Overview of ICRF Experiments in Alcator C-Mod

Transcription

1 Overview of ICRF Experiments in Alcator C-Mod 50 th APS Plasma Physics Conference November 17-1, 008 S.J. Wukitch, Y.Lin, P.T. Bonoli, A. Hubbard, B. LaBombard, B. Lipschultz, M. Porkolab, J.E. Rice, D. Whyte and the Alcator C-Mod Team Key Results: 1. First demonstration of efficient RF flow drive by mode converted waves.. Observed direct fast wave heating on electrons with low single pass absorption. 3. RF sheaths were observed despite insulating protection tiles and were larger in H-mode than expected. 4. Demonstrated real time matching utilizing ferrite stubs at high power. 50th APS Dallas TX 1

2 ICRF Mode Conversion Flow Drive (MCFD) Motivation: RF plasma flow actuator may offer a unique means to externally:» Manipulate transport via flow shear stabilization and» stabilize of certain high-beta MHD instabilities. Finite number of results have observed flow drive with RF waves.» Enhanced confinement attributed to ion Bernstein wave shear flow in PBX-M. 1» Plasma flow observed with low frequency RF heating in Phaedrus-T.» Sheared poloidal flow observed with IBW in TFTR. 3» Poloidal flow observed between mode conversion layer and minority resonance in TFTR. 4 Description: Utilize mode conversion (MC) to avoid coupling complications of ion Bernstein wave launching. Theory and simulation suggests flow occurs when significant damping on ions occurs and Will result in sheared poloidal flow B.P. LeBlanc et al., Phys. Plasmas, 741 (1995).. S.J. Wukitch et al., Phys. Rev. Lett. 77, 94 (1996). 3. B.P. LeBlanc et al., Phys. Rev. Lett. 8, 331 (1999). 4. C.K. Phillps et al., Nucl. Fus. 40, 461 (000). 5. F. Jaeger et al., Phys. Rev. Lett. 90, (003). 50th APS Dallas TX

3 Observe Toroidal and Poloidal Rotation with Mode Conversion Toroidal rotation, Vφ, profile is peaked and approximately twice that observed in D(H) minority heated discharge. For D(H) minority absorption the rotation profile is approximately flat and dominated by intrinsic rotation. Change in poloidal rotation profile is peaked off-axis in the ion diamagnetic drift direction. V θ is up to 1.5 km/s and localized: 0.3 < r/a < 0.7. For D(H) minority absorption, V θ less than 0.5 km/s and has no clear spatial structure. Discharge particulars: Upper single null, L-mode plasmas where the 3 He concentration was modest (n 3He /n e ~ 10%). D( 3 He) mode conversion layer is near the magnetic axis for 50 MHz. D(H) minority resonance is near the magnetic axis for 80 MHz. Injected power ~ 3 MW r/a 50th APS Dallas TX 3 Toroidal Rotation V φ [km/s] r/a -ΔV θ [km/s] Mode Conversion Minority Ohmic Mode Conversion Minority Ion diamagnetic drift direction

4 MCFD Scales with RF Power D(3He) MC D(H) Minority ΔVφ [km/sec] 60 ΔVφ [km/sec] ΔW/Ip [kj/ma] 1 3 P RF [MW]/<n e >[100 m-3] For D( 3 He) MC absorption, the change in toroidal rotation, ΔV φ, is approximately twice the empirical intrinsic rotation scaling. 1 For D(H) minority heating, the ΔV φ scales with ΔW/I p 1 For D( 3 He) MC absorption, the change in toroidal rotation, ΔV φ, is scales as P RF / <n e > not with RF E-field. 1. J. Rice Nucl. Fus. 41, 77 (001). 50th APS Dallas TX 4

5 MCFD is largely Independent of Antenna Phase The direction of rotation is independent of the antenna phase. V φ in co-current direction. V θ in ion diamagnetic drift direction. Phase scan showed only 10% variation between cocounter- and heating phase. The rotation magnitudes are similar for all antenna phases. 50th APS Dallas TX 5

6 Measurements Confirm Presence of MCICW Mode converted ion cyclotron wave (MCICW) detected by phase contrast imaging about ~ 4 cm away from the 3 He cyclotron resonance and on the HFS of magnetic axis. Wave number k R ~ 3-7 cm -1, consistent solution of dispersion equations and to previous MC experiments and also to the [Y. Lin et al, PPCF (005)]. 50th APS Dallas TX 6

7 TORIC Simulation Indicates Significant Power to Ions MW/m ECE Fit Simulation r/a Using a n( 3 He)/n e ~ 8-1% for TORIC simulation can reproduces measured MCICW profile from PCI And electron power deposition profile. The MC ICW is damped strongly onto 3 He ions through a substantially broadened IC resonance. Fast wave: k ~ 10 m -1 MCICW: k ~ m MW/m 3 ω = ω c + k, 3 v He 50th APS Dallas TX 7

8 Power to Ions Appears to be Important Experimentally, no flow drive has been observed when majority of power is absorbed by electrons. Flux averaged power deposition to 3 He from TORIC simulation is approximately at the same location where: Toroidal rotation profile decreases significantly and Poloidal rotation is observed and Suggests ICW - ion interaction is key to MCFD. [km/s] MW/m V φ ΔV θ 3 MC ICW power to He (flux surface averaged) r/a [km/s] (a) (b) 50th APS Dallas TX 8

9 Direct Fast Wave Absorption Experiments Motivation: Fast wave current drive is expected to provide central seed current for optimizing current profile in AT plasmas. Background: FW damping is dependent on electron β and electron temperature. 1 ImΔ A 1 T 1 e,, v k ω = = ζ = = Im π ω ω pi = βζ e e ω ω ci Estimates low single pass, 1-%, is accessible from existing discharges Fundamental D cyclotron resonance is present at 55 cm (r/a~0.5). e T m k x e e te kv te e e ζ Fundamental D resonance M. Porkolab, AIP Conf. Proc. 314, 99 (1994). 50th APS Dallas TX 9

10 Initial Results from Fast Wave Absorption Successfully observed FW heating. Utilized 50 MHz and heating phase (n φ =±10) Used D(H) minority at 80 MHz to heat plasma to >4 kev at top of sawtooth. Discharges were USN, L-mode with an H-factor ~1.3. Assuming L-mode scaling, effective absorbed power fraction ~0.5.» Central T e increased ~ 1keV.» Significantly higher density production with FW compared to minority heating.» Increase in neutron rate during FWEH.» Significant impurity production but radiated power remains under control B T ~5. T, I p =1. MA, USN P RF (MW) FW H minority W MHD (MJ) T e0 (kev) n e (10 0 m -3 ) neutrons (x10 13 s -1 ) P RAD (MW) Time (s) th APS Dallas TX 10

11 FW Absorption is Appears Dominant Current drive phasing resulted in little or no heating. Current drive phasing has lower n φ =7 and faster wave phase speed. FW absorption has nonlinear dependence on phase speed. Suggests fundamental deuterium absorption is not significant. Lowering target plasma temperature resulted in reduced heating efficiency for heating phase. Target discharge had lower T e (~3.6) which results in single pass <1%. 50th APS Dallas TX heating phase current drive phase P RF (MW) W MHD (MJ) T e0 (kev) n e (10 0 m -3 ) neutrons (x10 13 s -1 ) P RAD (MW) FW H minority B T ~5. T, I p =1 MA, USN Time (s) ,3

12 Reduction of ICRF Impurities Production is Challenging Previous prescription 1 to ameliorate impurity production was: Operate the antenna in dipole phasing, Align the Faraday screen with total magnetic field, and Utilize low Z film on antenna and plasma facing surfaces. Metallic plasma facing components (PFCs) present additional challenge. Allowable W concentration is <10-5 for ITER. W will radiate in the plasma pedestal region. Working hypothesis is that RF sheaths are responsible. RF heated discharges show faster erosion rate than ohmic H-mode. Measured increase in plasma potential and Deduced that the erosion of low Z films are linked to active antenna. Seek to characterize low Z film lifetime, sheath mitigation with BN tiles, and sheath characteristics. Expect RF sheath enhancement is proportional to E dl and will be proportional to square root of RF power. 3 Expect surfaces local to the antenna to be dominant impurity sources. 1. Jacquinot et al., Fusion Eng. Design 1, 45 (1990).. B. Lipschultz et al., J. Nucl. Mat (007). 3. D.A. D Ippolito et al., PPCF 33, 607 (1991). 50th APS Dallas TX 1

13 RF-Enhanced Sheaths Result in ICRF Impurity Production Simple electrical circuit model of RF- enhanced sheaths is: 1 Open field lines connect conducting surfaces and enclose RF flux. Electrons can respond to oscillating RF voltage and ions are relatively immobile, thus the electrons are lost preferentially. Field line plasma potential increases and the sheath potential is: V ~ V + 3T plasma RF e B T E r xb T E r Field lines closest to the antenna will charge to higher potential and a radial electric field will result. Estimate radial electric field is 1-5 kv/m. This results is a strong ExB flow ~ km/s poloidally. 1. Perkins, Nucl. Fusion 9, 583 (1989). D.A. D Ippolito, Phys. Fluids B 5, 3603 (1993) 50th APS Dallas TX 13

14 Low Z Film Lifetime is Limited Following a boronization, successive discharges are heated by ICRF. Boron is eroded when impurity control is lost during the H-mode Estimated erosion rate is ~15-0 nm/s with RF heating. JET utilizes beryllium coating on FS and limiter to control high Z impurity influx. Observed significant Be influx but did not estimate Be erosion rate. Assuming similar erosion rate for Be in ITER, lifetime of 5 mm layer in ITER is ~1000 discharges P rad (MW) 1st Discharge nd Discharge 4th Discharge H-mode Time (s) W MHD (MJ) th APS Dallas TX 14

15 Can Plasma Performance be Improved by Insulating Limiters? Replaced molybdenum protection tiles with insulating BN tiles to Remove local antenna molybdenum source. Eliminate sheaths on field lines that intersect the antenna protection tiles. Remove RF enhanced plasma potential. Approach was similar to BN limited installed in the Phaedrus-T tokamak. * Successfully eliminated metal impurities with installation of BN tiles and removal of Faraday screen. *Majeski et al., 11 th RF Power in Plasmas, AIP Conf Proc 44, 3 (199). 50th APS Dallas TX 15

16 Insulating Tiles did not Improve Performance Plasma performance was unimproved despite decrease in antenna limiter Mo source. Expected less RF power to obtain same stored energy. Suggests RF limiters are not the only source of core Mo. Significant RF enhanced plasma potential is observed despite BN tiles. BN tile impedance is 100x the plasma-pfc sheath impedance. RF voltage is dropped across insulating tile. Plasma potential ~100 V in H-mode and would have significant enhanced sputtering. Contrasts with previous results. In Phaedrus-T, little residual plasma potential increase was observed but power density is times lower than C-Mod. 1 In C-Mod, BN merely covered metallic surfaces whereas in Phaedrus-T the BN replaced the metallic antenna box th APS Dallas TX 16 P RF (MW) RF power required to obtain 160 kj < W Tot < 180 kj Year RF Power [MW] H-mode Plasma Potential [V] BN installed Time (s)

17 In L-mode, Sheath Voltage Scales as P 1/ only with Boronized Walls. Sheath voltage is expected to scale with antenna power as Vplasma = VantCn PRF where V ant is the antenna voltage, C n is constant depending on antenna loading, and P rf is the coupled power.* Post boronization, find the plasma potential is proportional to square root of power. In a poorly conditioned machine, plasma potential scales with RF power. *D.A. D'Ippolito et al., Nuclear Fusion 38, 1543 (1998). 50th APS Dallas TX 17 Plasma Potential [V] Plasma Potential [V] unboronized boronized 0. RF Power Density [MW/m ] 6 10 unboronized boronized ITER Power Density RF Power [MW] Sqrt(RF Power) [MW 1/ ] , ,7, , ,7,9

18 Sheath Potentials are Higher in H-mode For similar power, the sheath voltage is expected to increase with antenna voltage. Measured antenna voltage in H-mode is ~0% than comparible L-mode. Expect RF enhanced plasma potentials to be ~0% higher in H-mode Observe that plasma potentials are about twice as large in H-mode than L-mode. Caveats: RF power is affecting antenna loading.» Density profile is modified during power power ramp.» Antenna loading is constant above 100 kw. RF sheath model needs to allow for cross-field currents.,3» Ion flows across the magnetic field along the length of the flux tube are balanced by electron currents in and out of one end of the flux tube.» Integrated flux tube impedance is less than or equal to the impedance through the sheath. RF is creating an energetic edge electron population.» Small energetic electron population, ~0.1% can double the sheath voltage,» And a population of a few percent can increase the sheath potential by a factor of 10. 4» Possible mechanisms are Fermi 5 and near field 6 acceleration. H-mode L-mode H-mode with BN tiles RF Power [MW] 50th APS Dallas TX 18 Plasma Potential [V] P1/ Sorensen et al., Nucl. Fusion 36 (1996). D.A. D Ippolito et al., Nuclear Fusion 4, 1357 (00). 3 E. Faudot et al., Phys. Plasmas 13, 0451 (006). 4 D. Tskhakaya et al., Phys. Plasmas 9, 486 (00). 5 M. D. Carter et al, Phys. Fluid B 4, 1081 (199). 6 V. Petržílka et al., in Proc. 3nd EPS Conf. on Plasma Phys. 9C, P-.095 (005) ,,

19 Reduce coupled E by making antenna symmetric along a field line. Integrated E along a field line would vanish. Entire structure is perpendicular to field line. Install array of diagnostics to better understand plasma potential modifications with ICRF. Increase emissive probe coverage. Complementary retarding energy analyzers to measure electron distribution. RF probes to examine wave characteristics Edge reflectometry for density profile and fluctuations. Future Directions 50th APS Dallas TX 19

20 Challenge: Maintain Reliable Power Transfer Edge plasma density and density profile determines the antenna coupling efficiency. Sets the distance to propagation and propagation characteristics. Edge density and its profile is dependent upon : Plasma current, target density, magnetic field, confinement mode, wall conditions, and RF power. Transient phenomena modify the coupling on fast time scales: ELMs, Monster sawteeth, and Confinement mode transitions n e [m -3 ] LCFS Near SOL Cut-off density Main Plasma Limiter Far SOL Scrape-off Layer Limiter Shadow Antenna Limiter Rmajor [cm] 50th APS Dallas TX 0

21 Utilize Triple Stub System Ferrite Tuner #1 Ferrite Tuner # Fixed Stub Generator Antenna DC1 DCC System is installed in the E antenna matching network and operates at 80 MHz. Fixed stub is used for pre-matching to prevent excessive voltages in the ferrite stubs. Ferrite stubs were originally designed for 60 MHz for hybrid ring system at DIII-D and tested at ASDEX-U. Have directional couplers for forward power, reverse power, and phase at four locations to enable analysis. DC DC3 50th APS Dallas TX 1

22 Ferrite System Characteristics Electrical length is 35 cm for +/- 150 A. Swept end to end is 4 msec. Computational time is 00 μs and presently limited by computational speed. Calculating tuner settings from transmission line relations. Comprehensive arc protection to prevent coupling to an arc. Optical arc detection in the ferrite tuners. Reflected to forward power threshold set to 5%. Tuner #1 Tuner # Arc detection fiber Phase balance monitor based on current probes in at the antenna strap ground.» Detects loss of current in the current strap.» Primary defense against arcs at low voltage locations. 50th APS Dallas TX

23 Smith Chart Representation of Matching Range -Im(Γ) To XMTR Admittance Smith Chart Y = G+jB 3A B A 80% 40% 3B 0% 10% 5% 1% 1A -Im(Γ) To XMTR Example Admittance Smith Chart G=0.5 G=0.5 40% -0.5 To ANT 1B Re(Γ) <1% reflected <5% reflected -0.5 To ANT Re(Γ) Design range covers wide range of plasma conditions in both L and H- mode. 50th APS Dallas TX 3

24 Overall FFT Performance is Good With pre-matching, obtained 37 kv and 1.85 MW coupled into H- mode. Found voltage limit to be ~5 kv in one of the tuners. Other tuner is higher by at least 30%. Observed increased losses at high power. Related to net magnetic field in particular tuner rather than nonlinearity in losses due to power. Power Loss Ratio total loss in double stub loss through first FFT short transmission line loss P circulating (MW) FFT characteristics show that for coil current >100 A will result in increased losses. Pre-matching avoids: High voltage in FFTs and Avoids driving the net magnetic field to zero and increasing losses. 50th APS Dallas TX 4

25 FFT Load Following is Reasonable P forw (MW) Coil Current (A) Ant. Loading (Ω) D α (A.U.) L-mode H-mode Time (s) Tuner Tuner P relf (MW) FFT follows load transitions associated with discharge evolution and confinement transitions very well. Response is insufficient to maintain match <5% during entire ELM because it is <50 μs but sufficient to reduce mismatch < fault level. Currently limited by computational time but ultimately will be the FFT power supply. 50th APS Dallas TX Ant. Loading (Ω) D α (A.U.) 0.15 Γ = P refl /P forw Time (s)

26 Summary Observed significant toroidal and poloidal flow with D( 3 He) mode conversion. Toroidal rotation exceeds empirical scaling with instrinsic/spontaneous rotation. Scales with RF power. Majority of power is absorbed by ions. First observation of Fast wave electron heating in C-Mod with low single pass absorption. RF sheaths were: Still present for an antenna with insulating protection tiles and Observed increase with H-mode compared to L-mode was larger than expected. A triple stub, fast ferrite matching network reliably maintains power transfer to the plasma with low reflected power over wide range of conditions. To date, 1.85 MW coupled into H-mode. Further improvements are possible in the voltage limit and time response of these ferrite tuners. 50th APS Dallas TX 6