[en] The development of imaging interferometry and multichannel heterodyne scattering systems for fusion plasma diagnosis has dicated the development of high power, stable twin frequency sources in the 100 μm → 1000 μm spectral region. Output powers of 500mW (185 μm), 150mW (393 μm), 100mW (496 μm), and 25mW (1.22mm) have been measured directly on a calorimeter applying no correction factors. These powers have been achieved via the optimization of the CO₂ and FIR laser cavities together with the use of buffer gases (to relative bottlenecking) and variation of the pumped molecular gas temperature (to optimize the groundstate population). The source IF jitter on twin frequency FIR laser systems has also been investigated in detail. The effect of CO₂ and FIR feedback on the IF frequency has been studied with the view of producing low cost stable laser systems without complicated active stabilization. A comparison between a novel twin frequency laser involving a single optical cavity and a conventional system involving two separate cavities has been performed. The conclusions from these studies are discussed. They have been utilized in the design of laser systems operational on the TEXT tokamak and at UCLA

[en] Studies of reactor designs for inertial fusion energy (IFE) have indicated that some of the material requirements can be alleviated if the target chamber is filled with a low-pressure inert gas (<1 Torr). The experiments described in this paper examine the effects of such an ambient gas on the laser-target interactions relevant to laser-driven IFE. These experiments used the Nike KrF laser facility at the Naval Research Laboratory to irradiate uncoated 78μm thick polystyrene (CH) foil targets at focal intensities around 10¹⁴Wcm^-2. They compared the laser-target interactions in different inert gases (xenon, argon or krypton) at several ambient pressures up to 1 Torr with that of the vacuum (reference) case. The diagnostics included streaked rear-surface emission at wavelengths 460-540 nm and side-on shadowgraphy at 263 nm. The results of this study were encouraging. In spite of initial concerns about resonantly enhanced nonlinear optical effects in the Xe gas, we found no evidence of laser beam break-up, spreading, or attenuation. The shock breakout profiles (from emitted light) and the front and rear surface plasma profiles (from shadowgraphy) remained smooth and symmetric in all cases. The central shock breakout times, and thus the central laser irradiances, were about the same for the target in an inert gas as in vacuum. Our analysis indicates that the large nonlinear optical processes in atomic Xe are self-limiting because the converging beam ionizes the gas, thereby removing the atoms from the most intense part. (author)

[en] The limited viewing access and asymmetric D-shaped plasmas on DIII-D make millimeter-wave reflectometry an attractive diagnostic for density profile and fluctuation measurements. For a reflectometer the Abel inversion of the phase information is performed along the line of sight, so only a single viewing chord, and no symmetry assumption, is required for density profile measurements. The quantity required for the profile determination is a measure of the propagation delay (phase or time) to the cutoff layer and back, either continuously or at several points on the density profile. In its final form the reflectometer system for DIII-D will be composed of two independent reflectometers; a seven channel narrowband reflectometer for density profile and fluctuation measurements, primarily in the plasma edge region, and a broadband reflectometer for full density profile measurements. Laboratory testing of the broadband system is nearly complete, and we are aiming for installation on DIII-D in early 1989. In this paper we will restrict our attention to some of the results which have been obtained with the narrowband reflectometer. 7 refs., 4 figs

[en] A 50--75 GHz, X-mode, broadband reflectometer system is installed and operational on DIII-D. Density profiles have been obtained under a wide range of operating conditions, including Ohmic, L-, and H-mode operation. Good agreement has been found with profiles obtained by Thomson scattering, but only with the use of fast sweeps. Profiles are calculated from the line-integrated phase data using a stable numerical inversion algorithm. Using these techniques, and allowing for the reset time of the tube, density profiles can be measured every 2.5 ms. The system is still under development: By adding an additional 50--75 GHz BWO and mixers, the system will be reconfigured to also act as a correlation reflectometer

[en] Radial view interferometers on large fusion devices have experienced difficulty in compensating for large vibration-induced phase shifts. This compensation has previously been achieved using common path visible or near infrared interferometers. However, the high rate of phase change observed by the compensation interferometer combined with deterioration of reflecting surfaces located close to the harsh plasma environment limit the resolution and usefulness of such systems. This article describes laboratory tests of a vibration-compensating interferometer that operates at two wavelengths of the CO₂ laser: 9.2 and 10.6 μm. It is particularly well suited for application to CIT or other devices with either high density or large plasma dimensions

[en] The development of imaging interferometry and multichannel heterodyne scattering systems for fusion plasma diagnosis has dictated the development of high-power, stable twin-frequency sources in the 100→1000 μm spectral region. Output powers of 500 mW (185 μm), 150 mW (393 μm), 100 mW (496 μm), and 25 mW (1.22 mm) have been measured directly on a Scientech Calorimeter applying no correction factors. These powers have been achieved via the optimization of the CO₂ and far infrared (FIR) laser cavities together with the use of buffer gases (to relieve bottlenecking) and variation of the pumped molecular gas temperature (to optimize the ground-state population). The source intermediate frequency (IF) jitter in twin-frequency FIR laser systems has also been investigated in detail. The effect of CO₂ and FIR feedback on the IF frequency has been studied with the view of producing low-cost stable laser systems without complicated active stabilization. Finally a comparison between a novel twin-frequency laser involving a single optical cavity and a conventional system involving two separate cavities has been performed. The conclusions drawn from the above studies have been utilized in the design of laser systems operational on the TEXT tokamak and at UCLA

[en] Recent developments in plasma diagnostics such as multichannel collective scattering and phase imaging dictate the use of high-power, stable radiation sources in the far-infrared (FIR) spectral region. Towards this end, an optically pumped, twin frequency laser has been developed at UCLA. The CO₂ pump laser produces greater than 100 W in a single mode over the majority of the CO₂ wavelength range (9-11 μm). The twin frequency FIR laser utilizes a single cavity with two separate dielectric waveguides. The difference frequency (λ = 185 μm) is tunable from 0-5 MHz with a short-term stability Δ f ≤ +/- 20 kHz. EH₁₁ spatial modes are routinely achieved with output powers of 100 mW

[en] The limited viewing access and asymmetric D-shaped plasmas on DIII-D make millimeter-wave reflectometry an attractive diagnostic for density profile and fluctuation measurements. In its final form the refleoctometer system for DIII-D will be composed of two independent reflectometers; a seven channel narrowband reflectometer for density profile and fluctuation measurements, primarily in the plasma edge region, and a broadband reflectometer for full density profile measurements. Laboratory testing of the broadband system is nearly complete, and we are aiming for installation on DIII-D in early 1989. In this paper we will restrict our attention to some of the results which have been obtained with the narrowband reflectometer. (author) 7 refs., 4 figs

[en] Opacity calculations are generally restricted to single configurations approximation with no configuration interaction (CI). The theory for the inclusion of the CI effect on intensity distribution on transition arrays has recently been developed and added to the supertransition array model. However, in an experiment performed recently at NRL, presented in this work, it became apparent that the global shift and width of transition arrays, due to the CI effect, are significant and must be included in the calculations. This feature was also noticed in an LLNL experiment published recently on iron plasma. In these cases the dominant arrays originate from Δn=0 transitions where this effect is particularly significant. In this work we extend the theory, bypassing the impractical need for matrix diagonalizations, and derive analytic expressions for the CI-corrected array moments including CI shifts, widths, and the adjusted intensity distribution. Examples are presented comparing the theoretical results with detailed calculations and with the experiments. copyright 1999 The American Physical Society

[en] Reflectometric density profiles are obtained on DIII-D which are in good agreement with Thomson profiles, but only with the use of fast sweeps. The large changes in plasma profile which occur between ohmic and H-mode operation are observed to change the plasma VSWR. The system is still under development: by adding an additional 50-75 GHz BWO and mixers, the present system will be reconfigured to act as a correlation reflectometer, while still retaining the capability to measure density profiles. These alterations are currently underway, and it is hoped to obtain preliminary data by the time of the EPS conference. (author) 7 refs., 3 figs