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Advances in Edge Ion Heating Observed During HHFW Power Application in NSTX 2004

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12 diapositives · 270 Views · Nov 09, 2014

In the 2004 NSTX campaign, extensive measurements revealed significant edge ion heating effects linked to HHFW power application. High-energy edge ions, reaching up to 500 eV just inside the last closed flux surface (LCFS), were consistently observed. New diagnostics, including swept Langmuir probes and visible cameras, enhanced the analysis. The enhanced emissions of C.III and He.II ions correlated with RF heating, indicating IBW heating as a plausible mechanism. Data from various plasma conditions illustrate how increased RF power leads to observable heating of edge ions, advancing our understanding of plasma behavior.

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Advances in Edge Ion Heating Observed During HHFW Power Application in NSTX 2004
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  1. Edge Ion Heating by Launched HHFW in NSTX In 2003 run campaign, during the application of HHFW power, edge ions were observed to heat to high temperatures; 500 eV at 2 cm in from the LCFS. The 2004 run campaign: greatly expanded number of ERD measurements, with many successful RF experiments, and with new diagnostics (swept Langmuir probe, CHERS, visible camera). Edge ion heating is observed whenever significant HHFW power is applied; for 3.5, 7, 14 m-1 phasing, D2 & He plasmas, in He II and C III edge ions. IBW heating identified as a plausible explanation for edge ion heating. T.M. Biewer, R.E. Bell, S. Diem, P.M. Ryan, J.R. Wilson NSTX Results Review 2004

  2. Introduction/Outline • More extensive ERD data set from 2004 run. • Distortion to the edge He II and C III spectra. • Distortion correlates with application of HHFW. • Edge C III emission increase x10. • Visible camera images show enhancement of emission in front of RF antenna. • Edge Ti and v scale with applied RF power. • Heating observed in many cases. • IBW heating is plausible explanation. NSTX Results Review 2004

  3. Edge Rotation Diagnostic Poloidal View • 10 ms time resolution • 7 toroidal and 6 poloidal sightlines cover 140 to 155 cm at the outboard midplane. • Sensitive to intrinsic emission light of C III, C IV, and He II. • Measures velocity, temperature, and brightness of edge ions. • Spectral resolution of 0.22 Å/pixel with 75 m slits. • http://w3.pppl.gov/~tbiewer/ERD_RSI.pdf Toroidal View Toroidal View Neutral Beam Trajectories Poloidal View Poloidal Chords RF antenna NSTX Results Review 2004

  4. RF Power heats edge ions. Two successive time frames clearly show the spectral consequences of 30 MHz HHFW heating (4.3 MW input) on the edge plasma. This effect is apparent in edge C (impurity) ions and He (bulk) ions. Poloidal, rtan~146 cm Toroidal, rtan~146 cm RF on RF off RF on counts RF off Pixel # (~l) Pixel # (~l) Data is best fit with 2 Gaussian distribution function (hot and cold components). NSTX Results Review 2004

  5. Core e- and edge ion heating is observed. The launched HHFW is expected and observed to heat edge electrons, but hot edge ions are also observed. 1G fit 2G high 2G low NSTX Results Review 2004

  6. Edge light emission increases. RF (cold) Brightness He II (majority ion) C III (impurity ion) Tor. Pol. LCFS LCFS Tor. Pol. Ohmic Ohmic RF (hot) Brightness He II (majority ion) C III (impurity ion) LCFS Tor. Pol. LCFS Tor. Pol. cold pol Ohmic cold Ohmic • The powered RF antenna acts as a source of neutral particles at the edge of the plasma. • In the poloidal view (~20 cm from the RF antenna toroidally) the brightness of C III increases by a factor of 10. • In the toroidal view (~2 m from RF antenna toroidally) the brightness of C III increases by a factor of 3. • Charge exchange with these antenna sourced neutrals allows the formerly fully-stripped, hot helium ions to be seen by the ERD. • “Hot” component represent He2+ dynamics. • “Cold” component represent He+ dynamics. • Time scales allow two populations to exist simultaneously in non-equilibrium: • Emission: ~1 ns. • Ionization of hot He+: ~100 ms. • Thermalization between hot and cold helium: ~10 ms. NSTX Results Review 2004

  7. Visible camera confirms antenna emission. RF off RF on • Visible camera images show enhanced light emission near the RF antenna when power is applied. • Diagnostic capability added for 2004 run campaign. NSTX Results Review 2004

  8. More power leads to more heating. From NSTX Shot 110133 to 110145 the applied RF power was increased. Empirically, Ti increases as PRF0.47. Negative poloidal velocity is upwards on the outboard midplane. Negative toroidal velocity is opposite to the direction of Ip. NSTX Results Review 2004

  9. Edge heating observed in many plasmas. Antenna Phasing (kll) Antenna Phasing Bulk Gas Species Magnetic Configuration 14 m-1 7 m-1 3.5 m-1 Heating Counter-CD Co-CD DND USN LSN D2 He • 2004 run data spans a wide range of plasma conditions. NSTX Results Review 2004

  10. IBW heating of edge ions plausible. See following talk by S. Diem. The 30 MHz launched HHFW is attenuated in this spectrum with a 40 dB notch filter. A portion of the HHFW undergoes nonlinear parametric decay into a daughter IBW and an ICQM, which both damp in the outer 10 cm of plasma. IBW’s are observed as lower sidebands of the launched HHFW at the expected frequencies (dashed vertical lines) for the first 3 harmonics of the ion cyclotron quasi-mode. An Ohmic shot (no RF) is shown (in red), indicating the noise-floor of the measurement. Pick-up from a heterodyne network is present at 31 MHz in both spectra. NSTX Results Review 2004

  11. Summary • Distortion to the edge He II and C III spectra, correlated with application of HHFW. • Edge Ti and v increase with applied RF power. • More extensive ERD data set from 2004 run always shows edge ion heating when HHFW applied: • 3.5, 7, 14 m-1, co-CD, ctr-CD, heating antenna phases • Various Ip, BT, ne • D2, He plasmas • USN, LSN, DND • IBW edge ion heating is plausible explanation. • Parasitic to HHFW heating of core electrons. • Consistent with poloidal/toroidal anisotropy NSTX Results Review 2004

  12. Er from He II and C III (cold) Ohmic DEr~-5 kV/m RF on Ohmic v. RF heated The Er at the edge most region of the plasma (R>146 cm) is more negative during RF heated plasmas than during Ohmic. For R<146 cm the Er is similar (for the region measured) Implies that RF leads to ion loss at the edge of the plasma. NSTX Results Review 2004