Magnetic Probes

Magnetic probes are located all along the outer electrode of the experiment as shown below. Since these probes measure the magnetic field at the outer electrode, they do not provide information about the magnetic field within the plasma. However, the magnetic modes do provide us with information about the location and movement of the pinch. Below is a schematic of the ZaP Experiment, including locations of some of the magnetic probes. In some cases, there are multiple probes at a single z location, but with different azimuthal locations.

4-Chord Interferometry

Magnetic probes are located all along the outer electrode of the experiment as shown below. Since these probes measure the magnetic field at the outer electrode, they do not provide information about the magnetic field within the plasma. However, the magnetic modes do provide us with information about the location and movement of the pinch. Below is a schematic of the ZaP Experiment, including locations of some of the magnetic probes. In some cases, there are multiple probes at a single z location, but with different azimuthal locations.

Thomson Scattering

A q-switched ruby laser (694.3 nm, 50 ns pulse width, 10 J peak energy) is used for our Thomson scattering diagnostic. The measurements are made at z = 0, and the signal is measured by an array of photomultiplier tubes (PMTs). The amount and distribution of the scattered light can be used to determine the local temperature and density of the electrons at a single point in the plasma. A schematic of the Thomson set-up is shown below. Only one Thomson measurement can be made per plasma pulse. In the future, we’d like to expand to a multipoint system.

In this schematic of the Thomson scattering system, a ruby laser is fired through the plasma. Most of the light passes through the plasma unaffected and is collected in the laser dump. A small amount of light (1 in 10⁸ photons) is scattered by the electrons in the plasma. Some of this light enters the collection optics and is focused on the end of a fiber optic bundle. This fiber optic bundle then carries the signal to the spectrometer, which spreads out the light by wavelength. The output of the spectrometer is then binned by a set of fiber optic bundles. Each of these fiber bundles then carries its bin of light to a different PMT. The result is a measurement of the spectral distribution of the Thomson scattered signal from the plasma.

Spectroscopy

Spectroscopy is used to estimate ion temperatures and bulk velocities in the plasma. Ion temperatures are measured by identifying line radiation from ion impurities and measuring their Doppler broadening. These measurements are made through the 90-degree viewport. Bulk velocities are determined by measuring the Doppler shift of these line radiation, as seen through the oblique viewing port. Measurements for these calculations are made at 20 chord positions across the viewport.

In a spectroscopy system, light is collected in 20 chords by a telescope viewing the plasma through a viewport. This light is then transferred to a fiber optic bundle, which carries it to a spectrometer. The spectrometer separates the light by wavelength, and the output is collected by the ICCD camera. The resulting image (far right) is given with wavelength on the x-axis, chord on the y-axis, and intensity as color. The wavelength and distribution of the collected light can be used to determine plasma temperature and velocity profiles.