Design Features and Commissioning of Versatile Experiment Spherical Torus (VEST) at Seoul National University K. J. Chung, Y. H. An, B. K. Jung, H. Y. Lee, C. Sung, Y.S. Na and Y. S. Hwang Center for Advance Research in Fusion Reactor Engineering Department of Nuclear Engineering Seoul National University Hello, My name is YongHwa An from Seoul National University, Korea. Today topic of my talk is “Optimization of enhanced surface production by the negatively biased electrode in H- ion sources.” The 8th General Scientific Assembly of Asia Plasma and Fusion Association (APFA 2011) November 1-4, 2011, Guilin, China

Outline Background & Motivation Introduction to VEST Device Design Features Key Design Points with Operational Scenario Research Plans Commissioning Vacuum Chamber and Center Stack TF Coil System PF Coil System ECH Pre-ionization System Diagnostics Plan Port Assignment Summary

Background & Motivation Spherical Torus (ST) Spherical Torus (ST) : Low aspect ratio (A<2) fusion device Large aspect ratio (Conventional Tokamak) Small aspect ratio (Spherical Torus) a R Advantages Weakness High performance : High β, High plasma current Compactness Difficulty in start-up & sustainment due to the lack of space for solenoid Innovative start-up method is critical issue for ST! By developing new start-up and non-inductive current drive methods, ST can be a high performance fusion research device.

Introduction VEST Status Man Power, but NO Power yet 23 June 2011 VEST successfully installed at SNU

Introduction VEST(Versatile Experiment Spherical Torus) Objectives Basic research on a compact, high- ST (Spherical Torus) with elongated chamber in partial solenoid configuration Study on innovative partial solenoid start-up, divertor, 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] 0.1 0.3 Major Radius [m] 0.4 Minor Radius [m] Aspect Ratio >1.3 Plasma Current [kA] 30 100 Safety factor, qa 7.4 6.7 * Elongation : 3.3 assumed

Key Design Point Solenoid-Free Start-up Plasma injection and merging is one approach for solenoid-free start-up Existing injection scenario Compression-Merging method : Use in-vessel PF coil’s swing (START, MAST) Drawbacks of in-vessel PF coil Impurity Engineering problems Double Null Merging (DNM) : Use Outer PF coil swing (UTST) Limitation : Hard to get equilibrium

Key Design Point Double Null Merging Start-up with Partial Solenoids Relatively smaller space for central solenoid in ST. Hard to supply sufficient magnetic flux. Breakdown by Partial Solenoid Solenoid Start-up The most effective start-up method High shaping and equilibrium ability Hard to keep low aspect ratio Difficult to apply to spherical torus Plasma Merging Partial Solenoid Operation Inherits the merits of solenoid start-up Possible to maintain low aspect ratio Effective Start-up method in Spherical Torus Breakdown by Partial Solenoid Partial Solenoid

Operational Scenario Loop Voltage Calculation with Circuit Model eddy currents are accounted in this model Most severe at thick cover and wall close to PF2 Eddy current decay most slowly in thick cover wall Eddy current @ Chamber wall Loop voltage deceased by eddy current 38% Reduced

Operational Scenario Field Null Formation and Lloyd Condition Field null formation with circuit model PF2, 3, 5, 8 Current Waveform Field Null Lloyd condition: Lloyd condition Max. at ~15 ms > 100 V/m for 0.7 ms * Induced eddy current at the chamber wall considered.

Pressure Driven Current by ECH Heating Trapped Particle Configuration Operational Scenario Start-up Scenario utilizing Pressure Driven Current Breakdown by Partial Solenoid Pressure Driven Current by ECH Heating 2.45GHz 3kW ∙ Trapped Particle Configuration at midplane. Pressure driven current by ECH heating Opposite direction to the main plasma current induced by partial solenoid. But, it can be used as virtual coil for further optimization of null formation.   2.45GHz 6kW X Pressure driven current by ECH Trapped Particle Configuration 2.45GHz 3kW ∙ Breakdown by Partial Solenoid

Research Topics Sequential Tokamak Injection for Ramp-up PF #1 -0.044kAt PF #1 -0.044kAt PF #1 -0.044kA Small Plasma Solenoid 0A +40kAt 0A PF #2 -0.45kAt PF #2 -0.449kAt PF #2 -0.449kA PF #3 -1.83kAt PF #3 -1.833kAt PF #3 -1.833kA PF #4 -6.55kAt PF #4 -6.546kAt PF #4 -6.546kA Ip ~ 12kA Ip ~ 12kA Ip >12kA Solenoid charging Solenoid Swing Down Plasma injection +40kAt 0A Solenoid 0A Small Plasma Study on the feasibility of a sequential tokamak plasma injection method for maintaining plasma currents through partial solenoid operations.

Research Topics EC/EBW H&CD Use of ECRH is limited in ST device due to cutoff density. However, Electron Bernstein Wave (EBW) heating through mode conversion is possible since EBW has no cutoff density. 7.5GHz ECR 5GHz ECR 2.45GHz FHM 5GHz SHM 7.5GHz THM HFS Launching O-mode cutoff 2.45GHz System under Preparation EBW heating and current drive will be studied [V.F. Shevchenko, et. al., Nucl. Fusion 50 (2010) 022004] Schematic of the EBW assisted plasma current start-up in MAST LFS Launching 2.45GHz UHR Various EC Resonance Layer in VEST

Research Topics Innovative Divertor Concepts Control of heat flux and neutron flux is one of important issue in fusion research ST with high heat density and compactness is appropriate to heat flux study. VEST with enough space for divertor and sufficient number of PF coils is suitable for divertor research Innovative divertor concepts such as super-X and snow-flake divertors will be investigated. 8 pairs of PF coils Enough Space for Divertor Single Null (Left) and Snowflake (Right) Configuration in TCV [F Piras et al,Plasma Phys. Control. Fusion 52 (2010) 124010 ]

Vacuum Chamber and Center Stack Overall Dimensions of VEST Overall dimension of VEST Thickness of vacuum chamber wall PF3 PF4 ~ 1.1 m PF2 17mm 15mm 13mm 15mm PF5 0.6 m 3.4mm 6mm PF6 ~ 2.7 m PF7 0.8 m PF8 1.2 m 5mm 15mm PF1 15mm 13mm PF9 0.6 m 2.8mm 6mm PF10

Center Stack and Bottom Chamber Vacuum Chamber and Center Stack Design and Fabrication of Vacuum Chamber Lower Chamber Upper Chamber Rectangular Port 12” Port 10” Port Main Chamber 4” Port 6” Port Middle Chamber Center Stack Parts Center Stack V/V : S/S 316L Center Stack and Bottom Chamber

Vacuum Chamber and Center Stack Design and Fabrication of Center Stack Nipples for water cooling Cu block is brazed “Section A-A” PF1 Coil 2.4 m ~268mm G-10 Block Partial Solenoid (PF2) f87mm Epoxy molded A A f165mm Center Stack Chamber Wall 87mm Thin Solenoid (PF1) TF Coils (24ea) Inner TF Coils Inner Pipe (PF1 Bobbin)

TF Coil Design Parameters of TF Coil Design values BTF [T] at R0 0.1 Coil length [m] 2.7 Wire size [mm2] Inner: 124 (12 x 12) * Cooling channel : 6Φ Outer: 500 (50 x 10) No. of turns [#] 24 (12 x 2 pairs) ITF [kA] 8.33 Driving circuit Battery R [mΩ] 15 (18.8 measured) L [mH] 1.0 (0.93 measured) Battery bank 100 Ah battery x 200 ea Internal Resistance of battery bank ~ 10 mΩ (200 ea Total) V0 [V] 250 (20 batteries in series) Switching Magnetic Contactor Strand Specification for TF Coils Structure of VEST TF coil

TF Coil TF Coil Power Supply: Battery Banks VEST TF Coil Power Supply Pneumatic Switch MC Switch Battery Bank Designed to able to supply up to 8.3 kA Based on commercial deep-cycle battery 5 banks with 40 batteries for each bank (Total 200 batteries are used to make 0.1 T) Adv.: Cost effective and long flat-top Disadv.: Always contain large energy VEST TF Coil R = 15 mΩ L = 1 mH Emergency Safety Breaker Fuse 8.3kA for 4s

Commissioning Battery-based TF Power Supply TF coil current of 8.6 kA is achieved successfully by using 8 battery modules (Note: BT@R0 = 0.1 T for 8.3 kA TF coil current). To minimize high current load on batteries during the turn-off, the sequential switch-off is adapted.

PF Coil Overall Features of Various PF Coils Long Solenoid for plasma sustaining Null formation PF3 PF4 Vertical stability & Control of small plasma PF1 PF5 Null formation Equilibrium of small plasma PF2 PF6 Vertical stability & Control of small plasma PF7 PF8 Null formation PF9 Partial solenoid for plasma start-up Equilibrium of main plasma Strand Size - PF1 & PF2: 3.5mm*15mm - PF3 ~ 10 : 6.5mm*6.5mm (3.5ф hole) PF10 Pancake design of PF 3~10 Pancake module : 6*2 strands 4*2 strands for PF #3 & #4 Role of each PF coil

PF Coil Design Parameters for PF1 & PF2 Solenoid Coils Initial Goal Large plasma of 30 kA Small plasma of 10 kA Volt-sec [mV-s] ~ 55 ~ 56 (28 x 2) Required A-T [MA] 5.2 0.21 x 2 Rin / Rout [m] 0.045 / 0.063 0.08 / 0.125 Coil length [m] 2.4 0.5 x 2 Wire size [mm2] 56.0 (3.5 x 16) N [#] 632 (4 x 158) 250 (10 x 25) x 2 IPeak [kA] 7.3 0.84 Driving Circuit RLC double swing R [mΩ] 68 104 (52 x 2) L [mH] 1.6 7.4 (3.7 x 2) C [mF] 200 / 1 / 500 10 / 0.2 / 50 V0 [kV] +1.0 / +1.0 / -2.0 +0.7 / +0.7 / -0.5 Max. achievable V-s [mV-s] @stress limit 130 545 IPeak [kA] @stress limit 27.3 14.0 BPeak [T] @stress limit 7.4 8.0 Max. sustaining time @thermal limit (90oC) ~ 50 ms ~ 180 ms PF2 (Upper partial solenoid) PF1 (Long solenoid) PF2 (Lower partial solenoid) Stress limit: Tensile strength of Cu ~ 70 MPa

Commissioning PF2 Coil Power Supply Power Supply for PF2 Coils Scheme: Double swing circuit Switching: Thyristor with ferrite isolation Control: Optical trigger SW2 SW1 SW3 PF2 coil C1 (10 mF) C2 (0.2 mF) C3 (50 mF) HV Relay Module SW2 Gate trigger Module Capacitor bank Thyristor Module SW1 SW3

Commissioning ECH Pre-ionization for VEST Breakdown by Partial Solenoid with ECH Pre-ionization Magnetron VEST Preionization Upper & Lower Chamber Two cost-effective homemade magnetron power supplies (3kW, 2.45GHz) Low field side launching Main Chamber Commercial microwave power supply (6kW, 2.45GHz ) 2.45GHz 3kW Merged Plasma 2.45GHz 6kW 6” port WR284 Waveguide Vacuum Window Reducer WR340→WR284 WR340 3kW, 2.45GHz Magnetron ECH injection system for pre-ionization 2.45GHz 3kW Breakdown by Partial Solenoid with ECH Pre-ionization

Preparation for Operation Diagnostics Plan Diagnostic Method Installation Status Remark Magnetic Diagnostics Rogowski Coil Initial installation Fabricated Under fabrication 3 out-vessel 2 in-vessel Pick-up Coil Prototype Test Initially 16 pick-up coils Magnetic Probe Array Under design Vacuum field and merging plasma measurements Flux Loop Initially 10 loops Probe Electrostatic Probe Optical OES Monochromator Interferometry (Under design of phase comparator and supporting structure) 94 GHz Fast CCD camera Near Term Ordered 20 kHz Soft X-ray array AXUV16ELG Photodiode Thomson Scattering Long Term - Nd:YAG Laser 1.2J/pulse 10ns

Preparation for Operation Port Assignment MM 11 : Pumping Duct MM12 : 7.5 GHz Klystron 2.45GHz ECH Power, 6kW UU1/MM1/LD1 : Halpha Monitoring UT1/ LB1 : ECH (Magnetron, 3kW) MM10 : Vacuum BNC MM2 : NBI In UU3 : Interferometry MM3 : Laser In (TS,LIF) UU9/LD9 : Vacuum BNC MM9 : Soft X-ray UU8 : GDC MM8 : Fast CCD LD8 : Pressure Guage UT4 : Piezoelectric valve MM4 : Detector for CXS UT7/LB7 : ECH (Power Supply) MM7 : Laser Out (TS,LIF) UU5/LD5 : Fast CCD MM5 : Laser Detector (TS,LIF) MM6 : NBI Dump Yellow color in future plan.

Fist plasma is coming soon ! Summary Summary A new spherical torus named as VEST (Versatile Experiment Spherical Torus) has been built to be a low cost, compact, educational fusion research device at Seoul National University. Operation scenarios with two partial solenoid coils are prepared by using a simple circuit model accounting eddy currents. Various research plans such as double null merging start-up, sequential tokamak injection, innovative divertor concepts and non-inductive heating & current drive with EBW are considered. Coil and heating powers with basic diagnostics are under preparation. Discharge cleaning and breakdown-test are ongoing. Fist plasma is coming soon !

VEST Installation Movie Clip Thank you for your attention ! VEST Installation Movie Clip

Thank you for your attention ! Collaboration is strongly Welcome !

Center for Advance Research in Fusion Reactor Engineering (CARFRE) Lead the development of key technologies crucial for continuous and stable operation of fusion reactors Develop analysis tools and compile experimental database for fusion reactor design and systems integration Foster well-trained fusion research personnel Promote international collaborations in fusion research <Group 1> Fusion Reactor Systems Integration and Plasma Control Technologies <Group 2> Fusion Reactor Edge Plasma Technologies <Group 3> Advance Technologies of Fusion Energy Conversion System Organization: 10 Projects with 16 Principal Researchers from 5 Major Universities and 2 National Institutes Funding: ~1M US$/yr for 6.5years(+ 3 years optional) 28

Systems Integration and Plasma Control Technologies Phase 1 (Sep 2008- Feb 2012): Key Technology Development with Research Infrastructure Group 1 Fusion Reactor Systems Integration and Plasma Control Technologies Analysis tool for the integrated system Core plasma models 통합시스템 해석체계 노심 플라즈마 모델 Heat and particle flux, System integration data Neutron flux, System integration data Group 2 Fusion Reactor Edge Plasma Technologies Group 3 Advance Technologies of Fusion Energy Conversion System Edge plasma models PFCs property tests Blanket analysis model Tritium behavior analysis PFCs, Blanket High temp/ Low activation material 29

Systems Integration and Plasma Control Technologies Phase 2 (Sep 2013- Feb 2015): Integration of Key Technologies for Applications Group 1 Fusion Reactor Systems Integration and Plasma Control Technologies Analysis tool for the integrated system Core plasma models 통합시스템 해석체계 노심 플라즈마 모델 Heat and particle flux, System integration data Neutron flux, System integration data Group 2 Fusion Reactor Edge Plasma Technologies Group 3 Advance Technologies of Fusion Energy Conversion System Edge plasma models PFCs property tests Blanket analysis model Tritium behavior analysis PFCs, Blanket High temp/ Low activation material 30

Research Topics Sequential Tokamak Plasma Injection Suppose that 12kA main plasma initiated by merging two 6kA plasmas ① ② ③ ④ ① : Solenoid charging-up phase (12kA main plasma sustaining) ② : Plasma current ramp-up by merging (ramp up from 12kA to 21kA) ③ : Solenoid charging-up phase (21kA main plasma sustaining) ④ : Plasma current ramp-up by merging (ramp up from 21kA to 30kA)

List of Research Topics for VEST 1. Fusion Engineering Plasma startup and ramp-up Innovative divertor concept PFC surface coating and material test 2. Plasma Transport and Stability Confinement scaling Effect of SOL flow on plasma rotation Alfven wave characteristics in spherical torus Magnetic reconnection mechanism during merging phase Bootstrap current in spherical torus 3-D physics by internal coils like magnetic perturbations. Asymmetric plasma characteristics in same magnetic flux surfaces Transport & Stability studies during merging phase Shaping effect on plasma including negative triangularity Isotope effect as working gas 3. Heating and Current Drive ECH&CD including mode conversion Synergetic effect among heating schemes 4. Plasma-Wall Interactions SOL physics Dust