Overview of Versatile Experiment Spherical Torus (VEST)

Y.S. Hwang and VEST team

March 29, 2017 CARFRE and CATS Dept. of Nuclear Engineering Seoul National University

23rd IAEA Technical MeetingExperimental on Researchresults and plans of Using VEST Small Fusion Devices, Santiago, Chille Outline  Versatile Experiment Spherical Torus (VEST) . Device and discharge status  Start-up experiments . Low loop voltage start-up using trapped particle configuration . EC/EBW heating for pre-ionization . DC helicity injection  Studies for Advanced Tokamak . Research directions for high-beta and high-bootstrap STs . Preparation of long-pulse ohmic discharges . Preparation for heating and current drive systems . Preparation of profile diagnostics  Long-term Research Plans  Summary

1/36 VEST device and Machine status

VEST device and Machine status

2/36 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 start-up, non-inductive H&CD, high  and innovative divertor concept, etc

 Specifications

Present Future Toroidal B Field [T] 0.1 0.3 Major Radius [m] 0.45 0.4 Minor Radius [m] 0.33 0.3 Aspect Ratio >1.36 >1.33 Plasma Current [kA] ~100 kA 300 Elongation ~1.6 ~2

Safety factor, qa ~6 ~5 3/36 History of VEST Discharges • #2946: First plasma (Jan. 2013)

• #10508: Hydrogen glow discharge cleaning (Nov. 2014) Ip of ~70 kA with duration of ~10 ms • #14945: Boronization with He GDC (Mar. 2016) Maximum Ip of ~100 kA • # ??: Long pulse with coil power supply upgrade (March. 2017)

4/36 Discharge Status (Shot #14945) VEST Discharge Status : 100kA with Boronization (#14945)

• EC-assisted OH discharge – Plasma current 100 kA – Hydrogen plasma with Boronization – Tungsten & Graphite (inboard) limiter

• Equilibrium parameters – R=0.45m/a=0.33m (A=1.36) – Elongation: 1.6 – 2.0 – Edge safety factor: ~ 7

• Limited discharge length – Fast ramp-up  Require large Bv in the beginning – Wall eddy current  Limited time response of Bv – Excessive vertical field near current peak  early termination

5/36 Start-up Experiments

Start-up Experiments

6/36 Start-up experiments Widely used Field Null Configuration (Breakdown/avalanche)

The field null configuration has been generally used even for the ECH-assisted start-up of tokamaks.

Frascati Tokamak Upgrade KSTAR DIII-D [G. Granucci et al, Nucl. Fusion 51 (2011) 073042] [Y.S. Bae et al 2009 Nucl. Fusion 49 022001] [NUCLEAR FUSION, Vol. 31. No. 11 (1991)]

. Field null formation to maximize connection length and consequently sufficient EtBt/Bp . Null quality : size, sustaining time and location  loop voltage and pre-ionization . Notes:  KSTAR: not much help with higher ECH power in the ECH-assisted start-up  DIII-D: vertical field pre-bias helps ohmic start-up with ECH pre-ionization 7/36 Start-up experiments Trapped Particle Configuration (TPC) Y. An et al., Nucl. Fusion 57 016001 (2017) Field null Trapped particle Efficient and robust tokamak start-up demonstrated

• Mirror like magnetic field configuration – Enhanced particle confinement – Inherently stable decay index structure

– Wider operation range of ECH power, pressure and low Vloop 8/36

Start-up Experiment in VEST

Robust and Reliable TPC Start-up Applied to KSTAR Successfully

Earlier plasma colomn formation than field null

. Feasibility study of TPC in KSTAR • Even though low mirror ratio than ST, achieving efficient start-up with TPC • 2nd harmonic delay of 20 ms and ECH plasma density of 4x1018 m-2

• Ip formation with low Et less than 0.2 V/m

J. Lee, et al., Proc. IAEA-FEC 2016 EX/P4-14 Start-up Experiment in VEST H.Y. Lee Low Loop Voltage Start-up with different ECH Power at Low TF Field in TPC

 Earlier Ip initiation at low loop voltage with larger ECH power in double swing TPC start-up  Smaller resistivity initiate higher initial current to form sufficient size of closed flux surface  Threshold current for successful inductive start-up (reference for outer PF start-up)

10/36 EC / EBW pre-ionization VEST pre-ionization with LFS XB mode conversion

ECR 1.2 Coil Geometry 1.5 X-mode_2kW X-mode_3kW 1.0 X-mode_4kW X-mode_6kW 0.8

] n

-3 UH

1 m n

17 0.6 R

[

n 0.4

0.5 0.2

0.0 30 35 40 45 50 55 60 65 70 75 80 85 2.45GHz R [cm] 0 6kW CW Z (m) ECR ECH 24 X-mode_2kW 21 X-mode_3kW X-mode_4kW X-mode_6kW -0.5 18

15 [eV]

e 12 T

-1 9

3 -1.5 30 35 40 45 50 55 60 65 70 75 80 85 0 0.2 0.4 0.6 0.8 1 J.G. Jo R (m) R [cm] 11/36 Start-up experiments in VEST Pre-ionization with Low-Field-Side Launch EC/EBW Heating

• LFS perpendicular injection of ECW 10 kW – Over-dense plasma (X-wave tunneling) – Parametric decay with mode conversion – High density with collisional heating near UHR

J.G. Jo et al., Phys. Plasmas 24 012103 (2017)

• TPC configuration for Pre-ionization – Higher density plasma with broad density profile

– Different density profile with Bt strength

Low TF (Bo ~ 0.5 kG) High TF (Bo ~ 1 kG)

12/36 DC Helicity Injection DC Helicity Injection Start-up

20 15

[kA] 400.4 ms 401 ms 402 ms 403 ms

5 toroidal

I 0 -1.0 -0.8

-0.6

[kV] -0.4

inj -0.2 V

0.0 2.0 1.5

1.0

[kA] 0.5 inj I 0.0 8 Plasma field @ Z = 0, R= 0.089 6 4

Lower Gun Location [mT]

z 2 B 0 • R = 0.25 m -6 Vacuum field @ Z = 0, R= 0.089 • Z = - 0.804 m -7

-8

[mT] Magnetic Field Structure z -9 B • Stacking ratio, G = 7 ~ 8 -10 400 402 404 406 408 410 412 414 416 • Multiplication, M = ~ 16 Time [ms] The Electron Gun for DC Helicity Injection • High current electron beam based on arc discharge w/ washer stacks • Low impurity A toroidal current of up to ~20 kA has been generated by the electron gun • Increased multiplication factor confirmed as current streams reconnect • Very dynamic states of current sheet observed The electron gun J.Y. Park 13/36 Preparation for Advanced Tokamak Studies

Studies for Advanced Tokamak

14/36 Preparation for Advanced Tokamak Studies Scopes of Advanced Tokamak Studies in VEST Fusion reactor requires high beta (or Q) and high bootstrap current Simultaneously

Alpha heating dominant (high Q) High Bootstrap current fraction (high fb)  Centrally peaked pressure profile  Hollow current density profile (low li)

High power Current density profile control neutral beam heating Profile diagnostics Bootstrap/EBW/LHFW

Confinement and Stability? 15/36 Preparation for Advanced Tokamak Studies Experimental Research Direction for VEST Advanced Tokamak Study Scenario

 Powerful Core Neutral Beam Injection (NBI) resembling alpha heating tackles simultaneous achievement of high beta and high bootstrap current

High beta limit in spherical torus favors high beta operation + High bootstrap current fraction  reversed shear configuration

 Spherical Torus with Reversed Shear (RS) may provide AT regime ! • Sufficient confinement from ITB formation by RS • Possible high beta even with low li in RS due to low aspect ratio

High power NBI to center, forming RS in VEST! Advanced Tokamak Regime in VEST* * Estimated by 0-D system code

 Low toroidal field(0.1T) with high βN(~7)< βN,Menard(8.7) and Ip(0.08 MA).  Fully non-inductive CD with 80% bootstrap fraction may be possible with ~500kW.  High H factor(~1.2) needs to be attempted by forming ITB with MHD stability.

16/36 Preparation for Advanced Tokamak Studies S.M. Yang, C.Y. Lee and Yong-Su Na Simulations for the VEST Advanced Tokamak Scenario

 Total power loss is calculated using NUBEAM simulations in various conditions (퐈퐏 = ퟏퟎퟎ 퐤퐀)

19 −3 Total loss ( ne0 = 2 ∗ 10 푚 )

푬풃풆풂풎 R = 35 cm R = 40 cm 10푘푒푉 27% 33% 20푘푒푉 43% 70%

푃푒, 푃푖 at 15° injection 1.4 Pe-20keV,600kW 1.2 Pe-10keV,150kW 1.0 Pi-20keV,600kW 0.8 Pi-10keV,150kW (n = 2 ∗ 1019 푚−3) 0.6 e0 0.4 0.2

Heatingpower ( 0.0 0.0 0.2 0.4 0.6 0.8 1.0 푟/푎

• Based on the result, R = 35 cm equilibrium is chosen for better NBI power absorption.

• 15 degree of the injection angle is chosen considering both power loss and engineering issue in VEST and its heating power profiles are calculated. 17/36 Preparation for Advanced Tokamak Studies S.M. Yang, C.Y. Lee and Yong-Su Na Simulations for the VEST Advanced Tokamak Scenario  1-D transport simulation  NBI heating with NUBEAM code (Density and equilibriums are given)  1D integrated modelling with ASTRA code with TGLF module (Density profile is given)

 L-mode (edge at r=0.9) and H-mode cases are compared

 0-D system code analysis

 Low toroidal field 0.1 T  High beta N 7  Plasma current 80 kA

 Non-inductive 100%  Bootstrap fraction 80%

 Required NB power 500 kW 18/36 Preparation for Advanced Tokamak Studies Preparation for Long Pulse Ohmic Discharges  TF with Ultra Capacitor from Battery  PF in H-bridge circuit with Extra Capacitors

S.C. Hong and J.Y. Park 19/36 Preparation for Advanced Tokamak Studies J.H. Yang and S.C. Kim Development of Long Pulse Ohmic Discharges

+10ms +20ms +30ms -10ms 0ms

CE = 0.4

20/36 With great help from Dr. S.H. Hong (NFRI) Preparation for Advanced Tokamak Studies Discharge Improvement with Boronization • Boronization during Helium Glow discharge cleaning

– Carborane (C2B10H12) is used for non-toxic boronization – Water concentration recovery slowed down (RGA)

• Oxygen atom line emission (777.0 nm) reduced by 3 fold

– High density expected: More prefill gas (Hα line emission)

Helium flow (MFC 2.0 sccm)

Caborane oven (baked 50°C)

Diaphragm valve

Top flange (CF 6 inch)

21/36 Preparation for Advanced Tokamak Studies Diagnostics, Heating and Current Drive Systems in VEST

 Profile Diagnostic Systems  Heating and Current Drive Systems

Magnetics and Spectroscopy Neutral Beam Injection System with ~500kW at 15keV from KAERI Interferometer Interferometry with 94GHz, multi-channel from SNU/UNIST Low Hybrid Fast Wave H/CD with ~20kW at 500MHz from KAERI/KAPRA/KWU

Thomson Scattering with NdYAG laser Electron Cyclotron/Bernstein from SNU/NFRI/JNU/SogangU Wave H/CD with ~30kW at 2.45GHz ECH from SNU/KAPRA

22/36 High power central heating High Power NBI System : Dual Beam to Single Beam S.H. Chung (KAERI)  NBI beam loss calculation with NUBEAM code Beam at 20keV

100 90 80 70 60 50 40

Loss Ratio [%] 30 20 counter-injection 10 co-injection 0 5.0x1018 1.0x1019 1.5x1019 2.0x1019 2.5x1019 3.0x1019 Target Plasma Density(#/m3)

. Too large orbit loss for counter-injection  Dual beam to single beam . Large shine through loss even for co-injection  Require high plasma density 23/36 High power central heating High Power NBI System : System Design  Beam power density at beamline  Focusing beam profile calculation Calculated by BTR code 40

35 3.1 m Max Power 2 30 : 32.72 MW/m 0.7m 25 Max Power

: 28.12 MW/m2 2.6 m

] 20 Max Power 2 2016-10-22 : 35.16 MW/m2 2016-09-23 /m 15 2016-08-24 1.8 m 2016-07-25 Max Power 10 2016-06-26 : 36.16 MW/m2 2016-05-27 5 2016-04-28 1.4 m 2016-03-29 Max Power 0 2

Maximum Beam Power Density [ MW : 32.77 MW/m 0 1 2 3 Distance from Beam Exit [ m ]  Slit beam focusing  3D beam line design

Defl. angle, # of slits - 2 degree 29 - 1 degree 3 0 degree 13 1 degree 3 2 degree 29

24/36 High power central heating High Power NBI System : Ion Source Test Completed

• Neutral beam injector – Co-directional – Design power: 500 kW

• Ion source test completed – 750 kW at 15 keV

• Under commissioning – Power supply – Beamline

• Installation schedule – March~April 2017

25/36 Current density profile control Core Heating and Current Drive with LHFW

Accessibility for Lower Hybrid Fast Wave(LHFW)

FW path

For high density plasma in fusion reactor • Slow wave branch of LHW → Absorbed at the edge region • Fast wave branch of LHW → Possible absorption at the core region . Possible propagation regime in CMA diagram Proof of principle for current drive scheme - FW launching density ~ f(n||, w, wce) by fast wave branch of LHW in VEST - FW-SW confluence density ~ f(n||, wce)

S.H. Kim (KAERI) 26/36 Current density profile control In collaboration with KAERI and Kwangwoon Univ. LHFW H/CD Simulation and Component Preparation

• LH Fast Wave – Good core accessibility – Propagation at low density

• Hardware – UHF Klystron – Combline antenna

N|| = 3-5 / 500 MHz

27/36 Current density profile control EBW Heating and Current Drive with XB Mode Conversion ► Core heating and current drive with XB mode conversion

 GENRAY, CQL3D, mode conversion codes are used ECR  Perpendicular, LFS, X-mode injection • Short distance between R(X) cut-off and UHR

• High XB mode conversion efficiency with low n|| • Good central heating and current drive expected Absorption near EC fundamental resonance

EBW propagation

S.H. Kim (KAERI) 28/36

Current density profile control EBW Heating with XB Mode Conversion  EBW heating (6kW cw+10kW pulse at 402ms) on Ohmic plasmas

60 off 55 on 50 45 40 35

25 20

Plasma(kA)Current 15 10 5 0 400 401 402 403 404 405 406 407 408 409 410 411 412 Time (ms) • Boronization (impurity removal) and higher plasma current (higher electron temperature) • No change along the electron density profile with additional MW – no collisional damping • Plasma pulse duration is increased • Electron temperature rises two times near 3rd harmonics ECR resonance : the first H.Y. Lee resonance position after mode conversion Status and plan for VEST 29/40 Preparation for Advanced Tokamak Studies VEST Diagnostic Systems Diagnostic Method Purpose Remarks Plasma current & eddy 3 out-vessel & Rogowski Coil current in-vessel coils

Magnetic Pick-up Coil & Bz, Br & 56 pick-up coils Diagnostics Flux Loop Loop voltage, flux 9 loops Magnetic Probe B , B measurement z r Movable single array Array inside plasma 2 Triple Probes Electrostatic Probe Radial profile of T , n Probes e e Mach probe Fast CCD camera Visible Image 20kHz

Hα monitoring Hα Hα filter+ Photodiode Impurity monitoring O & C lines Spectrometer Optical Interferometry Line averaged n 94GHz, multi-channel Diagnostics e Reflectometry Radial profile of ne Edge density profile

EBE radiometer Core, edge Te BX mode conversion Charge Exchange Rotation and T DNB Spectroscopy i

Thomson Scattering Te, ne profile NdYAG laser 30/36 Profile diagnostics Movable Magnetic Probe Array System

• Equilibrium reconstruction with internal probe data

• Direct derivation of JT(R) – w/o vertical displacement – Plasma degradation ~ 10%

J.H. Yang et al., Rev. Sci. Instrum. 83 10D721 (2012) J.H. Yang et al., Rev. Sci. Instrum. 85 11D809 (2014)

J.H. Yang 31/36 Profile diagnostics In collaboration with UNIST and KNU 94 GHz Multi-channel Interferometer

• Microwave system with heterodyne interferometer

• 3.2 GHz IF (avoid EC noise) – EC 2.45 GHz

• Single chord  multi-chord

J. Wang 32/36 Profile diagnostics In collaboration with NFRI and Seogang Univ. Thomson Scattering System • Measurement target 18 -3 – ne: 5×10 m – Te: 10-200 eV • Laser energy: 0.65 J/pulse

Y.-G. Kim et al., Fus. Eng. Des. 96-97 882-885 (2015) D.Y. Kim and Y.G. Kim 33/36 Profile diagnostics Charge Exchange Spectroscopy/Beam Emission Spectroscopy • Ion temperature and rotation measurement

– Dichronic beam splitter: Hα and HeII share collecting optics – Transmission grating • He2+ density measurement • w/wo DNB • Fabry-Perot interferometry

Y.S. Kim, S.G. Ham and K.H. Lee 34/36 Future Research Plans

Long-term Research Plans

 Reactor-relevant Advance Tokamak research  AT scenario for high beta and steady-state operation  Innovative divertor to handle long-pulse high-performance operation  Energetic particle transport …

35/36 Summary

 VEST has achieved successful ohmic operation with plasma currents of up to ~100 kA, elongation of ~ 1.6 and safety factor of ~6 with efficient ECH pre-ionization.

 Efficient start-up method is developed with TPC(Trapped particle configuration) and EBW heating. . EBW heating with direct XB mode conversion from LFS launching by generating over-dense plasma in the pre-ionization phase. . Enhanced pre-ionization with TPC improves start-up with low loop voltage, low ECH power, and wider pressure window. . DC helicity injection startup experiments generate plasma current of ~20 kA.

 Preparation for the study of Advanced Tokamak is progressing . VEST Advanced Tokamak scenario is simulated with transport codes. . Long-pulse ohmic discharges are being prepared by improving PF coil power system. . High power (~ 500 kW) NBI system is under development. . EBW/LHFW heating and current drive experiments are under preparation by performing simulations and constructing hardware systems. . Profile diagnostics are under preparation

 Reactor-relevant AT research by developing both AT scenario and innovative divertor concept will be continued.

36/36 19th International Spherical Torus Workshop (ISTW 2017) will be held at Seoul National University, 19-22 September 2017

Previous ISTWs were held in Oak Ridge (1994), Princeton (1995), Culham (1996), St. Petersburg (1997), Tokyo (1998), Seattle (1999), Sao Jose dos Campos (2001), Princeton (2002), Culham (2003), Kyoto (2004), St. Petersburg (2005), Chengdu (2006), Fukuoka (2007), Frascati (2008), Madison (2009), Toki (2011), York (2013), and Princeton (2015). 37/36 Thank you for your attention !

38/36 Discharge Status (Shot #14945) VEST Discharge Status : 100kA with Boronization (#14945)

• Kinetic plasma parameters from magnetics – Spitzer temperature*: ~200 eV – Diamagnetic flux  Stored energy (perpendicular) <1 kJ – Estimated plasma density: ~2×1019 m-3 • Estimation from triple probe measurements – ~ 1.5×1019 m-3 at 100 kA from ~ 8×1018 m-3 at 60 kA  Greenwald density limit: ~3×1019 m-3

*Zσ = 3 / trapped particle fraction 88% 39/36 Start-up experiments Current initiation and field structure

-1

-2 PF1 Current

PF3&4 Current PF Current [kA]Current PF -3 PF9 Current

10 w/ Trapped Particle Configuration (PF3&4) w/o Trapped Particle Configuration (PF3&4) 8

Plasma[kA]Current 0 2

0 Flux Loop [V] Loop Flux -1 w/ Trapped Particle Configuration (PF3&4) w/o Trapped Particle Configuration (PF3&4)

390 392 394 396 398 400 402 404 406 408 410 Ip~8kA No Ip Time [ms] #5252 / #5255

 The field structure with no Ip does not provide stable field structure. 40/36 Start-up experiments Current initiation and field structure

-1

-2 PF1 Current

PF3&4 Current PF Current [kA]Current PF -3 PF9 Current

10 w/ Trapped Particle Configuration (PF3&4) w/o Trapped Particle Configuration (PF3&4) 8

Plasma[kA]Current 0 2

0 Flux Loop [V] Loop Flux -1 w/ Trapped Particle Configuration (PF3&4) w/o Trapped Particle Configuration (PF3&4)

390 392 394 396 398 400 402 404 406 408 410 Ip~8kA No Ip Time [ms] #5252 / #5255

 It was observed that negligible plasma current even with better Et*Bt/Bp and sufficient Vloop. 41/36 Start-up Experiment in VEST Decay Index for each scenario

Trapped Particle Configuration Stable Bv only Field Null

2.0 Trapped Particle • Conventional field null configuration Conventional w/ stable B 1.5 v does not provide stable B structure Conventional v at the onset of loop voltage 1.0 • The other scenarios provide stable B 0.5 v structure at the onset of loop voltage

0.0 0n 3 / 2 Decay Index Index Decay -0.5 R B n  z -1.0  0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 BRz  Major Radius [m] 42/36 Start-up Experiment in VEST Efficient volt-second consumption

Trapped Particle Configuration Stable Bv Conventional

25 25 25 #8556 #8553 #8230 20 20 20

15 15 15

10 10 10

5 5 5

Plasma Current [kA] Current Plasma

Plasma Current [kA] Current Plasma Plasma Current [kA] Current Plasma 0 0 0 8 8 8 6.9 mV∙s 8.5 mV∙s

6 4 mV∙s 6 6

4 4 4

2 2 2

Volt-second[mVs] Volt-second[mVs]

Volt-second [mVs] Volt-second Wasted V∙s 0 0 0 400.0 400.2 400.4 400.6 400.8 401.0 401.2 401.4 400.0 400.5 401.0 401.5 402.0 401.5 402.0 402.5 403.0 403.5 404.0 404.5 405.0 Time [ms] Time [ms] Time [ms]

1. Enhanced pre-ionization by TPC 1. Degraded pre-ionization 1. Stable Bv at the onset of Vloop 2. Stable Bv at the onset of Vloop 2. Not stable Bv at the onset of Vloop

1. When sufficient pre-ionization and proper field structure is provided wasted volt-second is not shown in field null configuration.

2. Use of trapped particle configuration further reduces the required volt-second

43/36 Start-up experiments Trapped Particle Configuration (TPC) Y. An et al., Nucl. Fusion 57 016001 (2017) Field null Trapped particle Efficient and robust tokamak start-up demonstrated

• Mirror like magnetic field configuration – Enhanced particle confinement – Inherently stable decay index structure

– Wider operation range of ECH power, pressure and low Vloop 44/36 Ohmic Start-up with ECH Pre-ionization

Higher dIp/dt and Wider Operation Regimes with TPC

Higher dIp/dt under TPC with identical Vloop and EtBt/Bp Wider operating windows for gas pressure and ECH power 10 with TPC 9 without TPC 8 7 6 5

4 /dt [kA/ms] /dt

p 3 dI 2 1 20 40 60 80 100 120 140 160 180 E *B /B [V/m] at R=0.4m 7 t t p with TPC 8 With TPC 6 without TPC 7 Without TPC 5 6 4 5

4 3

3 2

/dt [kA/ms] /dt p

/dt [kA/ms] /dt 2 p dI 1 dI 1 No current initiation 0 0 No current initiation -1 1 2 3 4 5 6 Y.H. An 10-5 10-4 ECH Power [kW] Filling Pressure [Torr] 45/36 Start-up Experiment in VEST Solenoid-free Start-up from Outboard with Outer Poloidal Field Coils H.Y. Lee  The closed flux surfaces have not been formed in the cases of both 6kW and 16 kW due to their high resistivity of > 1.0 E-5.  In the case of the resistivity of 1.0 E-5, the closed flux surface has

small minor radius 3~4 cm with very low Ip.  Once current column is formed, inward moving current can grow by utilizing decreasing external inductance even with small E field.  Plasma current may be initiated with resistivity of less than 1.0 E-5.

Resistivity Resistivity 1E-5 5E-6

 To reach target resistivity, higher ECH power of ~30kW may be needed to have sufficient density and temperature in the pre-ionization phase. 46/36 Start-up experiments Widely used Field Null Configuration (Breakdown/avalanche)

The field null configuration has been generally used even for the ECH-assisted start-up of tokamaks.

Frascati Tokamak Upgrade KSTAR DIII-D [G. Granucci et al, Nucl. Fusion 51 (2011) 073042] [Y.S. Bae et al 2009 Nucl. Fusion 49 022001] [NUCLEAR FUSION, Vol. 31. No. 11 (1991)]

. Field null formation to maximize connection length and consequently sufficient EtBt/Bp . Null quality : size, sustaining time and location  loop voltage and pre-ionization . Notes:  KSTAR: not much help with higher ECH power in the ECH-assisted start-up  DIII-D: vertical field pre-bias helps ohmic start-up with ECH pre-ionization 47/36 Start-up Experiment in VEST Mirror Ratio and Decay Index for Efficient Start-up in TPC

Stable Bv only Field Null

Trapped Particle Configuration (TPC) • Conventional field null configuration 2.0 Trapped Particle does not provide stable Bv structure Conventional w/ stable B 1.5 v at the onset of loop voltage Conventional 1.0 • Other scenarios provide stable Bv structure at the onset of loop voltage 0.5 0n 3 / 2

0.0 Decay Index Index Decay R Bz -0.5 n  BRz  -1.0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 • Efficient start-up with better pre- Major Radius [m] ionization at high mirror ratio 48/36 Ohmic Start-up with ECH Pre-ionization Double Swing Start-ups with TPC and FNC

PF#1 Double swing TPC

100 shot #14458, Full flux TPC shot #14597, Full flux FNC, same day 80

20 Plasma current [kA] current Plasma 0 402 404 406 408 410 412 414 Time [ms]

PF#1 full flux startup using field null vs. TPC Null: PF#1+PF#10, TPC: PF#1+PF#5 Fast plasma current ramp-up

49/36 J.W. Lee

Efficient ECH-assisted start-up using trapped particle configuration in VEST & KSTAR and application for ITER J. Lee, J. Kim, Y.H. An, M.-G. Yoo, Y.S. Hwang and Y.-S. Na (SNU and NFRI in Korea)

Reliable ECH-assisted start-up with

ITER-relevant Et in KSTAR

• BRIS-less start-up to counter Ip operation for various physics studies in KSTAR

• ITER-relevant Et level of less than 0.3 V/m • Unstable start-up with field null even with ECH-assistance

• Reliable low Et start-up with pre- ionization plasmas under TPC

Bp and pol. flux at 0.2 s Preliminary ITER scenario development using TPC

• Scenario development based on ITER_c_33NHXN_v3_4, appendix 6, half charging CS and P/S configuration B • With consideration of eddy current evolution driven by PF coils on conducting structure • Almost identical operation of solenoid coils, and newly determining the outer PF currents for TPC configuration

• About 0.2 s after solenoid swing, achieving the Et for Ip formation based on KSTAR result • Save V∙s and avoid operating solenoid coils up to its engineering limits, Et of 0.3 V/m Current density profile control Simulation Results for LHFW Heating and Current Drive S.H. Kim (KAERI)  TORIC Full Wave simulation : Absorption  LHFW RF system and - Frequency = ~ 500 MHz (UHF) - N∥ = 3 ~ 5, B0 = 0.2 T operation parameters 18 3 - ne0 ~ 1x10 #/m , Te0 ~ 1 keV  1D full wave coupling code

Ez field on Poloidal C.X. Power absorption dist. Driven current profile  GENRAY-CQL3D simulation : Current Drive

Driven current profile

Coupling cal. condition Coupling efficiency vs. den.

☆ In addition, 1D full wave code is developed to calculate LHFW ray tracing 17 3 LHSW ray tracing Velocity dist. on phase space by CQL3D condition for efficient LHFW coupling: nb~3x10 #/m 51/36 Current density profile control LHFW Components  Combline Antenna

 Klystron Frequency 470~520 MHz

 The klystron is prepared by refurbishing an VSWR < 3:1 old UHF broadcasting system of Korea which Peak n 3.25 has been hold by SNU and KAPRA. || Antenna size length = 486 mm, thickness = 36mm

Ey/Ez ratio 4~30 (average of 71.35%)

Peak E-field < 24.4kV/cm

S.H. Kim (KAERI) and B.J. Lee (KwangWoon Univ.) 52/36

EBW Heating Experiments in linear device Experimental Setup Bent HFS Injection Magnetic

Field(BT)

Quartz Tube ` Pump Stainless Steel Wall System W R 2 8 4 MW Loss LFS 3 Stub Protection Tuner Injection

Quartz Quartz Radial Field 2.45 GHz 950 Magnetron  Objectives : To find the feasibility of ECH from EBW via XB mode conversion. 900  Small cylindrical device with axial magnetic field bent 850 similar to tokamak

800  Possible to change the direction of MW injection mode

MagneticField(G) 875G HFS LFS – LFS&HFS 750  Easily changeable of magnetic field and MW power –

700 movable location of ECR -6 -4 -2 0 2 4 6 Distance (cm) 53/22 Joint meeting on VEST & QUEST on December Antenna 17th 2015, at : Kyushuopen Univ. waveguide(WR 284) LHY_JMVQ_DEC17

ECH Experiment in linear device 7.5 Over-dense plasma generation High Field Side X-mode Low Field Side X-mode

6.0 ]

-3 4.5

10 [

3.0 e n L-cutoff density 17 -3 1.45 x 10 m 1.5

0.0 300 400 500 600 700 800 900 Microwave Power [W]

 Diagnostics – measured at r = 0  The plasma with higher electron density generates from LFS X mode than from HFS X mode.  As shown in CMA diagram, LFSX/HFSX injection does not exceed R/L cutoff but in the experiment LFSX injection overcomes the R cutoff via mode conversion.  Feasibility of EBW via direct XB mode conversion 54/22 Joint meeting on VEST & QUEST on December 17th 2015, at Kyushu Univ. LHY_JMVQ_DEC17 EC / EBW pre-ionization Analysis on VEST pre-ionization plasma ECR ECR 1.2 24 X-mode_2kW ECR X-mode_2kW X-mode_3kW 21 X-mode_3kW 1.0 X-mode_4kW X-mode_4kW X-mode_6kW X-mode_6kW 18 0.8

] n

-3 UH 15 m n

17 0.6 R

[eV]

e 10

[ 12

e UHR n 0.4 9

0.2 6

0.0 3 30 35 40 45 50 55 60 65 70 75 80 85 30 35 40 45 50 55 60 65 70 75 80 85 R [cm] R [cm]

 Power absorption near the UHR(ne) by collisional damping and ECR(Te) by collisionless damping respectively.  Shorten density scale length by steep density gradient near outer limiter  1D full wave code has been developed and XB mode conversion efficiency is estimated to be ~20%.

J.G. Jo and S.H.Kim 55/36 EC / EBW pre-ionization 1D full wave simulation for QUEST

 1D Full wave simulation  XB Mode conversion efficiency calculation  Condition : MW frequency, toroidal magnetic field, density profile

Kim, S. H., et al. Physics of Plasmas 21(6) (2014). "One-dimensional full wave simulation on XB mode conversion in electron cyclotron heating."

28 Ghz 8.2 Ghz 6.00E+018 2nd 1st R cutoff 5.50E+018 UHR 5.00E+018 L cutoff O Cutoff 4.50E+018 2e18  Example : 1D Full wave simulation 4.00E+018 5e18

3.50E+018 8.2 Ghz  QUEST Simulation (8.2 GHz) 2nd

3.00E+018

 Density profile: Assumption Density 2.50E+018 2.00E+018 - Peak density (2e18, 5.5e18 #/m3) 1.50E+018  XB Mode conversion coefficient 1.00E+018 5.00E+017

- 2e18 #/m3: ~1% 0.00E+000 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1.0 1.1 1.2 - 5.5e18 #/m3: ~38% Distance (m) J.G. Jo and S.H.Kim 56/36 Profile diagnostics Imaging Fabry-Perot interferometry

• Gas and Ion temperature measurement

– Hα : Gas temperature

– Ov : Ion temperature at core • High resolution without scanning • Preliminary experiment is in progress

• Measure Isotope shift (Hα Dα lamp) – Take pictures by Nikon camera (f = 120 mm, mirror gap = 1.5 mm) – Real value : 0.178 nm – Measured value : 0.172 ± 0.014 nm

Y.S. Kim and S.G. Ham 57/36 Future Research Plans Innovative Divertor Studies Direct Energy Conversion concept applied in VEST divertor

Conceptual design of DEC applied on VEST

K.S. Chung and K.J. Chung 58/36