CIII TCV-78- CIII TCV-78- Good agreement in general with location and movement of CIII radiation distribution as plasma density increases. But C + location does not tell us from where the carbon source originates...
B-Eirene: Radiation Distributions- D Ecole Polytechnique α Fédérale de Lausanne Compare reconstructed measured emissivity for varying flux expansion at same er target integral DOD to isolate geometry effects. B-Eirene runs compared for cases with similar level of total recombination. Reasonable agreement (for increased recombination rate coefficients). Code fails to reproduce radiation max. away from strike point at high flux expansions. n sep =3.7x 9 m -3 n sep =3.x 9 m -3 n sep =.9x 9 m -3 D α D α D α Photons m -3 sr - s - TCV-78- TCV-783-9 TCV-78-7 #78, t =.8s #783, t =.s #78, t =.8s
Why do DIvertor Probes Measure T e too High? #783 Time: ms Time: 7 ms Time: 9 ms Fitted Probe Temperatures I [ma] I [ma] I [ma] I [ma] I [ma] I [ma] coll coll coll coll coll coll I sat = 89 ma E /I =. sat sat T =. ev e V = V fl I sat = ma E sat /I sat = NaN T e = 3 ev V fl = 9.8 V I = 83 ma sat E /I =. sat sat T e = 7. ev V = 7. V fl I sat = 8 ma E sat /I sat = NaN T e = ev V =. V fl I sat = 3 ma E /I = NaN sat sat T = 9 ev e V fl = V I = 8 ma sat E /I = NaN sat sat T = 3 ev e V = 8.7 V fl Prb.#.8 mm Prb.#.7 mm Prb.# 8 9.83 mm Prb.#. mm Prb.# 9.9 mm Prb.# 9. mm 3 3 V appl [V] I sat = 77. ma E /I =. sat sat T =. ev e V =. V fl I sat = 7 ma E sat /I sat =.8 T e = 8.9 ev V fl =.7 V I = 7 ma sat E /I =.7 sat sat T e = 7. ev V = 3 V fl I sat = ma E sat /I sat = NaN T e = ev V =. V fl I sat = 8 ma E /I = NaN sat sat T = 8 ev e V fl =. V I = 33 ma sat E /I = NaN sat sat T = ev e V =.3 V fl Prb.# 3 mm Prb.#.77 mm Prb.# 8 mm Prb.# 3.9 mm Prb.# 8.7 mm Prb.# 9 mm 3 3 V appl [V] I sat =. ma E /I =.9 sat sat T =.3 ev e V = V fl I sat = 7.7 ma E sat /I sat =. T e = 8 ev V fl =.8 V I = ma sat E /I =. sat sat T e = 7. ev V =.8 V fl I sat = 78 ma E sat /I sat =.9 T e = 7. ev V =. V fl I sat = 39 ma E /I = NaN sat sat T = ev e V fl =.3 V I = ma sat E /I = NaN sat sat T = ev e V =. V fl Prb.#.77 mm Prb.#. mm Prb.# 8 9.78 mm Prb.#. mm Prb.#.8 mm Prb.# 9. mm 3 3 V appl [V] Probe distance from separatrix mapped to the er midplane T e (min) fit Te (cut at V f ) fit
Influence of Parallel T e Gradient Midplane location at separatrix Approx. X-pt. position. mm,.8 mm,.7 mm, 9. mm: Distance from separatrix at er midplane n e [x 9 m -3 ] 3 3 8 8 3 Parallel Connection Length [m] Inner Target T e [ev] 3 3 3 3 3 3 3 Parallel Connection Length [m] Inner Target 3 3 3 3 n sep =.8x 9 m -3 n sep =.x 9 m -3 n sep =.x 9 m -3 n sep =.9x 9 m -3 n sep =.x 9 m -3 n sep =.7x 9 m -3 n sep =.7x 9 m -3 n sep = 3.x 9 m -3 3 3 Last 3 m Outer Target 9. mm.7 mm.8 mm. mm #783,. s Use parallel field T e gradients from B-Eirene in the absence of diagnostics.
Use Wesson s Analytic Model to Compute T eff Wesson s model based simply on mfp arguments and energy filtering effect of the sheath - in the presence of a T e gradient electrons from hotter regions upstream of the divertor target can dominate the probe characteristic. Compute a T eff - the uniform temperature which would give the same sheath potential fall as the actual temperature distribution. T eff /T > implies that the probe derived T e will be higher than the local value at the target, T. T eff /T T eff /T 8 3 Inner Target Outer Target. mm.8 mm.7 mm 9. mm.. 3 Separatrix Density [x 9 m -3 ] Distance from separatrix at er midplane Model in its simplest form (no n e gradient, no presheath), shows that TCV probes will read high at relatively low densities. Some interesting threshold phenomena seen when T e profile has extended flat, convective regions. J. A. Wesson, Plasma Phys. Contr. Fusion 37 (99) 9