3D Surface Probe Array
A magnetic surface probe array was included in the HIT-SI design at the time of manufacture. The array is capable of measuring the poloidal and toroidal components of the surface magnetic field at 96 locations in the machine. At each location there is a magnetic surface probe composed of three nested windings—normal, poloidal and toroidal—indicating for the direction they are sensitive to. The magnetic surface probes are mounted in the copper wall of the HIT-SI vessel, and are separated from the vacuum by a thin stainless steel disc. Each probe is electrostatically isolated and shielded from the experiment. A calibration curve corrects for the attenuating effects of the copper wall and recovers the field at the surface of the plasma facing wall.
The 96 probe locations form poloidal and toroidal arrays around the device. Shown above are two poloidal arrays with the surface probe locations highlighted in red. There are two more poloidal arrays offset 45 degrees from the two shown. These poloidal arrays form Amperian loops and allow for plasma current calculations in the confinement region. In addition the magnetic surface probe signals are useful for measuring fast surface dynamics (10 Hz – 200 kHz) and provide useful data for comparison to computer simulations.
HIT-SI is also equipped with a set of 20 flux loops (the flux loops are highlighted in red below). The flux loops are embedded in an external groove of the flux conserver that brings them closer to the plasma facing surface. The flux loops measure the toroidally averaged poloidal flux, yielding a measure of the toroidal currents in the confinement region.
Internal Magnetic Probe
The internal magnetic probe provides measurements of the poloidal, toroidal, and radial magnetic field profiles within the plasma. Three stems allow for measurements with a toroidal/poloidal displacement. This is a retractable probe with an insertion depth of 26.6 cm.
Far Infrared (FIR) Interferometer
The interferometer measures the electron density along a chord of the plasma. The HIT-SI interferometer, donated by Lawrence Livermore National Laboratory, uses a CO2 laser to optically pump two difluoromethane FIR lasers. A Martin Puplett configuration, shown below, is used to measure the phase change of the laser beam passing through the plasma. Measuring the density gives information on the particle confinement time, fueling dynamics, and our plasma performance in relation to the Greenwald limit.
An eight spatial channel, seven frequency channel Thomson scattering system is capable of measuring radial profiles of the electron temperature and density. A 20 J, Q switched ruby laser system, with a 1 GW pulse, is used. The Thomson scattering diagnostic’s range of operation is for electron temperatures of between 20 and 200 eV and electron densities greater than 1019 m-3.
A bolometer is a device to measure radiated power. There are presently three bolometers operated on HIT-SI. A midplane bolometer images a wide swath of the confinement region in order to give a "big picture" of the amount of power being radiated. The other two bolometers are mounted axially. One of these bolometers images an injector, while the other does not. By comparing these signals, the ratio of the power radiated in the injector versus the confinement region can be determined.
Camera (IR, visible)
Still cameras are used to observe areas of interest in the plasma. The first picture is of one injector mouth taken during plasma operation with a combination IR/visible light camera is below. The second picture is with a visible light only camera.
A fast framing visible camera system is being installed on the HIT-SI device to record plasma activity at an injector mouth. The planned studies include monitoring plasma dynamics and wall interactions, and using emission filters to single out specific ions and ionization states. In conjunction with the camera system, a movable, trace impurity injector will be installed near the injector mouth. By injecting fluorescent, trace impurities at different points in the injector mouth, the plasma dynamics at the injector can be studied.
The H-alpha diagnostic measures the time-dependent intensity of the H-alpha line, 656.3 nm.
Ion Doppler Spectrometer (IDS)
The IDS diagnostic is a passive optical system for measuring the temperature and velocity of ions in the HIT-SI experiment. Radiated emission along a chord through the plasma is coupled to a spectrometer by a fiber optic bundle. The spectrometer grating images a narrow wavelength range about an atomic emission line and a lens disperses the light over a 16 channel photo multiplier array. The width of the emission intensity is related to the temperature—an effect of Doppler line broadening. The bulk velocity of the ions results in a Doppler shift of the atomic line spectrum. The IDS diagnostic is used in the visible spectrum and can be tuned to wavelengths between 250 and 700 nm.
Spectroscopy—Poor Resolution Extended Domain (SPRED)
The SPRED spectrometer yields spectral information about the impurity content of the plasma. It disperses light from 200 nm (ultraviolet) to 550 nm (visible) wavelengths. Spectral lines present in the SPRED data allow us to compare the relative magnitudes of impurities from shot-to-shot.
Two VUV spectrometers give time-resolved measurement of a particular spectral line. Typically we use the VUVs to compare two oxygen or two carbon lines. The two spectrometers have the same field of view and have been cross calibrated to yield the same signal strength for the same wavelength setting.
Soft X-ray Camera
A 16-chord pinhole camera has the capability to detect soft x-rays due to magnetic reconnection and as a temperature monitor for plasma performance.
Z Effective Diagnostic
The effective ion charge of the plasma, or Zeff, indicates the average ionization state of the plasma. Bremsstrahlung radiation intensity can be determined from Zeff, the electron temperature and the electron density.
The Langmuir probe is a perturbative diagnostic. The local electron temperature and density are determined by measuring the voltage potential between the electrodes extending from the tip of the probe.