The ECH experiments in VEST(Versatile Experiment Spherical Torus)

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Geraldine Haynes
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1 The ECH experiments in VEST(Versatile Experiment Spherical Torus) January 28 th, 213 Hyunyeong Lee, Jong Gab Jo, Y. H. An, S. H. Kim, K. J. Chung and Y. S. Hwang NUPLEX, Dept. of Nuclear, Seoul National University, San 56-1, Shillim-dong, Gwanak-gu, Seoul , Korea

2 Contents v Introduction - VEST and necessity of ECH&EBW - Preliminary Experiment Setup & Result v ECH system and diagnostics in VEST - 3kW ECH system for VEST - 6kW ECH system for VEST - Triple probe system in VEST v ECH experiments result - Impedance matching - Pre-ionization experiment in VEST - The first plasma generation experiment in VEST - MW multiple reflection effect v Summary & Future Work 2/22

3 Spherical Torus as an Alternative Concept Spherical Torus [1] : Low aspect ratio (A<2) tokamak overcomes tokamak s drawbacks (Low beta, large size) Advantages of ST ü High performance (High plasma current, βlimit) ü Compactness Weakness of ST ü Difficulty in start-up & sustaining (Lack of space for solenoid) ü Innovative start-up method is critical issue for ST!! ü By developing new start-up and non-inductive current drive method, ST can be a alternative concept of fusion device [1] MAST, Culham homepage ( 3/22

etc. Ø Specifications Initial Phase Future Chamber Radius [m].8 : Main Chamber.6 : Upper & Lower Chambers Chamber Height [m] 2.

4 VEST : The First Spherical Torus In Korea Versatile Experiment Spherical Torus Ø Objectives Basic research on a compact, high-b ST with elongated chamber in partial solenoid configuration Study on innovative start-up, non - inductive current drive, conceptual divertor etc. Ø Specifications Initial Phase Future Chamber Radius [m].8 : Main Chamber.6 : Upper & Lower Chambers Chamber Height [m] 2.4 Toroidal B Field [T].1.3 Major Radius [m].4.4 Minor Radius [m].3.3 Aspect Ratio >1.3 >1.3 Plasma Current [ka] 3 1 Safety factor, q a /22

5 Start-up Using Partial Solenoid & ECH Pre-ionization 1 Coil Geometry 1.5 Breakdown by Partial Solenoid 3kW 2.45GHz MW for ECH Ø Innovative start-up method in VEST : Partial Solenoid Operation Inherits the merits of solenoid Possible to maintain low aspect ratio.5 Merging Ø This startup method makes two small plasmas using ECH pre-ionization & partial solenoid in lower & upper side and then main plasma is formed in merging together. Z (m) -.5 6kW 2.45GHz MW for ECH Ø Lloyd condition for reliable breakdown : decrease of 9% loop voltage using pre-ionization Bf Ef ³ 1 V / m Þ1 V / m B q ECH pre-ionization is essential for reliable start-up of VEST! -1 Breakdown by Partial Solenoid R (m) 3kW 2.45GHz MW for ECH Ø 2.45GHz magnetron of MW oven(3kw) : upper & lower 2.45GHz commercial MW source(6kw) : main Reason : Simple, safe, easily available, and economical 5/22

EBW Mode Conversion Ø Conventional ECH is not suitable to ST due to low cutoff density EBW encounters no cutoff density in the plasma Attractive for heating and CD in ST.

6 EBW Mode Conversion Ø Conventional ECH is not suitable to ST due to low cutoff density EBW encounters no cutoff density in the plasma Attractive for heating and CD in ST. -ph -ph Ø EBW power mode conversion coefficient : [2] C = 4 e (1 - e ) W é 1 1 Ø Budden parameter : ( / ) ( / ) 1 ú ú ù celne k + k - h = ê c k + Lne LB êë k + Lne LB + k û Ø EBW mode conversion : applicable for low magnetic field (ex. ST, reversed pinch) [2] Heinrich Peter Laqua, PPCF, 49, R1-R42, 27 [3] V.F. Shevchenko et.al., Nuclear Fusion, 5, 224, 21 6/22 Schematic of the EBW assisted plasma current start-up in MAST [3] Maximum mode conversion efficiency C versus L n at NSTX [2]

Preliminary ECH Experiment : Setup Magnetron ÑB T B T Ø Objective : Confirm the effect of ECH pre-ionization & EBW mode conversion Ø Stainless Steel wall & 3 stub tuner : protect MW loss and wall

7 Preliminary ECH Experiment : Setup Magnetron ÑB T B T Ø Objective : Confirm the effect of ECH pre-ionization & EBW mode conversion Ø Stainless Steel wall & 3 stub tuner : protect MW loss and wall reflection effect Ø Available to change the direction of microwave(lfs&hfs and O&X mode) Ø Antenna : Open waveguide(wr 284) Ø Diagnostics Axial Langmuir probe : planar type tip(r=1.5mm), perpendicular to the magnetic field : T e (linear fit slope of V-I curve), n(ion saturation current & T e ) 7/22

8 Density (#/cm3) 1.E+12 9.E+11 8.E+11 7.E+11 6.E+11 5.E+11 4.E+11 3.E+11 2.E+11 1.E+11.E+ Preliminary ECH Experiment : Result(1) Density (#/cm3) X cutoff O cutoff Power (W) Operation Pressure :.8mTorr LFSX 7.E+11 LFSO HFSX 6.E+11 HFSO 5.E+11 4.E+11 3.E+11 2.E+11 1.E+11 X(L)cutoff ECR UHR.E+ X(R)cutoff ECH power : 5W Ø In case of HFS launching, the O and X mode cutoff density exists. Ø In case of LFS launching, high density plasma generates(xb mode conversion). Ø In the profile of the preliminary chamber, steep density gradient exists due to the narrow space inside the quartz tube. Ø Budden parameter :.44 EBW Conversion coefficient C :.45 Ø The feasibility of EBW mode conversion may be possible. Distance (cm) Density Temperature Temperature (ev) LFSX 8/22

9 Preliminary ECH Experiment : Result(2) 7.5E+11 7.E+11 Density (#/cm3) 6.5E+11 6.E E+11 5.E+11 ECH power : 7W Operation pressure :.8mTorr Wall (#) Ø The density of the plasma is measured by eliminating the stainless steel wall along the LFSX injection. Ø The density is decreased with eliminating the wall covered with the quartz tube : Multiple reflection Ø Over dense plasma : EBW mode conversion might be occurred due to the multiple reflection and steep density gradient near the UHR. 9/22 Ø ECH pre-ionization system for VEST is installed in LFS and X mode launch based on this experiment result.

10 3kW ECH System For VEST Ø LFS injection system with two 3kW 2.45 GHz magnetron is being installed in upper and lower chamber of the VEST. Ø For cost effective and easily available purpose, homemade microwave power systems are built with two 3kW 2.45GHz magnetrons. WR34 Transition WR284 3 stub tuner Directional Coupler Magnetron Circulator WR34 Vacuum Window Magnetron 3 stub tuner Directional Coupler 1/22

11 Power & Control System For 3kW ECH System Ø To utilize the home-made magnetron for ECH, power and control system is needed. - Modifications of magnetron operation circuit - Power : Controlled by discharge current to the magnetron(resistors) - Time control : Discharge switch(mosfet & pneumatic) 2kΩ Dummy Pneumatic Switch Discharge Resistors 1.5kΩ Specifications of 3kW Magnetron Filament Heating Magnetron Item Filament Voltage Value 4.6Vac Filament Current 19.5A Anode Current Anode Voltage Anode Input Power Output Power Frequency 9mA 5.4kV 4.8kW 3W 2.45GHz 84μF MOSFET Switch 5V 4A Magnetron -6kV 1kΩ Charge Pneumatic Switch 1.5kΩ Capacitor 11/22

12 6kW ECH System For VEST Vacuum window Main Chamber Microwave Launcher Power Supply WR284 transition to WR34 Manual 3 stub tuner Directional Coupler Microwave launcher Ø 6kW 2.45GHz commercial microwave power supply installation - at the outboard midplane of the main chamber - objectives : not only pre-ionization but also heating and current drive - Manual 3 stub tuner : optimizing MW power absorption (minimizing the reflected power) 12/22

Diagnostics : Triple Probe System In VEST 1.

13 Diagnostics : Triple Probe System In VEST Triple probe circuit PXI system Electron Temperature Electron Density.5 Upper Z (m) -.5 Main R (m) Ø Triple probe system : Diagnoses of rapidly varying plasma : Three probe tips(r=.3mm, l=5mm, d=3mm) : Linear guide is used for profiles inside VEST : Installed in the main and upper chamber R =.4m : center of the plasma 13/22

14 Impedance Matching : 3kW & 6kW 4 Total Power Reflected Power 45 Total Power Reflected Power Total Power (W) Total Power (W) Time (ms) Before Time (ms) Reflected Power Total Power After Upper & Lower System 4 Power [W] Time [ms] Main Chamber System Ø Microwave impedance matching is optimized using manual 3 stub tuner. Ø Net ECH power of upper & lower ECH system increases. (125~175W è 25~3W) Ø Also Microwave matching of 6kW ECH system is optimized using 3 stub tuner. 14/22

15 Pre-ionization Experiment Result Resonance Layer t=71ms t=73ms t=9ms t=55ms t=57ms t=62ms TF coil current ramp up phase TF coil current ramp down phase Ø Due to the specifications of VEST TF coil power system, movement of resonance layer with toroidal magnetic field is observed from the CCD camera. Ø In TF coil current ramp up phase, resonance layer start to appear from center stack and then move outboard. On the contrary, When TF coil current is decreased, resonance layer move from outboard to inboard side. 15/22

16 Pre-ionization Experiment Result 2.4E+17 2.E+17 X-mode O-mode 5 4 lectron Density (m -3 ) El 1.6E E+17 8.E+16 X-mode Left-hand cutoff (1.45E+17) O-mode cutoff (7.45E+16) 3 2 Io onization Rate (%) 4.E+16 1 X-mode Right-hand cutoff (2.97E+15).E Forward Power (W) Operation pressure :. 1mTorr(MFC) Ø X-mode is more favorable than O-mode for pre-ionization. Ø Overdense Plasma might be observed by LFS X-mode injection. Ø Possibility of XB mode conversion in VEST device. 16/22

17 Density & Temperature Profile Fundamental ECR(875G, R~41.7cm) 25 Upper hybrid resonance (R: ~55.9cm) Right hand cutoff (R: ~57.2cm) 1 Te Ne T e [ev] O-mode cutoff density n= m n e [1 16 m -3 ] 5 2 Chamber Wall TF current: 7.6kA R [cm] X-mode ECH power : 4kW Operation pressure :.1mTorr(puffing) Ø Density & temperature profile during the pre-ionization is measured in shot by shot. Ø The density gradient near the UHR is smaller than the preliminary experiment. Ø Budden parameter :.9 EBW Conversion coefficient C :.2 Ø The lower efficiency(2.5 times) & power density(37 times) compared to the preliminary experiment device seems to be the reason of the disappearance of the over dense plasma. 17/22

18 The first plasma experiment in VEST PF1 curr rent [ka] PF1 current [ka] TF current [ka] PF8 current [ka] Time [ms] m -3 n e [1 18 ] Loop Voltage [V] I p [ka] ü The first plasma generation experiment with ECH preionization is performed successfully in conventional startup method : 3kA plasma current ü The more experiment will be progressed using partial solenoid operation. T e [ev] Time [ms] Shot #2766 Flux Loop: R=.139, Z=.85m Triple Probe: R=.65m, Z=m Triple Probe: R=.65m, Z=m 18/22

19 Plasma Current along ECH power ~6kW ~4kW ~3kW 6kW+3kW magnetron 2 Plasma Current [ka] Ø Conventional startup method in the main chamber Time [ms] Operation pressure :.1mTorr(puffing) Ø The more ECH power increases, the plasma current is larger. Ø The total ECH injection(including upper & lower chamber) is better than only main chamber ECH injection. 19/22

20 MW Detection : Multiple Reflection kW magnetron total power 3kW magnetron reflected power quartz window (9MM1, reflect sensor) quartz window (9MM1, forward sensor) 9MM1 : Sensor Power [W] UT6/7LB6 : ECH (3kW) Time [ms] Ø To confirm the possibility of the over dense plasma generation to the MW multiple reflection as shown in the preliminary experiment Ø Confirmation of reflected MW existence in 9MM1 when 3kW MW injection in the 7UT6/7LB6 port : 2~7W(Reflection) Ø To prepare the experiment plan for the tendency of reflected MW in plasma current ramp-up(ech power absorption rate along the discharge) 2/22

21 Summary ü Versatile Experiment Spherical Torus (VEST), the first Spherical Torus in Korea, is constructed. ü From the preliminary result, 3kW & 6kW 2.45GHz ECH pre-ionization system for VEST is installed in upper & lower and main chamber from LFS X mode launch. ü Over dense plasma generates in the preionization experiment with the feasibility of EBW mode conversion and multiple reflection. ü The first plasma generation experiment is performed successfully with 3kA plasma current in conventional startup scenario. ü Various ECH experiments and simulation will be performed in the future. 21/22

22 Future Work Ø The pre-ionization experiment Density profile measurement using interferometer Ø 7.5GHz MW source installation Plasma current ramp-up using second or third harmonics Study for the ECH harmonic efficiency Z (m) GHz FHM 1 7.5GHz SHM.5 Coil Geometry 2.45GHz FHM 7.5GHz THM O-mode cutoff Ø EBW mode conversion experiment Using movable limiter-antenna To make steep density gradient near UHR Ø Ray tracing code To make simple ray tracing code in cold & warm plasma Various EC Resonance Layer in VEST 2.45GHz UHR R (m) 22/22 Mode conversion rate versus L n at LATE [4]

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

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,