N.T. Lam

Electron cyclotron wave propagation, absorption, and backscatter measurements in a laboratory plasma

Broadband microwave propagation and absorption processes and backscatter from objects immersed in a magnetized, finite, warm plasma column is addressed. In particular, the propagation, absorption, and backscatter of electron cyclotron waves are measured and compared with bounded vacuum hot plasma wave propagation, absorption, and ray tracing theory. The nonreciprocal nature of the transmission and absorption in an anisotropic plasma is measured. A homodyne technique which isolates the scattering from a single object in the plasma from the scattering from all other objects in the plasma and the walls of the containment device is developed and utilized. The range of absorption frequencies and nonreciprocity of the transmission signal are shown to be well correlated with wave trajectories in the associated regions of the Clemmow-Mullaly-Allis (CMA) diagram. It is shown that quasi-parallel propagation of electron cyclotron waves near resonance is present and that the transverse effects of wavenumber on propagation in the cylindrical plasma are small. >

Analysis of Dielectrc-Filled Waveguide Coupling to Plasmas in the ICRF

Dielectric-filled waveguides may be advantageous for heating reactor-grade tokamak plasmas, due to their compactness and power-handling capability. We present a theoretical analysis of coax excitation and plasma impedance for a dielectric-filled rectangular waveguide. The plasma reflection coefficient is obtained by matching plasma and waveguide fields at the interface. The numerical results show that the reflection coefficients can be made small by careful tailoring of the waveguide dimensions to the density profile for heating at the second or third harmonic of deuterium. We present a scattering matrix approach for the design of a coaxial feed to match the waveguide in the presence of a wide range of plasma loading. For a waveguide filled with a high permittivity dielectric, a shorted probe gives better coupling than an open-ended probe.

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.

Nonlinear analysis and simulation of a millimeter range free electron laser with a helical quadrupole wiggler

Abstract A free electron laser (FEL) with a helical quadrupole wiggler and guide field is studied via analytic theory and computer simulation in the Compton regime. Analytic work on the stability of ideal orbits in the quadrupole wiggler with axial guide field has been carried out. An NRL code which was developed to examine helical dipole wiggler amplifiers is modified to treat both a helical quadrupole wiggler and the resulting relativistic orbit equations. Earlier work which examined the effects of near-axis self-fields on the stability of betatron oscillations and linear gain is extended to include waveguide modes, electron orbits far off axis, axial guide field and the 3-D nonlinear gain to saturation. The numerical simulation of nonlinear gain for the TE 21 mode without wiggler taper yields moderate efficiency (5%), high gain (66 dB) and broad bandwidth (50%) at 300 GHz with an annular beam.

Experiments and analysis of backscatter for microwave propagation in a plasma

Summary form only given. Measurements and analysis of the absorption, reflection, and scattering of electromagnetic waves due to electron cyclotron absorption zones and the presence of modulated scatterers in a plasma are presented. Measurements using a modulated homodyne detection system have been made that are sensitive to the backscatter from a movable probe in the plasma. The phase sensitive system is used to measure the local k-spectrum in the inhomogeneous plasma and is compared with WKB (Wentzel-Kramers-Brillouin) theory. A 12-cm-diameter by 2-m laboratory plasma is produced by a 2.45-GHz microwave source. Wideband wave absorption of 30-50 dB due to the electron cyclotron resonance zone in the 1.5-3.0 GHz range is measured. Experiments examining the microwave scattering frequency sensitivity of modulation-launched magnetoacoustic modes from a ceramic encapsulated PIN diode and from a disc Langmuir probe scatterer have also been performed.

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

Ion cyclotron range of frequencies (ICRF) heating of fast ions in fusion plasmas

We consider the problem of fundamental and second harmonic deuterium and fast tritium beam heating for ITER and TFTR plasmas in the presence of a population of fast alpha particles. For the fundamental deuterium heating case, we justify the replacement of a 0.1–0.4% fast alpha slowing down population by an absorption equivalent Maxwellian of Tα=600–800 keV. The absorption, transmission, and reflection coefficients are calculated using XWAVE, a code which treats full wave effects and perpendicular magnetic field gradients from a fundamental definition of power conservation. A range of D/T ratios and deuterium tail temperatures are examined for a range of alpha particle concentrations. We find that the region near the two‐ion D‐T hybrid resonance can provide a substantial enhancement of the alpha particle absorbed power and wave reflection. We also have used XWAVE and the SEMAL code to describe second and higher harmonic heating of the deuterium, tritium, and alpha particle for the case of the TFTR supersho...

Effects of the L‐ to H‐Mode Transition in BPX on a Recessed Coil Antenna

We study the effects of a simulated L‐ to H‐mode transition for the Proposed BPX (Burning Plasma Experiment) device on the coupling of a recessed coil antenna in the Ion Cylotron Range of Frequencies (ICRF). We use two computer codes: ANTIMP, a code we have developed which uses a Runge‐Kutta method of solution, and a finite‐element code developed by Brambilla, which we have modified to examine four‐feed antenna coils. The radiation resistance is calculated for antenna parameters consistent with a design proposed by the Oak Ridge National Laboratory (ORNL). Both finite cavity size and finite phase velocity along the current straps are modelled. The plasma density is simulated using both piece‐wise linear and parabolic profiles. For anticipated BPX values of the density gradient at the separatrix, the radiation resistance decreases by a factor of 2 to 3 during an L‐ to H‐mode transition. A reduction in the value of the radiation resistance due to finite phase velocity can be minimized by using a four‐feeder...

ICRF Heating in BPX

The problems of fundamental 3He and D heating scenarios including the effects of equivalent Maxwellians representing slowing down α particle distributions are addressed for BPX utilizing XWAVE, a code which treats full wave effects including reflection, mode conversion and absorption. The power conservation relation follows from a definition of local power absorption and a companion general expression for kinetic flux based on fundamental principles. Ohmic, intermediate and fusion burn cases are discussed which examine wave absorption, reflection and mode conversion. Minority concentration levels, bulk and tail temperatures, and α particle absorption are examined for different launched wave spectra.

Analysis and experiments for a waveguide launcher in the ICRF for tokamaks with divertors

We present an analysis of a rectangular dielectric‐filled waveguide, suitable for ICRF heating. To simulate the H‐mode, we take the edge plasma to consist of a pedestal of variable length followed by a region with either a parabolic or a gaussian variation. We present numerical results for the waveguide reflection coefficient, the equivalent surface plasma impedance and the electric field profile at the aperture, for a wide range of edge plasma conditions. A dielectric‐filled rectangular waveguide launcher has been designed, fabricated and tested for the ICRF wave coupling. Our theoretical analyses and measurements over the 60–130 MHz range indicate that a very high power coupling efficiency (≥90%) can be obtained for a matched launcher with appropriate tuning of the noncontacting sliding short. An input reflection coefficient model has been developed for a matched or plasma‐loaded waveguide launcher and compared with measurements.