1. The toroidal current is consistent with numerical estimates of bootstrap current The measured magnetic diagnostic signals match predictions of V3FIT Comparisons with and without toroidal bootstrap current show agreement Helical nature of Pfirsch-Schlüter current in HSX has been confirmed Future modeling and measurement Temporal evolution of toroidal current will be studied – Poloidal flux diffusion Effects of toroidal current on rotational transform will be studied Configuration Flexibility: HSX can alter the magnetic spectrum with a set of auxiliary coils Mirror: The helical symmetry is spoiled by introducing (n,m) = (4,0) component (and harmonics) into the magnetic spectrum, affecting the equilibrium and bootstrap currents Well/Hill: Rotational transform profile may be raised/lowered, adjusting the location of the = 1 resonant surface The Helically Symmetric eXperiment has Quasi-Helically Symmetric axis: (n,m)=(4,1) No toroidal curvature The Pfirsch-Schlüter current Rotates with toroidal angle Is reduced by a factor of compared to a conventional stellarator The bootstrap current Is in opposite direction than that of a tokamak Reduces the rotational transform in HSX VMEC Calculates free-boundary MHD Equilibrium Inputs are measured T e & n e profiles and assumed T i & n i (Z eff ≈ 1) BOOTSJ Calculates bootstrap current profile from VMEC results. Results may be input back into VMEC as toroidal current profile. V3FIT Computes response function for diagnostic coils Expected magnetic signals are computed from response functions and VMEC output BOOTSJ calculates the bootstrap current; Assumes LMFP regime BOOTSJ provides an upper limit to the bootstrap current The toroidal current rises throughout the majority of the shot Steady state reached only in coldest plasmas Decaying exponential growth is observed in many cases: projections are based this model Measurement of the Pfirsch-Schlüter and Bootstrap Currents in HSX HSX Magnetics and Computational Modeling J.C. Schmitt, J.N. Talmadge, P.H. Probert, S.F. Knowlton*, D.T. Anderson HSX Plasma Laboratory, Univ. of Wisconsin – Madison, WI USA*Physics Department, Auburn Univ. – Auburn, AL USA Summary + Future Directions Pfirsch-Schlüter Current Operating Parameters & Diagnostics Bootstrap Current Numerical Model and Measurement B 0 = 1 Tesla 50 kW ECR Heating (1 st Harmonic) 10-chord Thomson Scattering T e (ρ) and n e (ρ) profiles Rogowski coil – External Toroidal current Low-frequency response dB/dt triplet array – External MHD and bootstrap currents Low-frequency response Poloidal and toroidal measurements The Pfirsch-Schlüter current exhibits dipole behavior and rotates with the |B| contours Temporal Evolution of Currents in HSX Majority of HSX plasmas are in long mean free path (LMFP) regime Magnetic diffusivity varies across the confinement volume 16 Poloidal Station # 16 2 1 1 2 BOOTSJ: 478 A Diagnostic signals and toroidal current evolution t = 10. ms: MHD equilibrium established, bootstrap current still small t = 50. ms: ECH turn-off, bootstrap current has grown to 450 A The diagnostic signals agree well with the V3FIT numerical results Toroidal current is still evolving – Radial profile information is not yet known VMEC equilibrium, J BS =J BOOTSJ The bootstrap current is the upper limit for the measurements to date Projected steady state values exceed bootstrap current estimate Decaying exponential growth is not appropriate for most cases – may overestimate steady state value 1/2 FP 1/6 FP Poloidal Rotation of Null Point in B θ 50 kW ECRH heated plasma Profiles obtained during resonance heating location scan Special thanks to Steve Knowlton and the V3FIT team HSX Vacuum Vessel and Diagnostic Coils Off-axis ECRH Near-axis ECRH J PS 1/2 FP 1/6 FP 49 th Annual Meeting of the Division of Plasma Physics, November 12-16, 2007, Orlando, Florida Offset in measured B θ due to toroidal bootstrap current