M.H. Bettenhausen

Microwave reflections from a VUV laser produced plasma sheet

Summary form only given. A 20 ns vacuum Ultra-Violet (VUV) laser pulse is used to create a plasma sheet in an organic gas. A bistatic antenna system is used for transmitting and receiving X-band microwaves which interact with the plasma. Reflected signals are measured for amplitude and phase analysis. Amplitude and phase shifts are compared to an aluminum conducting sheet placed in the same position as the plasma. The working gas is tetrakis (dimethylamino) ethylene (TMAE) with an ionization energy of 6.1 eV. The ionizing source is an excimer laser (W/sub max/=300 mJ) operating at 193 nanometers (6.4 eV). The laser beam is transformed into a sheet using VUV thin-film matched lenses. A plasma sheet with a peak density of 2.5/spl times/10/sup 13/ cm/sup -3/ and T/sub e/=0.3 eV is formed with dimensions 0.7-5 cm/spl times/7.8 cm/spl times/30 cm. Additional measurements of transmitted signals are utilized to determine plasma density and collision frequency. A finite-element computer model of the plasma profile to determine microwave transmission and reflection levels has been developed to optimize reflected signal levels as a function of density and thickness and to interpret experimental results. Comparison between the experimental results and the model show that this system is attractive for use as a microwave reflector. In addition, studies are being carried out to explore plasmas created with air components with this microwave.

Experiments and analysis of wave absorption, reflection and scattering in plasmas

Summary form only given. Measurements and analysis of the absorption, reflection, and scattering of electromagnetic waves due to electron cyclotron resonance zones and the presence of scatterers in the plasma were carried out. The 12-cm-diameter by 2-m laboratory plasma was produced by a 2.45-GHz microwave source in a local mirror field, with wave measurements carried out in a larger mirror region. The plasma density that could be created was in the range of 5×109 to 5×1011/cm3 with electron temperatures in the 3-6-eV range and 0.5-2-kG magnetic fields. Wave absorption measurements of 25-50 dB due to the electron cyclotron resonance zone in the 1.5-3.0-GHz range were performed. A modulated homodyne detection system that isolates the backscatter from a single object in the plasma and suppresses the scattering from the walls. probes, and other background was constructed. This system was used to measure the shielding produced by the plasma, including the shielding by cyclotron resonance. The effect of collisional absorption for particular density profiles in weakly magnetized plasmas was analyzed to determine their effect on reflectivity. For collisional. low-magnetic-field plasma conditions comparable to those that can be obtained in the present experiments, the electromagnetic wave reflectivity was calculated using various Epstein profiles

Laser and radiofrequency tailored gas plasma sources

Summary form only given. We are carrying out plasma source research utilizing vacuum ultraviolet excimer laser ionization of an organic gas and microwave scattering and fast Langmuir probe measurements of the plasma formation and decay. We find that peak plasma densities of 2/spl times/10/sup 13//cm/sup 3/ in 5 cm/spl times/10 cm/spl times/20 cm volumes can be created with a 300 mJ, 20 ns pulse of 193 nm radiation. The plasma is formed in 10 ns and decays on microsecond time scales. We have analyzed, designed and constructed a plasma radiofrequency source (f=2-200 MHz, p=1-3 kW) to examine highly collisional plasma production and sustainment of the laser produced plasma. Wave modelling results utilizing our ANTENA II and MAXEB codes which incorporate both fast (helicon) and slow (Trivelpiece-Gould) modes are utilized to interpret the experimental results. One- and 2-D plasma density effects as well as the 2-D coupling antenna are incorporated and show that a moderate magnetic field can enhance wave propagation and absorption for collisional plasma sources. We present initial experimental results for argon and other gas components including air to provide an efficient plasma source.

Two-dimensional warm plasma wave field and absorption simulations for helicon plasma sources

Summary form only given, as follows. We have modified the MAXEB computer code which was developed to model and study Inductive sources. MAXEB, a two-dimensional (r, z) simulation code, calculates the electromagnetic wave fields and power absorption in an inhomogeneous, cold, collisional plasma immersed in a nonuniform magnetic field. The current distribution of the launching antenna provides the full antenna spectra which is included in the model. We have modified the code to include warm plasma thermal effects and the contribution of collisionless (Landau damping) wave absorption by electrons. We present studies of the wave fields and electron heating profiles which include low magnetic fields (B<100 G) where the electrostatic effects of the Trivelpiece-Gould (TG) mode as well as the helicon (H) mode are important. The effect of an applied magnetic field, 2-D (r, z) density profiles and the antenna spectrum on collisional and collisionless field solutions and power absorption is investigated. It is found that at low collisionaiity /spl nu///spl omega/<0.1 and for appropriate antenna spectra that Landau damping can dominate the absorption process. Benchmark cases for which field solutions and power absorption agree with the 1-D density and magnetic field profile for the ANTENA II code are also presented. We examine cases in which the primarily electrostatic surface wave dominates the heating and is absorbed near the edge region and cases in which the primarily electromagnetic wave absorbs power in the core plasma region. The code illustrates the 2-D wave phase velocity evolution as the wave is launched from the antenna excitation region. Our helicon simulations are directly compared with selected experimental data.

Laser and radiofrequency air plasma sources

Summary form only given, as follows. We are carrying out plasma source research utilizing 193 nm laser ionization of an organic gas (Tetrakis-dimethyl-amino-ethylene) in constituents of air (N/sub 2/, O/sub 2/) and other gas (He, Ar, Xe) components. Microwave scattering, fast Langmuir probe and photodiode measurements are used to diagnose the plasma formation and decay. Initial peak plasma densities of 5/spl times/10/sup 13//cm/sup 3/ are obtained with volumes of 10 cm/spl times/5 cm/spl times/20 cm. Results of these measurements with an emphasis on the role of the addition of air and other gas constituents on plasma lifetime will be presented. A comparison of the experimental results with air plasma chemistry codes will be discussed. We have analyzed, designed and constructed a plasma radiofrequency source to examine highly collisional plasma production and sustainment of a laser produced plasma. Wave modelling results utilizing the ANTENA II and MAXEB codes in the 2-200 MHz range show that a moderate magnetic field can enhance wave propagation and absorption away from the antenna coupling region for highly collisional plasma sources. We examine the role of metastable atoms and electron attachment and detachment processes for our 1-3 kW radiofrequency plasma source sustainment experiments for different Ar, N/sub 2/ and O/sub 2/ gas components. The use of the laser for creation of accurate plasma density spatial profiles which are sustained by the radiofrequency source for air plasma will be discussed.

Dynamics of a laser produced plasma and its properties for microwave reflection

Summary form only given, as follows. A large plasma sheet is created by a 193 nm laser ionizing a low ionization potential molecule tetrakis-(dimethylamino)ethylene (TMAE). The plasma density is diagnosed by a fast response single Langmuir probe, and special features of the probe measurements at early times in the laser produced plasma are discussed. On microsecond time scales, the plasma decay is dominated by a two-body recombination process, and other plasma decay processes, such as plasma diffusion, electron attachment, three-body recombination with either another molecule or an electron as the third body, can be neglected. The initial plasma densities vary for different experimental conditions, but all the density traces at later times approach a same limit, which is only sensitive to the two-body recombination coefficient. Based on plasma density measurements, a dielectric model is developed to calculate the properties of microwave reflection from the plasma sheet. The critical reflection angles for different plasma densities, as well as the polarization effects on the microwave reflection are discussed. The calculations of microwave reflection from the plasma density measurements agree well with those obtained by direct microwave measurements.

Antenna coupling and absorption mechanisms in a helicon source operation

Summary form only given. The ANTENAII computer code, a modified version of the ANTENA code written by McVey, is used to study and model helicon sources. A drifting beam with a small Maxwellian spread characterizing the fast electrons (E/sub e-beam/=30 eV and a width of T/sub e-beam/=3 eV) along with the bulk Maxwellian electron distribution (T/sub e-bulk/=3 eV), comparable to what has been observed experimentally has been added to the code to examine their influence on helicon wave power absorption. We have studied the effects of electron cyclotron damping on power absorption that can occur at low magnetic fields (B/spl ap/1-50 G). The contribution of the off-diagonal hot plasma dielectric tensor terms and that of the transit-time damping to the power dissipated is also examined. We examine different helicon antenna structures and describe the relative processes of wave absorption by collisional, Landau, transit-time and cyclotron damping for a wide range of parameters. Preliminary studies of plasma transport as well as radial plasma uniformity in helicon sources are presented with the aid of the interface of our ANTENAII code with the 2-D fluid transport model, INDUCT95, developed by Vittelo et al. (1995).

Theoretical and computational characterization of multi-tone helix traveling wave tubes

Summary form only given, as follows. Initial results are presented for theoretical characterization of multi-tone, helix traveling wave tubes (TWT). The goal of this effort is to determine techniques for supression of intermodulation and harmonic effects between multiple signals operating within a given band and somewhat below saturation. This will be done by developing tools for simulating TWT operation including non-linear effects and saturation characteristics. The results presented here obtain the vacuum dispersion relations and an analysis of reactive loading of the helix. Linear gain characteristics are also presented using a model which treats a circular electron beam as a uniform bounded plasma. Plans for development of advanced computational models are discussed. Preliminary comparisons with experimental results and comparisons of benchmark results with existing codes will be presented.

Parasitic coupling of ion Bernstein waves with ICRF fast wave excitation in tokamaks

Summary form only given, as follows. Analysis and computer simulation is presented for investigating the coupling of ion cyclotron range of frequency (ICRF) waves to tokamak plasmas using phased coil antenna arrays. The model accounts for three-dimensional antenna, and feeder current effects, an idealized Faraday shield, an antenna cavity which is finite in all three dimensions and warm plasma effects. The WIGS computer code, developed by the authors, is used to calculate the antenna radiation resistance and the spectrum of the ICRF power coupled to the plasma. Parasitic coupling to the ion Bernstein wave during fast wave heating experiments is investigated. Parasitic coupling to the ion Bernstein wave in the edge plasma region is shown to be important for low parallel wavenumbers during minority ion heating with large gradients in the plasma density near the edge of the plasma. The sensitivity of the parasitic coupling to changes in the cavity dimensions, current feeder configuration, coil current phase velocity and edge plasma profiles is discussed. The fraction of the launched power coupled to the ion Bernstein wave is to vary substantially with the antenna and plasma parameters and the phase difference between the excitation of adjacent antenna coils. Specific results are shown for the antenna and plasma parameters of the TFTR and JET tokamaks.

VUV laser plasma formation and microwave agile mirror/absorber

Summary form only given, as follows. Agile microwave reflectors are of interest as potential replacements for large phased arrays on mobile crafts. Preliminary measurements of microwave reflection from a VUV (/spl lambda/=193 nm) excimer laser produced plasma (W=20 mJ, /spl tau/=17 ns) in an organic gas (TMAE). This work investigates the reflection of X-band microwaves at oblique incidence by a planar TMAE sheet of dimensions: 8 cmW/spl times/4-15 mm H/spl times/20-cm L, produced by a new (W=300 mJ) excimer laser. Fast Langmuir probes are used to determine the laser produced plasma density and temperature profiles, at TMAE neutral pressures in the 25-60 m Torr range. Densities and temperatures in the ranges of n=5/spl times/10/sup 12/-2/spl times/10/sup 13/ cm/sup -3/ and T/sub e/=0.5-0.7 eV are obtained. The amplitude and phase of the transmitted and reflected microwave signals are measured using both homodyne and heterodyne systems with hybrid-tee mixers. A time history of the plasma temperature and density is obtained with Langmuir probes and compared with measurement based on microwave transmission through the plasma. Reflected and transmitted signals are compared with those from a calibration aluminum sheet of the same dimensions as the plasma.