Power balance of Lower Hybrid Current Drive in the SOL of High Density Plasmas on Alcator C-Mod

Transcription

1 Power balance of Lower Hybrid Current Drive in the SOL of High Density Plasmas on Alcator C-Mod I.C. Faust, G.M. Wallace, S.G. Baek, D. Brunner, B. LaBombard, R.R. Parker, Y. Lin, S. Shiraiwa, J.L. Terry, D.G. Whyte MIT Plasma Science and Fusion Center October 29, 2014 I.C. Faust, et. al (MIT PSFC) APS 2014 New Orleans, LA October 29, / 12

2 Step response of SOL parameters used to deduce LH power deposition Core - ICRF Edge - LHCD By definition, core-deposited power is lost at τ E timescales. Change in SOL parameters due to LHCD occur on times much shorter than the confinement time. RF Power [MW] Rapid modulation of the applied LH power isolates the deposition of LHCD via background subtraction. q [MW/m 2 ] τe τe I.C. Faust, et. al (MIT PSFC) APS 2014 New Orleans, LA October 29, / 12

3 Fast change in SOL characteristics occur with applied LHCD at low CD efficiency q [MW/m 2 ] LH power [MW] Lyα ne [10 20 m 3 ] Low n e q [MW/m 2 ] LH power [MW] Lyα ne [10 20 m 3 ] Transitional n e q [MW/m 2 ] LH power [MW] Lyα ne [10 20 m 3 ] High n e Amplitude of SOL values change with density, not in time response. I.C. Faust, et. al (MIT PSFC) APS 2014 New Orleans, LA October 29, / 12

4 Fast modulation results yield multiple conclusions. Level of modulation in heat flux increases with density, however it does not modify the time response. Trends in the peak heat flux modulation amplitude correspond to an increase in the total power in the plasma. The loss of efficiency is from direct loss of power at the edge of the plasma. Calculations of ionization power loss from electron cooling suggest a significant but not dominant loss channel. Modulation in the Ly α emission is most significant in the active divertor, suggesting divertor localization in power loss. Profile shapes of n e, T e, q of SOL are self-similar with application of LHCD. Damping of power is not in the far SOL. It is likely correlated with increase of divertor fuelling. I.C. Faust, et. al (MIT PSFC) APS 2014 New Orleans, LA October 29, / 12

5 The diagnostic set describes target, upstream conditions. I.C. Faust, et. al (MIT PSFC) APS 2014 New Orleans, LA October 29, / 12

6 LHRF at high density supplements the total input power At low density, the P tot is constant, high density P tot supplimented by P LH. P tot = (V surf L Ip t )Ip + P LH The LH contribution to the power balance is density-dependent for constant I p, B t, etc. Change in P tot due to LH should be reflected in loss mechanisms (conduction, radiation). Change in total power Ptot/ PLH ne [m 3 ] I.C. Faust, et. al (MIT PSFC) APS 2014 New Orleans, LA October 29, / 12

7 LH power conducted to divertor rises with average density Conducted LH power fraction rises with density until detachment (as measured by surface thermocouple) Similar trend in P tot and q states that an increasing fraction of LH power conducted out decreasing CD efficiency. The change in heat flux must manifest itself in SOL parameters. heat flux q / P LH [m 2 ] Change in peak heat-flux n e [m 3 ] probe Te I.C. Faust, et. al (MIT PSFC) APS 2014 New Orleans, LA October 29, / 12

8 Electron cooling accounts for 10% of LH power losses above [m 3 ] Calculation of loss to ionization has several assumptions: ε = PH rad,iz σv iz,h ev/ionization P iz = ε(s/xb Lyα ) P Lyα E Lyα No In/Out asymmetry. Ionization layer closely localized to SOL. Magnitude of power loss suggests that ionization is a significant, but not dominant loss mechanism. Electron cooling power εiz [kw] n e [m 3 ] n e [m 3 ] time [s] I.C. Faust, et. al (MIT PSFC) APS 2014 New Orleans, LA October 29, / 12

9 Ly α emission enhanced in active divertor high power low power Fast enhancement of Ly α emission in high density near x-point in USN, LSN and DN. Increase in ionization must manifest itself as increasing density, seen in 5-10% variation in n e Change in particle flux is coming from the divertor. Z [m] Lyα Brightness [kw/m 2 ] I.C. Faust, et. al (MIT PSFC) APS 2014 New Orleans, LA October 29, / 12

10 SOL n e, not T e, changes with LH power at high density High LH power, ne Low LH power, ne High LH power, ne Low LH power, ne n e,t e measured with scanning mirror langmuir probe. Monotonic density profile suggests lack of far SOL damping. Neligible change in density is seen at low density ( [m 3 ]. Negligible change occurs to upstream T e with applied LH. Te [ev] ne [m 3 ] ρ ρlcf S [mm] PLH [MW] ρ ρlcf S [mm] I.C. Faust, et. al (MIT PSFC) APS 2014 New Orleans, LA October 29, / 12

11 Conducted power profile does not change at high density Langmuir probe-derived heat flux (q = γj sat T e ) profile shape unchanged with applied LH. Similar behavior seen in H-modes- J. Terry (Talk JI0005) Heat flux profile shape consistent across LHCD density transition. q[mw/m 2 ] Divertor Heat flux profile Peak in heat flux is maintained near strike point, suggests SOL or near LCFS loss of power ρ[mm] q [MW/m 2 ] I.C. Faust, et. al (MIT PSFC) APS 2014 New Orleans, LA October 29, / 12

12 Fast modulation results yield multiple conclusions. Level of modulation in heat flux increases with density, however it does not modify the time response. Trends in the peak heat flux modulation amplitude correspond to an increase in the total power in the plasma. The loss of efficiency is from direct loss of power at the edge of the plasma. Calculations of ionization power loss from electron cooling suggest a significant but not dominant loss channel. Modulation in the Ly α emission is most significant in the active divertor, suggesting divertor localization in power loss. Profile shapes of n e, T e, q of SOL are self-similar with application of LHCD. Damping of power is not in far SOL, likely correlated with increase of divertor fuelling. I.C. Faust, et. al (MIT PSFC) APS 2014 New Orleans, LA October 29, / 12