J.J. Moschella

Resonant, heterodyne laser interferometer for state density measurements in atoms and ions

A resonant, two-wavelength heterodyne interferometer has been developed to measure state densities of specific atomic populations. Using a tunable diode laser, one of the wavelengths is set near an absorption line and is sensitive to resonant enhancement of the refractive index. The other wavelength is at least several linewidths away and is used to measure nonresonant effects. The subtraction of the phase shifts of the two interferometers allows one to measure the population density in the lower state of the line being investigated. The interferometer system has been tested using a plasma, created by an inverse pinch plasma source, to measure the state density of Hn=2 by making resonant observations near the Hα (6563A) transition. Average line-of-sight densities of 5.4×1011cm−3 were measured with a minimum resolvable density of 7.3×109cm−3. The transverse spatial resolution was 1–2mm. The technique described is applicable for measuring lower state densities in any atom or ion where the oscillator strengt...

Spectroscopic diagnosis of the temperature profile of an Al:Mg Z-pinch

We consider the question of how the temperature profile of a Z-pinch plasma can be determined and/or constrained by the requirement that its observed K-shell spectrum be replicated. As a case study we employ spatially integrated, time-resolved K-shell data obtained from the implosion of 30-wire Al:Mg alloy arrays on the Saturn driver at Sandia National Laboratories. Given the measured pinch size, its K-shell power and line intensities are compared with the predictions of a collisional-radiative-equilibrium plasma model whose temperature profile is varied in seeking agreement with the data. The Al data rules out a large range of possible temperature profiles, but two quite different temperature distributions can both match the measurements. These are: a uniform temperature, or, one with a sharply dropping temperature near the pinch outer edge. However, the measured ratio of the Mg /spl alpha/ resonance lines to those of Al, even though time-integrated, excludes the possibility of a uniform temperature distribution.

Comparison of POS-load dynamics using different plasma sources on Hawk

Hydrogen Plasma Opening Switch (POS) experiments on Hawk show potential for increasing bremsstrahlung radiation on DECADE. Interferometry indicates that the density of POS plasma is greatly reduced over a large fraction of the interelectrode region (>1 cm out of 2.2 cm) at the time of opening. This low-density region is much larger for the hydrogen POS than for flashover-source POSs used previously on Hawk (and DECADE), and may result in a significantly larger effective vacuum gap, improving the efficiency of energy coupling to loads. Species separation may be responsible for the difference in plasma dynamics between single- and multispecies POSs.

Streaked laser absorption measurements of density and temperature profiles in a high density Z pinch

A nonresonant laser absorption technique is described for studying the density and temperature profiles in a high density z‐pinch discharge. A long pulse (700‐ns) dye laser backlights the pinch and a streak camera records the transmitted laser intensity. Spatially and temporally resolved absorption coefficients are inferred from the fractional transmission through the plasma. The measured values are related to plasma density and temperature profiles using theoretical calculated absorption coefficients and equations of state. The results demonstrate the fine spatial resolution that can be obtained with this technique, reveal a complex radial structure in z‐pinch discharges not observable in emission light streak photographs, and establish the practicality of using the technique to evaluate and benchmark theoretical model predictions.

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.

Resonant laser diagnostics of a planar plasma opening switch

The plasma opening switch (POS) is an integral part of inductive store pulsed power systems. Using flashboards coated with BaO and a dye laser tuned to the 493.4 nm ground state transition of the singly-ionized barium ion, resonant laser diagnostics have been employed to image the switch plasma and provide a measurement of the plasma density during conduction and opening. Gaps open during the conduction phase with their position in the inter-electrode region depending upon the initial fill plasma. There is little axial motion of the plasma, contrary to the predictions of analytical hydro model calculations performed using the measured switch parameters. This discrepancy may be due to a finite lifetime for ions in the switch that is less than the conduction time resulting in a larger effective mass. From a functional point of view modified bipolar model calculations best fit the data. >

Plasma opening switch-load coupling on ACE 4

Plasma opening switches (POS) are a key technology in inductive energy storage based pulsed power applications. It has been observed on HAWK, DPM1, and ACE 4 that the geometry downstream of the POS plasma injection region can have a significant effect on POS-load coupling. However, the correlation between performance and geometry is not well understood. POS-load coupling has been investigated for the 9 cm radius, ACE 4 coaxial POS. This study provides insight into the coupling of the POS driving an inductive load and guidance in the design for coupling to an e-beam diode. It also complements the work reported at this meeting by Coleman et al. and Don Parks et al.

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.

Time-resolved spectroscopy and energy-resolved imaging of coated wire arrays as the interwire spacing is decreased

We have spectroscopically observed, as suggested by Sanford et al. (1996), a transition from discrete wire implosions to the implosion of a more continuous plasma shell in aluminum-wire-array Z pinches driven by a 7-TW electrical generator. The time history of magnesium dopant K-shell X-ray emission depended on the interwire spacing, as well as whether the dopant was introduced as an alloy in or a coating on the aluminum wires. We diagnose the final assembly of the hot, dense K-shell emitting core, a region of the pinch not accessible to laser interferometry or optical/UV emission measurements.