First Experiments Testing the Working Hypothesis in HSX:

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Presentation transcript:

First Experiments Testing the Working Hypothesis in HSX: Does minimizing neoclassical transport also reduce anomalous transport? J. N. Talmadge HSX Plasma Laboratory University of Wisconsin-Madison Special acknowledgement to J. Canik, K. Likin, C. Clark and the HSX Team, D. Spong of ORNL

Higher QHS Te with same absorbed power HSX has already demonstrated reduced thermal conductivity and reduced viscous damping Higher QHS Te with same absorbed power Thermal diffusivity at r/a=.3 ~1m2/s for QHS, ~3 m2/s for PSM Central Te increases by 200 eV with quasisymmetry Mirror configuration close to neoclassical at core; QHS is anomalous QHS has longer confinement time: τEQHS ~ 1.5 ms, τEPSM ~ 0.9 ms

Larger Flow Speed in QHS HSX has already demonstrated reduced thermal conductivity and reduced viscous damping Larger Flow Speed in QHS Slow decay rate reduced in QHS QHS Mirror Time (ms) Higher flow for less drive with quasisymmetry Slow decay rate lower in QHS, but anomalous momentum damping also exists (as in tokamaks) Potential for higher shear flow with quasisymmetry

How we test the working hypothesis in HSX …. Goal is to compare anomalous thermal conductivity in two configurations with different effective ripple, but with most parameters fixed Adjust absorbed power to match density and temperature profiles No need to scale anomalous transport when profiles match Match plasma volume, transform, well depth as much as possible Minimize movement of magnetic axis between configurations Power deposition profile should be similar Don’t want magnetic axis to be out of line with the Thomson laser path

How we test the working hypothesis in HSX …. Minimize nonthermal contribution to stored energy At low power and high density, stored energy from integrated Thomson profiles is similar to flux loop measurements When Wflux loop > WThomson , ECE and soft X-ray data confirm presence of nonthermal electrons From the power deposition and the temperature profile obtain the experimental thermal conductivity Calculate anomalous component by subtracting neoclassical contribution Does the configuration with lower effective ripple have reduced anomalous transport?

Symmetry is broken with auxiliary coils Phasing currents in auxiliary coils breaks quasihelical symmetry (n=4, m = 1) with n = 4 & 8, m = 0 mirror terms Neoclassical transport and parallel viscous damping increased + + + - - - ‘Old’ Mirror - + + + - - ‘New’ Mirror Minimal displacement of magnetic axis at ECH and TS ports 1 2 3 4 5 6

‘New’ mirror excites n = 4 and 8, m=0 modes ‘Old’ Mirror QHS ‘New’ Mirror

εeff increases by factor of 8 at r/a ~ 2/3 New mirror configuration increases effective ripple while keeping magnetic axis stationary New Mirror Configuration allows for both on-axis heating and on-axis Thomson profiles εeff increases by factor of 8 at r/a ~ 2/3 Thomson Scattering Laser Path (separated by 1 field period) ECRH Beam

Verification of ~ 1 mm shift in magnetic axis inboard outboard Large collector 1 mm collector Electron emitter QHS QHS peak Mirror 1 mm Shift Mirror peak QHS peak

…. while transform, well depth and volume remain almost fixed Rotational Transform Well Depth QHS ‘New’ Mirror Transform (r/a = 2/3) 1.062 1.071 Volume (m3) 0.384 0.355 Axis location (m) 1.4454 1.4447 εeff (r/a = 2/3) 0.005 0.040 < 1% < 10% < 1 mm shift factor of 8

New hybrid transmission line doubles absorbed power Second harmonic X-mode at B = 0.5 T tests confinement of electrons at low collisionality Focusing Mirror - Switch, Dummy Load Mode Converter, 2 Focusing Mirrors + Polarizer Hybrid transmission line (waveguides plus mirrors) has better performance compared to old waveguide line Higher mode purity Accurate wave polarization Less power in the side-lobes Higher arc threshold 28 GHz / 200 kW Gyrotron New hybrid transmission line doubles absorbed power

Confinement time normalized to ISS04 similar to W7-A power Normalized confinement in HSX increases with power, as in W7-A At similar powers, normalized confinement for HSX and W7-A are also similar

Confinement time higher in QHS than Mirror Flux Loop data: n ~ 2 x 1012 cm-3 QHS Mirror Confinement time is weak function of power divergence from ISS04 scaling law

Matching profiles requires more power in Mirror Thomson QHS Mirror Flux Loop data: n ~ 2 x 1012 cm-3 To obtain similar Te and ne profiles in QHS and Mirror Gyrotron power: QHS: 25 kW Mirror: 70 kW Almost 3x power needed in Mirror over QHS More power leads to larger nonthermal fraction of stored energy Discrepancy between flux loop and Thomson larger for Mirror at higher power

Increasing power in Mirror flattens density profile Temperature Density QHS Mirror With increasing power, density profile in QHS remains roughly fixed Mirror density profile is peaked at low power, then hollow at higher power D12 is smaller in QHS due to quasisymmetry

Neoclassical Thermodiffusion Accounts for Hollow Density Profile in Mirror Configuration Figure shows experimental particle flux from Hα + DEGAS, neoclassical prediction In region of hollow density profile, neoclassical and experimental fluxes comparable The T driven neoclassical flux is dominant

Electron temperature profiles can be well matched between QHS and Mirror Density QHS Mirror Temperature profiles match with 70 kW in Mirror, 25 kW in QHS Density profiles don’t match because of thermodiffusion in Mirror

Experimental thermal conductivity lower in QHS than Mirror in the core Neoclassical Spong: PENTA code Error bar due to uncertainty in profiles and absorbed power Mirror conductivity is close to neoclassical in the core Neoclassical thermal conductivity in QHS is far below Mirror Does this decrease translate to a reduction in anomalous?

Anomalous conductivity is difference between experimental and neoclassical Mirror QHS At the plasma core: Mirror experimental conductivity = QHS anomalous + Mirror neoclassical At these operating conditions: unclear there is difference in anomalous transport between QHS and Mirror

More to come …. New Mirror configuration allows factor of 8 variation in effective ripple Lower effective ripple in QHS leads to: Reduction in momentum damping, thermodiffusion, energy transport Anomalous transport, as of now, is similar for QHS and Mirror: → nonthermal population is a concern Improvements in testing working hypothesis Increasing field to 1 T Fundamental heating with ordinary mode at densities up to 1 x 1013 cm-3 should reduce nonthermal problem Electric field and turbulence measurements: Edge probes, reflectometer, 16 channel ECE, CHERS (summer 2007). Comparisons of flows and electric field to PENTA code (Spong)