Internal Magnetic Field Measurements and Langmuir Probe Results for the HIT-SI Experiment (First Evidence of Spheromak Generation and Sustainment) Roger J. Smith Plasma Dynamics Group University of Washington, Seattle, Washington
Abstract HIT-SI is a spheromak device in which the plasma is generated and sustained by steady inductive helicity injection. Helicity injection is maintained at a constant rate by means of two AC RFP sources phased in quadrature and connected to the main chamber so as to drive a rotating n=1 mode at 5kHz frequency. A magnetic probe consisting of three spatially separated radial arrays of 3d pickup coils has been designed to allow the direct measurement of the plasma current and induced electric fields using finite differences of the magnetic field components. The probe is insertable at the mid-plane to a depth of 15cm. Langmuir probes have been built to study the edge plasma at the midplane and the injector openings. Measurements of the internal magnetic field structure, plasma current, poloidal and toroidal flux distributions along with Langmuir probe results are presented for HIT-SI operations over an operation space of varying injector voltage, injector flux and fill density.
HIT-SI Team Faculty/Staff Support Staff Graduate Students Thomas R. Jarboe Daniel E. Lotz William T. Hamp Brian A Nelson Dennis Peterson Griff O Neill Roger Raman Dzung Tran Paul E. Sieck Aaron J. Redd Matthew Fishburn Jonathan Wrobel John A. Rogers Susan Griffith George Sutphin George R. Andexler Andrew Cassidy Rabih Z. Aboul Hosn Undergraduates Annamarie Askren James Newman Amer Fejzic Ling Yu
The HIT-SI Spheromak Device HIT-SI is a spheromak generating and sustainment device with Bow-tie cross section main chamber for higher beta. The spheromak is generated and powered by opposing Helicity injectors that reconnect plasma into the main chamber with an n=1 perturbation. The Helicity injectors generate RFP plasmas by means of a solenoid and voltage biased end plates. The injectors are operated at 5kHz in quadrature for a constant Helicity injection rate into the main chamber. Overview of the Helicity Injected Torus Program, Aaron Redd, GP1.00090
HIT-SI Machine and External Magnetic Diagnostics
Injector Overview for Shot 104388
First Evidence of Spheromak Generation in HIT-SI Poloidal and Toroidal Equilibrium Fields are Observed
Discharge: 104314 Poloidal and Toroidal Fields
Spheromak Equilibrium Fields Radial and Equilibrium Fields There is a noticeable change in the B r field fluctuations: the high frequency fluctuations disappear.
Force free Equilibrium of the HIT-SI Machine Valerie Izzo, PhD Thesis, UW Ongoing MHD Simulations of HIT-SI Using NIMROD, George Sutphin,GP1.00096
Comparison of Computed and Measured Equilibrium Fields Best agreement is achieved with a constant λ profile with Ip ~10-15kA. The measured Spheromak is more expanded than the computed equilibrium.
Measured Poloidal and Toroidal Flux The toroidal and poloidal fluxes of the n-=0 Spheromak equilibrium are becoming comparable to the n=1 component in the main chamber.
Local λ (J B/ B 2 ) and Cos(θ) Distributions Measured J and B are more parallel when Spheromak (3-5ms) is present. The λ profile is constant over the probing region and is ~13m -1. The measure λ increases when the Spheromak appears.
Comparison of Measured J and B Distributions Relaxation produces a straight scaling between J and B. The J r is large and un-relaxed before Spheromak formation. J p and J t fluctuation levels decrease but J r fluctuation level remains high.
The Internal Magnetic Probe Diagnostic Measures radial profiles of all three: poloidal, toroidal and radial magnetic field components. Also has the capability of measuring local plasma current, J, using the off-axis stems. SS vacuum enclosure using 5.33mm dia., 0.13mm(5 mil) thick tubing provides electrostatic shieldig. A 10 linear actuator is used to insert and retract the probe over a useable 6 range. Origin stem and two-toroidal/poloidal displaced stems for gradient measurements.
Boron nitride sheaths are the plasma facing material, they are 0.1cm (0.04 ) thick. The Origin stem is a radial array of 13, 3d Bdot coils, ~300mm 2, spaced 1.27cm(0.5 ) apart, covering the entire 15cm (6 ) length of the probe Off axis stems are radial arrays of 5, 3d Bdot coils. The stems have a poloidal and toroidal offset on a square format (side=1.33 ) to measure gradient of the magnetic fields. Bandwidth in excess of 120kHz
Shot 104338 Internal Magnetic Probe Image during a Discharge
Langmuir Probe Measurements on the HIT-SI Plasma Two Langmuir probe diagnostic systems are being installed on HIT-SI. I) New Injector Probe: An array of 4 W tips, providing differential floating potential measurements spanning the port of one injector. Radial electric fields are measured which is important for determining the efficiency of the Helicity injection process. Helicity Injection Rate dk / dt ~ E B II) Commissioning of the HIT-SI Langmuir probe diagnostic. An insertable, to 5cm, array of triple probes to measure the edge plasma density and temperature and floating potential. An edge radial electric field probe. Both internal magnetic probe and the Langmuir probe will be instrumental in determining S. Lundquist Parameter S = µ V L /η o A
New Injector Probe Diagnostic A new Langmuir probe array is being added to the openings of the injector to measure the radial electric field across the injector openings.
Injector Probe Design: 4 Tungsten tips spanning edge to center of the injector. Ability to rotate Injector probe to a position not obscuring the injector opening. Two symmetrical injector probes on both ports of an injector. Boron Nitride structure and sheathing as a plasma facing material. Differential measurement of floating potentials.
Injector Probe Assembly Successfully completed a four tip probe in the BN structure. Just the last two BN sheaths need to be machined. s Shown above are: the BN probe with twisted pairs of HVAC kapton insulated wire connected between the four W tips, the elbow is mounted on the BN probe, and the SS connection between the probe and rotary actuator shaft.
Injector Probe Placement on the HIT-SI Machine
Langmuir Probe Diagnostic A diagnostic platform with 3 separate Langmuir triple probe stems. The platform is insertable to 5cm depth. BN sheaths cover the probes. The stems are displaced by 18cm toroidally and 4.5cm poloidally from the origin corner stem. Recently a radial array of Langmuir probes was added to the center of the platform. 8 floating potential measurements spaced 6mm apart provide a measurement of the edge radial electric field distribution. The edge diagnostic platform will measure T e, n e, floating potential fluctuations, and radial electric fields with the possibility of magnetic field sensors being added in the future.
Langmuir Probe Diagnostic Platform
Magnetic probing Conclusion A Spheromak Equilibrium with Ip =10-15kA has been generated and sustained for 2 ms in the HIT-SI device. Internal magnetic probing has verified the internal magnetic structure of the equilibrium and measured the distribution of key parameters: vector J, vector B, J/B, and poloidal and toroidal flux. Langmuir Diagnostic development A new Injector Probe is being built to better quantify the amount of helicity being injected per cycle into the equilibrium region The Langmuir probe platform measures edge plasma parameters: ne, Te and edge radial electric field. The diagnostic is close to being commissioned
Future Work Optimizing the Spheromak equilibrium for higher currents and higher ratios of n=0 to n=1 fields is the immediate goal. Internal magnetic probing will play a significant quantifying any advances. Diagnose the injectors with the new injector probes. Continue to bring the full complement of diagnostics to bear on the HIT-SI plasma: Langmuir probe diagnostic for local edge n e, T e ; Interferometry for n e, Thomson scattering for T e.