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University of Wisconsin Physics Department Department of Energy National Science Foundation

As the UW-LTRX was designed with the goal of employing a rotating solid wall along the boundary of the experimental volume, diagnostic access is necessarily much more constrained than in comparable devices. With the exception of ports near the ends of the experimental volume, no radial access is available. Thus, all diagnostics and feedthroughs must ultimately enter from either end of the machine.

Amperage Accounting and Segmented Anode

Cross section of the copper segmented anode, illustrating both the support scheme as well as the complete coverage of the target. The thickness of this section, along with the highly conductive material used, maintains the line-tied condition at the anode.

The linear nature of the UW-LTRX allows for spatial and temporal accounting of the currents in the machine. Utilizing shunt resistors on the plasma source and bias capacitors, the amount of current entering the machine is well known. Currents to the end bells and central tube of the machine are measured by Rogowski coils. The anode of the machine is comprised of a thick copper disk surrounded by two concentric rings. Rogowski coils are also placed on the leads exiting each anode segments, allowing all currents leaving the machine to be accounted for. The segmented anode gives a three-point view of the current profile within the machine which can then be related to the safety factor profile, the critical parameter in current driven MHD stability studies.

Magnetic Fluxloop Arrays

For the study of MHD stability and MHD modes, the central diagnostic is the flux loop. The UW-LTRX employs 120 of such coils measuring all components of the magnetic field at the edge of the experimental volume. Within the vacuum vessel, three rings of ten B_theta coils are mounted at the plasma edge. This allows Fourier decomposition of the mode spectrum azimuthally. Ten B_z coils are also contained within the vacuum vessel at one theta position. Outside the vacuum, 80 B_r saddle coils are mounted to the vessel. These coils are regularly spaced to allow for Fourier decomposition in azimuth. Fourier analysis in the z-direction is not amenable due to the line-tying condition. Three fluxloops encircling the entire plasma cross section are also in place to measure the volume averaged diamagnetism. All signals are integrated prior to digitization by Sterling Scientific analog integrators.

Axial probes and 2-D drive mechanism

Cartoon illustrating the articulating knuckle used to position radial armature of probe after insertion through a 13 mm tube. The retractable PEEK shield slides flush with the semicircular section, locking the joint into a right angle. This is done in situ with a custom rod tool through a vacuum window.

Magnetic probe cross section, illustrating the thermally fit, interlocking SS316 skeleton providing rigidity and electrostatic shielding, as well as the boron nitride and quartz plasma shielding. Dimensions are in mm.

The UW-LTRX plasma is ideally suited to internal probe work as the heat fluxes are modest and the discharges are highly reproducible. A defining feature of the UW-LTRX geometry is the inaccessibility of the experimental volume from the radial direction, due to the topological constraint of the rotating wall. To access the vessel an axial probe drive mechanism is used that allows probe insertion from the anode of the machine. Due to geometric constraints of the probe feed through, inserted probes must have a diameter of no greater than 13 mm. A 90-degree articulating joint has been developed that allows the probe to swing into the radial direction once it has passed the feedthrough and entered the experimental volume. Using stepper motors, the articulated probe is then able to sweep an arc in the (r, theta) plane. Assuming azimuthal symmetry and using probe mobility in the axial direction, (r,z) contour maps of the plasma can be created. This technique relies on the established shot-to-shot repeatability of the discharge when dealing with slow (< 1 kHz) dynamics.

Several probe heads to utilize the same insertion and control system. A single-tip sweeping Langmuir probe has been extensively used to characterize electron temperature, density, and plasma potential. The relatively small diameter of the UW-LTRX plasma (~ 15 cm) drives the design of ever smaller probes to minimize perturbations to the plasma. The Langmuir probe utilizes a tungsten wire of 0.4 mm diameter. It is insulated from the plasma by a quartz stalk of 4 mm diameter, filled with boron nitride powder. In addition, a three-axis magnetic fluctuation (B-dot) probe has been designed and deployed, shown in Bdot. The probe has been constructed with a thermally fit stainless steel mechanical structure which also serves as an electrostatic shield, with boron nitride insulating the coil region and quartz insulating the stalk. A two and four tip Mach probe is also in development

Optical and Spectroscopic

Optical diagnostics are also employed on the UW-LTRX. A compact Ocean Optics survey spectrometer has been absolutely calibrated using an Optronics Laboratories 455 integrating sphere. It is commonly used to qualitatively diagnose impurity line radiation and can extract quantitative measurements from line intensities and line ratios when interpreted with a collisional radiative model suitable to the UW-LTRX plasma. A larger, 1.5 m focal length Czerny-Turner spectrometer has recently been acquired. It will have the ability to determine information from the shape of line radiation emitted from the plasma. It is predicted that at the temperatures and densities measured, Stark broadening should dominate this line-shape and thus provide an independent confirmation of the electron density. Doppler flow velocities can also be measured using this instrument by looking at line emission wavelength shifts. Development of an ion Doppler spectrometry probe is also planned.