E.P. Carlson

A Fast Exhaust-Gas Analyzer for the ITER Fusion Experiment Divertor

This paper presents a first demonstration of a radio-frequency (RF)-excited optical gas analyzer (RF-OGA) designed to quantitatively measure minority species inside the neutralization region of the ITER fusion experiment divertor. The sensor head, which creates its own plasma excitation and plasma light emission, is designed to operate in a strong magnetic field, and the RF coupling leads to bright light emission. It also allows for operation at low voltages, avoiding the radiation-enhanced breakdowns expected when high voltages are present in the ITER environment. Furthermore, the preferred sensor head features full isolation of the metal RF electrodes from the induced plasma. This ¿electrodeless¿ operation will permit long operation without frequent maintenance. The testing of a first experimental RF-OGA with an electrodeless design in a strong (~2-T) magnetic field showed a mostly linear response of the He I-6678 A¿ line emission to the He concentration in a hydrogen background, which would produce a He concentration measurement accurate to within 2% of the helium-to-hydrogen ratio.

Interferometric measurement of physical phenomena during the implosion phase of a puff-on-puff Z-pinch load on Double-EAGLE

Theoretical studies have predicted that the disruptive role of the Rayleigh-Taylor (R-T) instability on the current conduction and implosion characteristics of annular Z-pinch loads will be mitigated by mass accretion if uniform fill or multiple annular shell loads are used. Holographic interferometry was used to study these physical processes during the implosion phase of puff-on-puff loads on a terawatt accelerator. Both axial (r-z) density perturbation and azimuthal (r-/spl theta/) filamentation modes of the R-T instability were observed. Significant ionization (Z/spl ap/3-10) of the inner gas puff atoms was observed below the anode grid before the outer puff had imploded to this radial position. Radiation hydrodynamic calculations indicate that photoionization by radiation from the outer current carrying shell could not account for this ionization. Current flowing on the inner gas puff could be the source of this ionization. The effect of these physical processes on the radiation yield from z-pinches warrants further investigation.

A holographic lateral shearing and conventional interferometer for studying PRS loads

Summary form only given. A multiple reference beam holographic system has been developed to study PRS loads. This diagnostic employs three, spatially distinct reference beams that are used to create three holograms stored on the same film plate. One of the holograms records the undistorted laser wavefront (reference), while the other two record the distorted laser wavefront after passage through the plasma at two different times on the same shot and along the same path. An optical time delay of up to 50 ns is implemented using a polarizing beam-splitter, two steering mirrors, and a pair of prisms. The use of spatially distinct reference beams during hologram formation allows one independent access to each of the images when the hologram is reconstructed. As a result conventional interferograms and lateral shearing interferograms can be reconstructed from the same exposure. During reconstruction, one can also adjust the frequency and orientation of background fringes for conventional interferograms as well as the shear angle and direction for shearing interferograms. Only minor changes in the optics are required to switch between the two analysis modes. At the heart of the diagnostic system is a 170 ps, 250 mJ pulsed Nd:YAG laser that uses a doubling crystal to operate at 532 nm. Examples of reconstructed interferograms will be presented.

A fast exhaust-gas analyzer for the ITER divertor

This paper presents a first demonstration of an rf-excited optical gas analyzer (RF-OGA) designed to quantitatively measure minority species inside the neutralization region of the ITER divertor. The sensor head, which creates its own plasma excitation and plasma light emission, is designed to operate in a strong magnetic field and the rf coupling leads to strong light emission. It also allows for operation at low voltages, avoiding radiation-enhanced breakdowns expected when high voltages are present in the ITER environment. Furthermore, the preferred sensor head features full isolation of the metal rf electrodes from the induced plasma. This “electrodeless“ operation will permit long operation without frequent maintenance. First tests of the RF-OGA in a strong (∼2T) magnetic field show a mostly linear response of the HeI-6678 Å line emission to the He concentration that would produce a He measurement error of ±2%.

Time resolved measurements of radial implosion velocities, ion temperatures and implosion characteristics of double-shell argon gas-puff loads with on-axis jet

Summary form only given. Tailoring the radial density profile of gas loads on radiation simulators is expected to increase the X-ray yield by mitigating the adverse effects of the Rayleigh-Taylor instability on axial uniformity of the current carrying sheath during the implosion phase of the load. Multi-shell gas puff loads, with and without on axis jets, have been studied to achieve this objective. The role of the inner shell and on-axis jet on the radiation characteristics of the pinch have been studied during the assembly phase of an argon load with a time resolved Johann spectrometer. The radial distribution of K-shell lines emitted by trace elements, added to the individual plenums, were recorded. The linear current ramp of the simulator imploded loads with a total mass of 325-375 mug/cm in 210 to 225 ns. The Si(III) spectrometer crystal was tuned to record the optically thin Lya line of S and He-like lines of Cl produced by 2% Freon and 2.5% hydrogen sulfide tracers. The micro channel plate strip lines were gated on with 5 ns pulses having an inter-frame time of 5 ns to obtain 3 radially resolved, but axially averaged, spectra. Spectra recorded during the rising edge of the X-ray pulse clearly show that the emission is from a thin cylindrical shell that is imploding with a radial velocity in the range of 24 to 42 cm/mus. Furthermore, the radial distribution from these trace elements is counter intuitive. The S Lya line is emitted from an on-axis region whose radius is approximately one half of that occupied by the Cl Hea line even though the S is initially in the inner shell and the Cl is in the on-axis jet. These measurements reveal detail features about the implosion, thermalization, and heating of Z-pinch gas puff loads as they assemble on axis. They can be compared with theoretical calculations to benchmark the latter and provide a better understanding of the physical processes involved

Possible Evidence for Non-Thermal Electron Energy Distributions in Z-Pinch Plasmas

Summary form only given. Radially resolved X-ray spectra were recorded with a time resolved Johann spectrometer on Double EAGLE. Four percent Cl was added as an optically thin trace element to the Ar in individual shells of multi shell gas-puff loads to study the implosion and radiation characteristics of these shells. The ratios of observed Cl line intensities were compared with predicted values using CRE codes to determine radial distribution of electron temperature, Te and ion density, Ni. Preliminary analyses give a higher value for Te (1280 eV) and lower value for Ni (7times1019 cm-3) when using the ratio of the resonance lines (Lyalpha/Healpha+IC) together with the IC/Healpha line ratio than is obtained with the ratio of the jkl satellites to the IC line intensity together with the IC/Healpha ratio (Te<600 eV, 1020<Ni<1021 cm-3). The apparent two temperature plasma composition implied by the analysis will be discussed to determine if it represents two axially separated radiating regions or if it is a possible indicator of a non-thermal electron population.

Role of Insulator Length in Plasma Focus Operation

Summary form only given. he plasma focus has been studied for several decades, but significant aspects of this device are still not well understood. One example is the importance and role of the insulator length as a parameter for machine operation. Generally, the community knows that the first few nanoseconds are critical in the formation of the plasma sheath, which may be the most critical time in the overall operation. Historically, the insulator length has either been used for fine tuning the timing of a particular electrode geometry for the capacitive energy source or selected on the basis of other considerations that are not directly related to the plasma processes. In a series of experiments that were performed for other purposes, we appear to have resolved the direct role of the insulator length for operations with a Mather-geometry plasma focus. There are two distinct electric fields associated with the insulator in the coaxial gun geometry, namely the electric field radially through the insulator and the electric field coaxially along the insulator surface. The radial field serves the purpose of initiating the discharge from the cathode. The axial field, directly related to the insulator length, is much smaller than the radial field, but its effect has not been clear. In our machine, we use a field enhancement initiation electrode, similar to the one developed by Nardis group several years ago. In the experiments that we were performing, the behavior of several different gases in the focus were being evaluated. For gases with progressively lower ionization states and the same electrical conditions, the focus operation shifted modes and even ceased to form a workable plasma sheath. We realized that the difference in operating mode resulted from the discharge striking directly through the fill gas, as opposed to initiating along the insulator surface. Further analysis of the effects of this parameter will be presented.

Magnetic field measurements in Z-pinches using polarization separation Zeeman techniques

Summary form only given, as follows. Measurements of magnetic fields in current carrying plasmas using Zeeman splitting spectroscopic techniques are difficult when the splitting is much less than the line width of the transition. HY-Tech has employed a technique, developed by Jahoda et al. (1963) to record separately the spectrum of the right circularly polarized (RCP) /spl sigma/(+) and left circularly polarized (LCP) /spl sigma/(-) Zeeman components when viewing the plasma along the magnetic field direction (the /spl pi/ components are absent in this case). The Zeeman shift of the /spl sigma/(+) components, corresponding to /spl Delta/M/sub J/= /spl plusmn/1 transitions, is larger than that of the /spl pi/ components (/spl Delta/M/sub J/=0). The polarization separation is accomplished using a quarter wave retarder to convert the RCP and LCP components into orthogonal linearly polarized components which are spatially separated using a polarizing beam splitter. Linear optical fiber arrays select identical spatially resolved locations in the RCP and LCP images of the pinch which are focused onto the input slit of a spectrometer. An image intensified CCD camera records separately the spectra of the two polarization components at a number of spatial locations, from which the Zeeman splitting is inferred. Spectral measurements on the Hawk accelerator using annular gas puff neon loads containing 10% N/sub 2/ indicate that the splitting of the 4619 NV line to be 2 times the splitting of the 4603 NV transition. This is in reasonable agreement with the predicted splitting ratio of 1.36 for these lines. The technique conclusively demonstrates that the Zeeman splitting can be accurately inferred using this technique even when the splitting is one-tenth the line width and the signal is noisy.

Radial density and temperature distributors in a Z-pinch inferred from time and space resolved spectra on Decade Quad

Summary form only given, as follows. The radial distributions of the electron density and temperature in the compressed phase of a Z-pinch have been inferred from X-ray spectra obtained with spectrometers employing film recording. The rapid temporal variation of the pinch plasma during the radiation phase necessitates time resolved measurements of the emission spectrum to understand the dynamics of the radiation process. HY-Tech has constructed a space and time resolved Johann spectrometer for this purpose. The spectrometer employs four (4) imaging slits in the dispersion plane of the spectrometer to produce four (4) one dimensional images of the spectrum. A 40x45 mm micro-channel plate with 6 mm wide gated strips is used to record these spectra. Radially resolved but axially averaged spectra are obtained if the slits are aligned with the pinch. A limited range of Bragg angles (/spl theta//sub 0/13.50) can be viewed in the Johann geometry. The crystal angle /spl theta//sub 0/, and detector can be rotated from 250 to 550 in the present design to access spectral lines of interest while preserving the Johann geometry. Electron densities and temperature, inferred from Ly/sub /spl alpha//, and He like lines of Cl, added as a 2% dopant to Ar, are reported for puff-on-puff loads on Decade Quad.

Measurements of ion temperatures of Al wire arrays in short and long current pulse Z-pinches

Summary form only given, as follows. The Doppler broadening of ion line profiles emitted by a Z-pinch plasma provides information about the thermalization of the implosion kinetic energy and the radiation efficiency of the pinch. Measurement of these line profiles is often complicated by source broadening in the instrument and opacity broadening of the emitted radiation. A high resolution concave crystal spectrometer in the Johann geometry was used to record the time averaged spectra of optical thin trace elements in the load. An imaging slit provided radially resolved but axially averaged spectra. Source broadening is overcome by imaging monochromatic X-rays emitted from all axial pinch positions to one point on the Rowland circle. The recording film is located on the Rowland circle whose radius is half the crystal radius and whose circumference is tangent to the center of the crystal. The spectral resolution of the spectrometer, /spl lambda///spl Delta//spl lambda//spl sim/1000, was measured with K radiation generated by a 10 keV electron beam incident on a Si or Al target. Measurements are reported for Al wire loads on both the Saturn and Double Eagle accelerators in both a short current pulse mode (60-100 ns) and a long pulse mode (125-225 ns). On Double Eagle we observed ion temperatures in the 3-5 keV range in the short pulse mode and 20-30 keV in the long pulse mode. On Saturn, on the other hand, we observed the larger values for both current modes.