1. 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, March 2016

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

3. Observation of GAM in SINP Tokamak
Observation of Coherent Mode in the range of kHz in floating potential spectrum during high qedge plasma discharge using Langmuir Probe Diagnostics
Poloidally and Toroidally Coherent mode (m=n=0)
R: 30 cm , a: 7.5 cm, Bt: 0.2 – 0.5 T
Ip: 10 – 12 kA, Vloop: 30 V
Te(edge): 2-12 eV, ne(core): 1-2x1013 cm-3
qedge:
SINP tokamak parameters

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

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

6. Disruption avoidance by electrode biasing
Biased Electrodes are known to produce sheared radial electric fields
leading to generation of sheared poloidal rotations in edge region
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
Typical repetitive discharges of ms are deliberately disrupted by puffing sufficient amount of H2 gas during the Ip flat-top to obtain successive repetitive disruptive discharges.
Capacitor Bank
Electrode Current
Plasma
Vacuum Vessel
Limiter
Flexible
bellow
Gate Valve
SCR
Ceramic
Electrode
High Field Side
Vbias
Material: Molybdenum
Diameter: 5 mm
Tip position inside limiter ~ 3 cm (near qedge ~ 3)
Exposed length ~ 2 cm
Experimental Set-up

7. Disruption avoidance by electrode biasing
Shot # without bias – Disrupted (Black)
Shot # with bias (~ 220V) - 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, (2014)
R.L. Tanna et al., Nucl. Fusion 55, (2015)

8. 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
Increased poloidal flow shear stabilizes both m/n=2/1,3/1 modes
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
Tearing modes generated due to gas puff are stabilized and the disruptions caused by these modes are successfully mitigated

9. SOL turbulence studies in the presence of neutral gas
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
Electron impact ionization of neutral gas and charge exchange processes are considered
Radiative recombination rate has been neglected as its magnitude is small in typical SOL temperature
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, (2015)

10. 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.

11. 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, (2009)

12. CUTIE two fluid nonlinear simulations – ELMs, RMPs
No RMPs
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
No RMPs
RMPs

13. CUTIE two fluid nonlinear simulations – ELMs, RMPs
No RMPs
dTi/Ti0 273 ms
RMPs
dTi/Ti0 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 are 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, (2010)
Reiser and Chandra POP 16,  (2009)

14. Simple 1D model for ELM dynamics
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, (2003) 

15. Simple 1D model for ELM dynamicsEP
(α=0.5, γ=0.5, δ0 =0)
(α=0.5, γ=0.5, δ0=0)
<ψ>
<ξs> Eψ
<ξm>
<J> EP EP EP
(α=1, γ=0, δ0=0)
(α=0.5, γ=0.5, δ0=1e-3)
(α=0.5, γ=0.5, δ0=1e-7) Eψ Eψ Eψ
Simple 1d 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

16. Summary Thank you for your attention!
Observation of Coherent Mode (15-21 kHz) in floating potential spectrum during high qedge plasma discharge using Langmuir probe diagnostics – GAM
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
SOL turbulence studies in the presence of neutral gas
Both density (n) and potential () fluctuations are reduced in the presence of the neutral gas
CUTIE two fluid nonlinear simulations for ELMs, RMPs
RMPs penetrate the plasmas with time and their amplitudes reduce due to screening
push ELMs inside
Development of a simple 1D model for studying ELM dynamics
can demonstrate characteristic ELMs oscns-thermal, current cycles
Anamolous viscosity stabilizes
Thank you for your attention!