Helically Symmetry Configuration Evidence for Alfvénic Fluctuations in Quasi-Helically Symmetric HSX Plasmas C. Deng and D.L. Brower, University of California,

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Helically Symmetry Configuration Evidence for Alfvénic Fluctuations in Quasi-Helically Symmetric HSX Plasmas C. Deng and D.L. Brower, University of California, Los Angeles; A. Abdou, A.F. Almagri, D.T. Anderson, F.S.B. Anderson, S.P. Gerhardt, K. Likin, S. Oh, V. Sakaguchi, J.N. Talmadge, K. Zhai, University of Wisconsin-Madison; D.A. Spong, Oak Ridge National Laboratory Key Results Summary Mode Frequency Scaling 1.Alfvén Continua for QHS and Mirror Mode Plasmas (conventional stellarator) on HSX 2.Characteristics of observed fluctuations in Quasi-Helically Symmetric plasma 3.Evidence for fast-electron driven GAE mode HSX Interferometer Channels  Conventional stellarators exhibit poor neoclassical transport in low- collisionality regime due to magnetic field ripple  HSX: helical axis of symmetry, no toroidal curvature, no toroidal ripple |B| Symmetry in |B| :,  Symmetry in |B| leads to small deviation of trapped particles orbits from a flux surface and, as a result, to improved neoclassical confinement in low collisionality regime reduction in ‘1/ ’ transport Alfvén continua: n = 1 mode family GAE Gap: kHz, for m=1, n=1 B=0.5 T, n e (0)=1.8x10 12 cm -3 STELLGAP code (D. Spong) Quasi-Helical Symmetry: Helical axis of symmetry, no toroidal curvature Quasi-Helically Symmetric (QHS) configurationMirror Mode (MM) configuration Mirror Mode: toroidal mirror term introduced to magnetic configuration, equivalent to conventional stellarator operation 10% mirror perturbation (STELLGAP code) GAE Gap: kHz, for m=1, n=1 B=0.5 T, n e (0)=1.8x10 12 cm -3 Density Fluctuations Mode Frequency Scaling with Electron DensityMode Frequency Scaling with Ion Mass Dispersion and Density and mass scalings consistent with Alfvénic mode mode frequency decreases with ion mass Fluctuations with symmetry Breaking Fluctuation no longer observed for Mirror perturbation >2% (conventional stellarator configuration: ~10% mirror perturbation) Soft X-ray (0.6-6 keV) Emission is 3x higher for QHS over Mirror mode Hard X-ray emission in QHS plasma is much higher than in Mirror Mode Implies fast particles are better confined in QHS Fluctuation decreased with introduction of Symmetry Breaking (toroidal mirror) term Soft, Hard X-rays Emission for QHS and Mirror The GAE Mode Growth Rates The GAE Mode Driving Growth Rate Formula ( Cylindrical Plasma)HSX Plasma where and With, being the temperature, density, beta value and thermal velocity of the non thermal electrons. is the non-thermal electron distribution function. is diamagnetic drift frequency of the non-thermal electrons Case I. Passing ParticlesCase II: Trapped Particles Flux Surfaces and Interferometer ChordsDensity Evolution for QHS Plasma Interferometer System: 1.9 chords kHz B.W cm chord spacing Density Fluctuations in QHS Fluctuation Features mode observed only in QHS plasmas Noise: f < 30 kHz fluctuation noise 1. only observed in QHS plasmas 2. coherent, m=1 (n=?) 3. localized to steep gradient region 5. Satellite mode appears at low densities,  f~20 kHz 6. Electromagnetic m=1 core localized Observed Fluctuations Associated with ECRH Mode disappears ~1 msec after ECRH turn-off, same timescale as W E and soft x-rays No Mode In Mirror Configuration B=1 T GAE Frequency chirping sometimes observed Fluctuation Observed in Different Heating Locations fluctuation observed when ECRH resonance near plasma center -large  e_non-th and  e_non-th increase growth rate - low shear (1.05<  and cold ions (no ion Landau damping) may also play a role HSX plasma produced/heated by 2 nd Harmonic X-mode ECRH; can produce non-thermal electrons with mode unstable when  * e /  mode >1 [first term Eq.(1)], and 2.V A ~V e ~ 10 9 cm/s; occurs at r/a=0.2 for thermal population. To drive mode unstable at larger radii, non-thermal electrons must provide the drive For L n = 3 cm, non-thermal electron population 30%, and From, Eq. (1), perpendicular temperature needed to drive the mode unstable is Energetic electrons are able to drive fast particle instabilities mode unstable when  GAE  De_nonth, where  De_nonth is the precession drift frequency If GAE mode frequency is ~50kHz, - for r eff =0.1*a= 1.3cm, R=120cm, perpendicular energy required - for r eff =0.5*a=6.5cm, ECRH produces non-thermal electrons with Instability depends on energy of fast particles, not mass Experimental Evidence for non-thermal Electrons 1. ECE measurement: shows the perpendicular energy of the non-thermal electrons ~4-16 keV at n e =0.5- 2*10 12 cm -3, at 50kW ECRH power [see K.M. Likin’s Poster ] 2. Soft X-ray measurement: (0.6-6kV) 3. Hard X-ray measurement: shows non-thermal electrons have energy up to 200keV GAE B=0.5 T 1.Calculations of Alfven Wave Continuum by 3-D gyro-fluid code shows the possibility of GAE mode in HSX 2.Density fluctuation characterized as m=1 global mode 3.Electromagnetic component: Fluctuation observed on Magnetic probe is coherent with the density fluctuation 4.frequency ~ density scaling and frequency- mass scaling of the fluctuations show the mode is Alfvénic 5.The growth rate calculations and experiment show that the fluctuations is most likely driven by non-thermal electrons, 6.Mode is only observed for QHS configuration in HSX, not for Mirror Configuration (i) non-thermal electrons are better confined (ii) the damping of the mode in Mirror Configuration is likely greater than in QHS case 7.Experimental evidence suggests the observed fluctuation is a GAE mode Fluctuations with ECRH Power Fluctuation amplitude increases with P ECRH Fluctuation frequency increases slightly with P ECRH No fluctuations observed when P ECRH < 27kW Coherent Fluctuation observed over broad range of density When heating location at the magnetic axis, and when n e 3.0x10 12 cm -3, no density fluctuation is observed For certain percentage of Hill term, when n e < n crit, plasma transits to improved confinement; W E and n e increased greatly. During improved status, the fluctuation mode appears. n crit decreases with Hill mode percentage and the transition becomes less frequent. When Hill percentage greater than 7.5%, no transition Explanation:  ~ 1  >1: see fluctuation  <1: no fluctuation (mode stable) Fluctuations in Hill Configurations (1) (2) (3)