Overview of HIT-SI Diagnostic Systems

R.G.O'Neill, R.J.Smith, A.J.Redd, P.E.Sieck, W.T.Hamp, G.L.Sutphin, J.S.Wrobel, C.Akcay, T.R.Jarboe,

and the HIT-SI Team

Plasma Dynamics GroupUniversity of Washington, Seattle, Washington

Abstract

The HIT-SI device is a spheromak which uses Steady Inductive Helicity Injection (SIHI) for current drive. An objective of the HIT-SI program is to understand the magnetic relaxation and reconnection associated with forming an axisymmetric spheromak object using non-axisymmetric inductive helicity injection. This requires a comprehensive diagnostic set. The diagnostic compliment on HIT-SI includes: Magnetic diagnostics consisting of 96 surface mounted 3d probes, 20 toroidal flux loops, a retractable midplane internal magnetic probe and an injector magnetic probe array; a multi-point Thomson scattering system; a scannable tangentially viewing FIR interferometer; emission diagnostics consisting of a 16-channel ion Doppler spectrometer, SPRED, bolometry, H-alpha, HXR detectors, Zeff detector, and dual symmetrically-viewing VUV spectrometers. Diagnostics under development include a midplane Langmuir probing array, an injector Langmuir probe, a multi-chord SXR camera, and a microwave interferometer/polarimeter.

Faculty and StaffThomas JarboeBrian NelsonAaron ReddRoger SmithJohn RogersRoger RamanGeorge AndexlerDzung TranDaniel LotzDennis PetersonSusan GriffithMatthew FishburnAndrew Cassidy

UndergraduatesJames Newman Anna Askren Rorm ArestunNora Nguyen Aaron Siirila

Graduate StudentsPaul SieckWilliam HampGeorge SutphinJonathan WrobelRabih Aboul HosnCihan AkcayGriff O’Neill

The HIT Team

HIT-SI Spheromak

• HIT-SI is a spheromak device with ‘Bow-tie’ cross section main chamber for higher beta.

• The spheromak is generated by opposing Helicity injectors that inductively drive plasma current in an n=1 configuration.

• The injectors are operated at 5 kHz in quadrature for a constant Helicity injection rate.

HIT-SI Diagnostics • External Magnetics – 3d pick-up coils and flux loops• Internal Magnetic Probe – Radial arrays of 3d pick-up coils• FIR Interferometer Diagnostic– Line integrated electron density.• SPRED Diagnostic – Spectrally resolved line radiation.• Symmetrically viewing VUV Spectrometers – Monitors ion

charge states.• Bolometer Diagnostic – Measures total radiated power• Zeff Diagnostic – Measure of impurity level• Hα Diagnostic – PMT detected Hα radiation• CCD imaging Camera – Time integrated image of visible light. • Swivel Imaging Diagnostic – fibers incorporating filters,

collimation and direction.• Ion Doppler Spectrometer Diagnostic – Ion velocities Ti.

Diagnostics Under Development• Thomson Scattering Diagnostic – Multi-point

measurement of Te and Ne• Langmuir Probe Diagnostics – Edge Te, Ne• Radial Langmuir Probe – Edge radial electric field

distributions• Soft X-ray Camera Diagnostic – Te indicator and magnetic

reconnection detector• Microwave interferometer/polarimeter – Electron density

and magnetic field

External Magnetic Diagnostics

Magnetic Probe data for Shot 102749

Bolometer Diagnostic• The bolometer diagnostic

measures total radiated power from the plasma based upon the FOV and responsivity of the detector over the visible and UV regions of the spectrum.

• The diode responsivities are 0.22 A/W for 50 eV photons, and 0.26 A/W for 10 0eV photons based on an AXUV-100 photodiode.

Symmetrically Viewing VUV Spectrometer Diagnostic

• The two spectrometer image have the same FOV and have been cross calibrated to yield the same signal strength for the same wavelength setting.

• Typically the ratio of OV/OVI is used as a figure of merit of plasma performance.

Internal Magnetic Probe• Measures radial profiles of all three field components: poloidal, toroidal

and radial magnetic fields. Also has the capability of measuring local plasma current, je, with off axis stems.

• Three stems for measuring fields with a toroidal/poloidal displacement• Boron nitride shield, 0.1 cm thick.• Central radial array of 8, 3-d pickup coils, ~300 mm2 area, spaced 1.25

cm apart• Off axis radial arrays of 5, 3-d pickup coils with poloidal and toroidal

offsets to measure local current• Bandwidth in excess of 150 kHz.

Internal Magnetic Probes Shows Evidence of Spheromak Formation

Soft X-ray Camera Diagnostic

• Tangentially viewing 16 chord pinhole camera to detect soft x-rays due to magnetic reconnection and as a temperature monitor for plasma performance.

• 16 channel AXUV photodiode detector.

Microwave Interferometer/Polarimeter

Polarimetry has been proposed on HIT-SI as a non-perturbativeinternal field measurement.A 2 mm heterodyne microwave system based on three IMPATT diode sources is being considered. The system will use circularly polarized beams transmitted through the plasma and measure the chord averaged index of refraction for a left and right hand circularly polarized beam.

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SPRED Diagnostic

• Spectrally resolved line radiation over the 220-350 nm range, integrated over time.

Zeff Diagnostic on HIT-II

• The Zeff diagnostic measures the impurity content of the plasma.

• Used together with Thomson Scattering and interferometry.

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Ion Doppler Spectroscopy Diagnostic

• IDS system uses a 16 channel PMT for time resolved measurement.• Measures line averaged ion velocities and temperatures, Ti.• Focal length: 1m, 1800 grooves/mm grating, slit width: 10- 5000 µm.• Quartz fiber optic for improved spectral response in the near UV.• Wavelength range: 200-700 nm.• Electronic bandwidth: >20 kHz.

FIR Interferometer Diagnostic• Tangentially viewing FIR Martin-Puplett interferometer

system, measuring line integrated density with shot-to-shot scanning capabilities.

• Difluoromethane FIR laser, 184 µm wavelength with heterodyne detection to 200 kHz.

• Improved system will have heterodyne detection to 1 MHz.• Martin-Puplett layout providing ease of chord selection

between shots• Double pass system, 1 fringe corresponds to 0.61 x1019 m-2

• Martin Puplett Cartridge interferometers for ease of alignment developed at UW.

Martin-Puplett Cartridge Interferometer

• Complete interferometer with the exception of the scene end mirror developed at the UW.

• Pre-aligned and stable .• Scene beam is aligned and

cartridge module is installed to complete the interferometer.

• Multi-chord systems beyond two chords are now conceivable.

Density Traces for HIT-SI operation

Thomson Scattering Diagnostic• An 11 spatial channel, 5 frequency channel Thomson

scattering system. Measures radial profiles of Te and ne.• 20 J Q switched ruby laser system, ~ 1GW pulse.• Multi-chord system using a 11x5 MCP detector head in

either orientation.• Range of operation, Te: 20-200 eV and density >1x1019 m-3.• Laser and collection optics are completely contained on a

movable platform.• Innovative laser dump to absorb laser pulse in 1m from

focus.

Langmuir Probe Diagnostic• Edge diagnostic platform for

measuring Te, ne.

• 3 separate Langmuir probe stems, 5 cm insertion depth.

• The stems are displaced by 18 cm toroidally and 4.5 cm poloidally.

• A recently added Langmuirprobe with a radial array of 8 floating potential tips spaced 6 mm apart will measure the edge radial electric field.

Injector Probes

• A new pair of injector langmuir probes will be used to measure potential at the inector openings.

• Probes will rotate in and out of the injector plasma.

CCD Camera

• An Olympus E-10 camera, takes time integrated exposure of the plasma.

• Neutral density filters are used to attenuate the light level.

• Mounted to view the injector plasma or midplane plasma through the 2” break in the shell.

Visible Light Swivel Diagnostic

• Optical system with lens and filter for monitoring and localizing line radiation. (C,O,Hα, etc)

• Directional ball movement to image different areas of the plasma.

Summary and Conclusions• Several of the diagnostic systems from the previous HIT-II

device have been carried over to HIT-SI.

• The operation and performance of HIT-SI is now developing with this set which is sufficient to characterize the plasma behavior.

• In future work, key diagnostics such as Thomson scattering, and a multi-chord soft X-ray camera, upgraded interferometry, and microwave polarimetry will be added to quantify the performance of the spheromak.