Overview of Indian activities in PEP Santanu Banerjee

Overview of Indian activities in P&EP Santanu Banerjee Contributors N. Bisai, D. Chandra, L. Lachhvani, J. Ghosh, P. Dhyani, R. L. Tanna, D. Sharma, A. Sen, P. K. Kaw, A. Thayagaraja, ITPA P&EP meeting, IPR, Gandhinagar, 16 -18 March 2016

Outline q Observation of GAM in SINP Tokamak q Disruption avoidance by electrode biasing in ADITYA tokamak q SOL turbulence studies in the presence of neutral gas q CUTIE two fluid nonlinear simulations for ELMs, RMPs q Development of a simple 1 D model for studying ELM dynamics 2

Observation of GAM in SINP Tokamak q Observation of Coherent Mode in the range of 15 -21 k. Hz in floating potential spectrum during high qedge plasma discharge using Langmuir Probe Diagnostics q Poloidally and Toroidally Coherent mode (m=n=0) SINP tokamak parameters R: 30 cm , a: 7. 5 cm, Bt: 0. 2 – 0. 5 T Ip: 10 – 12 k. A, Vloop: 30 V Te(edge): 2 -12 e. V, ne(core): 1 -2 x 1013 cm-3 qedge: 1. 6 -6 3

Observation of GAM in SINP Tokamak Variation of frequency and poloidal wavenumber with edge safety factor • Driving strength of GAMS – poloidal updown asymmetry L. Lachhvani et al. , Submitted to Nucl. Fusion (2016) 4

Disruptions in ADITYA tokamak q Disruptions in typical discharges of Aditya tokamak are analysed thoroughly (Contributed to ITPA Disruption database) q Majority (> 95 %) of disruptions in Aditya show ü MHD growth prior to disruptions ü Identified as m/n = 2/1, 3/1 resistive tearing modes ü Cessation of mode rotations and locking ü Growth of neighbouring chains of islands leads to loss of confinement ü Total termination of plasma current q Disruption can be induced by controlled gas puffing ü Edge cooling leading to generation of resistive tearing modes – causing disruptions 5

Disruption avoidance by electrode biasing q Biased Electrodes are known to produce sheared radial electric fields – leading to generation of sheared poloidal rotations in edge region q Sheared rotations are known to suppress the MHD fluctuation – Hence, MHD generated disruptions in Aditya tokamak are targeted with sheared rotation induced by biased electrode Experimental Set-up Capacitor Bank Typical repetitive discharges of 70 - 80 ms are deliberately disrupted by puffing sufficient amount of H 2 gas during the Ip flat-top to obtain successive repetitive disruptive discharges. SCR Electrode Current Vbias Flexible bellow Gate Valve High Field Side Ceramic Material: Molybdenum Diameter: 5 mm Tip position inside limiter ~ 3 cm (near qedge ~ 3) Exposed length ~ 2 cm Limiter Electrode Plasma Vacuum Vessel 6

Disruption avoidance by electrode biasing Shot # 26714 without bias – Disrupted (Black) Shot # 26719 with bias (~ 220 V) - Disruption avoided (Red) With Gas puffing at t ~ 42 ms MHD Oscillations increases with gas puff in Disruptive discharge Growth of m/n = 2/1, 3/1 modes Mode rotation ceases With Application of bias at t ~ 41 ms Modes do NOT grow Mode rotation continues And Disruption does NOT occur !!! • • P. Dhyani et al. , Nucl. Fusion 54, 083023 (2014) R. L. Tanna et al. , Nucl. Fusion 55, 063010 (2015) 7

Disruption avoidance by electrode biasing With Application of bias voltage Plasma potential profile gets modified and Radial Electric field Er and its shear increases Leading to As the bias voltage is increased ü Increased poloidal flow shear stabilizes both m/n=2/1, 3/1 modes ü Saturated island width and stability index ’a decreases slowly with the increase in poloidal flow shear When the flow shear becomes equal or more than the magnetic shear at / r 0. 45 at bias voltage 180 V ü Tearing modes generated due to gas puff are stabilized and the disruptions caused by these modes are successfully mitigated 8

SOL turbulence studies in the presence of neutral gas q Modeling: Scrape-off layer turbulence has been modelled using two-dimensional fluid model that consists of – Continuity equation – Quasi-neutrality condition – Neutral gas continuity equation q Electron impact ionization of neutral gas and charge exchange processes are considered q Radiative recombination rate has been neglected as its magnitude is small in typical SOL temperature q Further the model is extended to include the effects of finite electron temperature gradient through the – Electron temperature equation • N. Bisai et al. , POP 22, 02251 (2015) 9

Density and potential fluctuations Density (n) and potential ( ) data as a function of time in the presence of neutral gas (WNG) and without neutral gas (WONG). Both fluctuations are reduced in the presence of the neutral gas. 10

Comparison of freq. spectrum in simulation and expt. Left figure: Comparison of frequency spectrum with neutral gas (WNG) and without neutral gas (WONG). In WNG frequency shifts towards lower frequency side. Right figure: Aditya experiment • R. Jha et al. , Plasma Phys. Contr Fusion 51, 095010 (2009) 11

CUTIE two fluid nonlinear simulations – ELMs, RMPs No RMPs • n=2 static RMPs applied at wall for all m values – • RMPs penetrate the plasmas with time and their amplitudes reduce due to screening- • push ELMs inside 12

CUTIE two fluid nonlinear simulations – ELMs, RMPs No RMPs d. Ti/Ti 0 contours @ 273 ms Redistribution of mode energies among different mode numbers due to RMPs (Lower amp. Pushed in) • • RMPs amplitude increased up to 417 ms and then decreased to same value, observed hysteresis in states. There also thresholds beyond which confinement changes. At much larger amplitude confinement starts degrading but it recovers when amplitude is reduced • Thyagaraja et al, POP 17, 042507 (2010) • Reiser and Chandra POP 16, 042317 (2009) 13

Simple 1 D model for ELM dynamics q Objectives: To understand the effects of different physical quantities on the complex ELMs relaxation cycles Model equations Hysteresis parameters • Tangri et al, Phys. Rev. Lett. 91, 025001 (2003) 14

Simple 1 D model for ELM dynamics <P> (α=0. 5, γ=0. 5, δ 0 =0) EP (α=0. 5, γ=0. 5, δ 0=0) <ψ> <ξs> Eψ <ξm> <J> EP EP (α=1, γ=0, δ 0=0) (α=0. 5, γ=0. 5, δ 0=1 e-7) EP (α=0. 5, γ=0. 5, δ 0=1 e-3) Eψ Eψ Eψ • Simple 1 d model can demonstrate characteristic ELMs oscns-thermal, current cycles • Very Preliminary results: Key terms and physics yet to be identified • Regular thermal oscillations-current cycle introduce big oscillations-viscosity stabilizes 15

Summary q Observation of Coherent Mode (15 -21 k. Hz) in floating potential spectrum during high qedge plasma discharge using Langmuir probe diagnostics – GAM q Disruption avoidance by electrode biasing in ADITYA tokamak – Plasma potential profile gets modified and radial electric field Er and its shear increases – Increased poloidal flow shear stabilizes both m/n=2/1, 3/1 modes – Tearing modes generated due to gas puff are stabilized and the disruptions caused by these modes are successfully mitigated q SOL turbulence studies in the presence of neutral gas – Both density (n) and potential ( ) fluctuations are reduced in the presence of the neutral gas q CUTIE two fluid nonlinear simulations for ELMs, RMPs – RMPs penetrate the plasmas with time and their amplitudes reduce due to screening – push ELMs inside q Development of a simple 1 D model for studying ELM dynamics – can demonstrate characteristic ELMs oscns-thermal, current cycles – Anamolous viscosity stabilizes Thank you for your attention! 16