H. Y. Lee, J. W. Lee, J. G. Jo, J. Y. Park, S. C. Kim, J. I. Wang, J. Y. Jang, S. H. Kim, Y. S. Na, Y. S. Hwang

Transcription

1 Study on EBW assisted start-up and heating experiments via direct XB mode conversion from low field side injection in VEST H. Y. Lee, J. W. Lee, J. G. Jo, J. Y. Park, S. C. Kim, J. I. Wang, J. Y. Jang, S. H. Kim, Y. S. Na, Y. S. Hwang Center For Advanced Tokamak Study Department of Nuclear Engineering Seoul National University Korea-Japan Workshop on H&CD December 15 th, 2016 NUPLEX, Dept. of Nuclear, Seoul National University, San 56-1, Shillim-dong, Gwanak-gu, Seoul , Korea

2 Introduction VEST : the first Spherical Torus in Korea Versatile Experiment Spherical Torus Objectives Basic research on a compact, high- ST (Spherical Torus) Study on innovative start-up, non-inductive H&CD, and innovative divertor concept, etc Specifications Initial Phase Future Chamber Radius [m] 0.8 : Main Chamber 0.6 : Upper & Lower Chambers Chamber Height [m] 2.4 Toroidal B Field [T] Major Radius [m] Minor Radius [m] Aspect Ratio >1.3 >1.3 Plasma Current [ka] ~100 ka 100 Elongation ~ Safety factor, q a ~3.5 ~3 2/15

3 Introduction Motivation Electron Cyclotron Heating(ECH) is widely used for various purposes in fusion device that the pre-ionization, local heating and current drive. Especially non-inductive current drive and startup using ECH is essential for Spherical Torus (ST) which has lack of space for the center stack. But ECH in ST shows the limitations due to low toroidal field. An EBW(Electron Bernstein Wave) which has no cutoff density, has been proposed as a promising alternative for heating and current drive in ST that it is impossible for ECH due to density limit. EBW Pros Cons OXB (O cutoff & UHR) OXB (CS & UHR) Excellent results in theory and experiment Complex Scenario Density fluctuation Angular dependant Complex Scenario Need : polarizer Limit of O cutoff XB (UHR) Simple design Single Mode conversion Limit of R cutoff tunneling effect Control on density profile Device MAST, NSTX, QUEST LATE, TST-2, CDX-U 3/15

4 Experimental Setup in VEST 2.45 GHz ECH/EBW System of VEST 1.5 Coil Geometry (a) (b) Z (m) GHz ECH 2.45 GHz ECH/EBW system (a) CW 6 kw (b) Pulse 10 kw R (m) CW ECH/EBW System Commercial microwave power supply (6 kw 2.45 GHz: 1ea) Low field side launching Vertical injection(x/o) Pulse ECH/EBW System Cost-effective homemade magnetron power supplies (10 kw : 1ea, 3 kw : 4ea) Low field side launching Vertical injection(x/o) Pulse duration, trigger time 4/15

5 Experimental Setup in VEST 2.45 GHz pulse ECH System of VEST Filament Heating 10 kw Magnetron Cathode MOSFET Switch Anode Electro magnet Antenna 20 kω Pneumatic Switch 12 kv 220 uf 110 kω Pneumatic Switch 100 Ω kW CW Forward Power 10kW Pulse Forward Power 10kW Pulse Reflect Power 13 V 53 A /15

6 EBW Pre-ionization Experiments in VEST ECH Pre-ionization Experiment with Pure TF 2.70E E E+017 Toroidal field ~ 0.1 T at R = 0.2 m ECR TF 3.8 ka with ECH 6 kw TF 3.8 ka with ECH 10 kw Toroidal field current ~ 0.1 T at R = 0.45 m 2.70E E E+017 ECR TF 8.2 ka with ECH 6 kw TF 8.2 ka with ECH 10 kw Density (#/m3) 1.80E E E E E E+016 Density (#/m3) 1.80E E E+017 L cutoff 9.00E+016 UHR R cutoff6.00e E+016 L cutoff UHR R cutoff 0.00E Radius (m) Pure TF + ECH pre-ionization experiment 0.00E Radius (m) In case of 3.8 ka, high density plasma generates near ECR with ECH 6 & 10 kw In case of 8.2 ka, over dense plasma generation over L cut off density and plasma density peak exists near UHR : Collisional damping with XB conversion In case of 3.8 ka, the density peak exists near inboard but in case of 8.2 ka, the mode conversion efficiency increases with steep density gradient near UHR and collisional damping of EBW makes the density peak 6/15

7 EBW assisted Start-up Experiments in VEST TPC Start-up Scheme Field Null Configuration Significant enhancement of pre-ionization under TPC (Trapped Particle Configuration) Inner Wall Trapped Particle Configuration Field Null Configuration TF only B T ~0.05 T at R=0.4 m Outer Wall Trapped Particle Configuration n e [10 17 m -3 ] LFSO R [m] Prompt I p Initiation under TPC Save Voltsec consumption & Stable Decay index LFSX 7/15

8 Density (#/m3) 2.50E E E E E+016 EBW assisted Start-up Experiments in VEST Start-up Experiment with low TF (~4 ka) (1) ECR TF only 6kW TPC 3kW TPC 5kW TPC 6kW TPC 6+10 kw L cutoff UHR R cutoff Current Rampup Rate (MA / s) E only TF 6 kw TPC 3 kw TPC 5 kw TPC 6 kw Enhancement of pre-ionization under TPC ( Trapped Particle Configuration) The enhanced plasma density overcomes the cutoff density : Collisional damping of EBW (Electron Bernstein Wave) via direct XB mode conversion from the low field side injection The plasma density increases rapidly after encountering UHR resonance layer In case of higher ECH power 16 kw, the density peak exists near outboard side due to converted EBW collisional damping The enhanced pre-ionization plasma near inboard side which has relatively strong electric field must be helpful for successful plasma current formation. The enhanced plasma density has on influence to the plasma current ramp-up rate with same loop voltage ~ 1.5 V 3.0 8/15

9 EBW assisted Start-up Experiments in VEST Start-up Experiment with low TF (~4 ka) (2) 650 Current Rampup Rate (ka / s) TPC 3kW TPC 5kW TPC 6kW Based on these experimental results, extremely low loop voltage startup scheme is suggested. The plasma current has not been generated with only TF case. With low loop voltage, the pre-ionization density affects to the plasma current ramp-up rate dominantly. With extremely low electric field ~0.16 V/m, the startup experiment has been performed successfully with enhanced pre-ionization plasma via EBW collisional heating. The TPC start-up scheme has the feasibility applicable to low electric field start-up machine including KSTAR and ITER(~0.3 V/m). 9/15

10 Density (#/m 3 ) 4.00E E E E E E E E+016 EBW assisted Start-up Experiments in VEST Start-up Experiment with high TF (~8.3 ka) (1) ECR 0.00E Radius (m) TPC with ECH 6 kw TPC with ECH 6&10 kw L cutoff UHR R cutoff In case of only TF with 6 kw ECH power, density peak exists near outboard side due to collisional damping of EBW via direct XB mode conversion from LFS injection. Solenoid free startup scheme utilizing outer PF coils (6&9) is suggested The loop voltage of 6~7 V has been adopted to the over-dense pre-ionization plasma utilizing TPC but the plasma current has not been grown up. The lower plasma resistivity for plasma current formation is necessary for successful solenoid free start-up to overcome the vertical field. Loop voltage [V] Time [msec] R~0.66 R~0.68 R~0.70 R~0.72 R~0.74 R~ /15

11 Plasma Current [ka] /15 EBW assisted Start-up Experiments in VEST Start-up Experiment with high TF (~8.3 ka) (2) R = 0.75 m, a = 0.2 m The simulation for simple LR circuit with loop voltage from outer PF coils Shape : Circular single filament current with major and minor radius This simulation result with assumption of no wall More powerful enhancement of pre-ionization is necessary for formation of plasma current I p overcoming B v E E-5(TPC 6+10 kw) E-5(a=0.05m) Time [ms] Estimation : over 40 ev T e and #/m 3 (resistivity : T e dominant) If the plasma current is generated, the plasma moves inward and the current and shape increases with help of negative voltsec consumption with decrease of external inductance. Additional simulation and start-up experiments will be investigated for CS-free start-up. Plasma Current [ka] R = 0.70 m, a = 0.50 m I p overcoming B v E-5 (TPC 6+10kW) E-5 (TPC 6kW) E-5 (a=0.1m) Time [ms]

12 EBW Heating Experiments in VEST EBW collisional heating during startup Density (#/m 3 ) 1.60E E E E E E E+017 Additional 10 kw ECH on TPC Startup with 6 kw (R=0.5 m) TPC Startup with 6&10 kw (R=0.5 m) Plasma Current (A) TF 8.2 ka TPC Startup with 6 kw (R = 0.5 m) TF 8.2 ka TPC Startup with 6&10 kw (R = 0.5 m) 2.00E E Time (ms) Time (ms) EBW collisional heating occurs during TPC startup The additional 10 kw pulse ECH injection has occurred at 402 ms The feasibility of strong heating in the closed flux surface (R=0.5 m) is confirmed and EBW collisional damping (density increase). Electron temperature does not change : Low collisionality is necessary High electron temperature and low impurity for reducing the effect of collisional heating 12/15

13 Density (#/m3) Plasma Current (ka) 5.00E E E E E E E E EBW Heating Experiments in VEST EBW heating Experiments (1) 1st ECR 1.00E (on) 406(off) 5.00E (on) 0.00E (off) Time (ms) 2nd ECR Radius (m) off on 3rd ECR Temperature (ev) st ECR 2nd ECR (on) (off) 5 408(on) 0 408(off) Radius (m) Boronization (impurity removal) and high plasma current (high electron temperature) Additional 10 kw MW injection at 402 ms 3rd ECR No change along the electron density profile with additional MW no collisional damping The electron temperature rises two times near 3 rd harmonics the first position after mode conversion 13/15

14 EBW Heating Experiments in VEST EBW heating Experiments (2) Temperature (ev) st ECR 2nd ECR 3rd ECR (on) (off) 5 408(on) 408(off) Density (#/m3) 5.00E E E E E E E E E E ms 408 ms 0.00E The Electron heating of EBW via direct XB MC from LFS The electron temperature arises at 406 ms but has not been changed at 408 ms. Hollow density profile due to fast plasma current ramp-up rate 406 ms : ramp-up phase steep density gradient 408 ms : current peak phase broad density profile The more detailed electron heating experiments will be performed along the UHR and ECR position for the change of local electron heating 14/15

15 Summary The pre-ionization plasma with TPC has been enhanced assisted by EBW collisional heating and the density peak has been observed near inboard and outboard side along the toroidal field. Based on the TPC pre-ionization results, two start-up schemes are suggested for extremely low loop voltage start-up and solenoid free start-up utilizing outer PF coils. The plasma current ramp-up rate is determined by the enhanced pre-ionization plasma density and based on the results, the startup experiments have been performed successfully with the extremely low loop voltage of ~ 0.2 V/m. The solenoid free startup experiments utilizing outer PF coils have been performed but the necessity of the more intensified pre-ionization plasma with analyzing the plasma resistivity has been addressed. The EBW heating experiments during ohmic discharges have been conducted with low collisionality plasma and it is observed that the electron temperature arise near the 3 rd harmonic ECR from EBW via direct XB mode conversion from low field side injection. 15/15

16 Reference [1] V. F. Shevchenko et al., Nucl. Fusion, (2010) [2] A. K. Ram et al., Phys. Plasmas, (2000) [3] Idei Hiroishi et al., FEC2012, EX/P6-17 (2012) [4] Uchida Masaki et al., FEC2012, EX/P6-18 (2012) [5] S. Shiraiwa et al., Phys. Rev. Lett., 96, (2006) [6] G. Taylor et al., Rev. Sci. Instrum. 72, 285 (2001) [7] S. Pesic, Physica C, 125, (1984) [8] Josef Preinhaelter et al., Rev. Sci. Instrum. 77, 10F524 (2006) [9] S. J. Diem et al., Rev. Sci. Instrum. 79, 10F101 (2008) [10] S. H. Kim et al., Physics of Plasmas, 21, (2014) 16/15

17 Backup Thank you for your attention! 17/15