H. Ikezi

High‐power soliton generation at microwave frequencies

We point out that the creation of a soliton pulse train in a nonlinear‐dispersive system can be used for the generation of high‐power microwave bursts. A modulated transmission line with an appropriate nonlinear dielectric is proposed. The nonlinear wave evolution is analyzed and requirements for the dielectric material are found. Problems associated with the material are discussed. Low‐power experimental demonstrations of these concepts are presented.

Temporal contraction of solitons in a nonuniform transmission line

We present experimental and numerical evidence of temporal contraction of electromagnetic pulses propagating in nonuniform, nonlinear dispersive transmission lines. The pulses are both contracted and amplified by orders of magnitude.

Millimeter wave polarimeter for characterizing high-power plasma heating systems

We have built a millimeter wave polarimeter which measures wave polarization parameters: the polarization angle, and the ellipticity including field spin direction in an evacuated high-power system. The polarimeter was applied to diagnose the 1 MW level electron-cyclotron plasma heating system at 110 GHz for the DIII-D tokamak. We have observed the time-dependent behavior of the gyrotrons and have characterized and calibrated the high-power transmission system which consists of grooved mirror polarizers, miter bends, switches, and corrugated wave guides. This article describes the principle of operation and the design method of the polarimeter and the examples of measurements.

Interaction of fast waves with ions

To fully utilize the available power sources in DIII–D (FW, NBI, ECH), understanding of the synergism between the heating mechanisms is important. In this paper the ion distribution, under simultaneous application of NBI and FW, is calculated from Fokker‐Planck code CQL3D coupled to ray‐tracing code CURRAY. It is found that interaction between energetic ions and FW can be minimized or maximized by adjusting various parameters such as magnetic field, density, beam energy, and FW frequency. Specifically, in DIII–D, we find negligible interactions above 1.8 T and above 80 MHz, while the interaction increases at lower fields and frequencies. The results are compared with experiments in DIII–D including the calculated neutron rate. Energetic ion orbit losses may play an important role in the ion distribution, and this effect is being investigated.

Electrostrictions in high‐power soliton generator

Electrostriction effects are considered in a high power dielectric loaded transmission line designed to generate solitons. The Cerenkov emission of elastic waves from the metal‐dielectric interface produces the largest strain among the strain producing mechanisms. The expression for strain as a function of electric field strength is found. The signal loss due to the emission of elastic waves is shown to be insignificant.

First demonstration of a traveling wave antenna in a tokamak and relevance to the JFT‐2M combline

First observations of a traveling wave antenna (TWA) in a tokamak are reported. For a fractional bandwidth (δω/ω) of about 20%, TWA experiments using a four‐element antenna in DIII–D showed less than 1% reflected power and excellent stability of the phase velocity of the launched wave—even during giant ELMs. The power radiated into the plasma varied from about 40% during ELM‐free H‐mode to almost 90% during L‐mode and ELMing H‐mode. Theoretical modeling validated by these low power observations is used to predict the plasma load resistance of a high power combline being built for JFT‐2M.

Fast wave density and species mix diagnostic (abstract)

Since fast Alfven waves propagate across a plasma at the Alfven speed, the plasma mass density can be determined through interferometry. In previous measurements on the DIII-D tokamak,1 fast waves (∼100 MHz, ∼5u2002W) were launched from an antenna at the outer midplane, but detection of the signal was hampered by poor sensitivity of the receiving antenna, which was mounted behind protective graphite tiles on the inner wall. We modified several graphite tiles to act as more sensitive receiving antennas. At lower frequencies (∼25 MHz), fast waves can reflect from the ion–ion hybrid cutoff layer. The position of this layer is sensitive to the ratio of hydrogen to deuterium in the plasma. Receiving antennas on the outer wall will measure the hydrogen concentration through reflectometry. Launching other frequencies may yield impurity density ratios as well. These techniques may be useful for measuring relative densities if D, T, and α particles in burning plasmas.

Experimental studies of fast wave propagation in DIII‐D

Fast Alfven waves radiated from the phased array antenna in the DIII‐D tokamak and used for heating and current drive are studied by employing a B‐loop array mounted on the vacuum vessel wall. The wave propagation direction controlled by the antenna phasing is clearly observed. A small divergence of the rays arising from the anisotropic nature of the fast wave is found. Comparison with a ray tracing code confirms that the ray position calculated by the code is accurate up to at least one toroidal turn of the rays. Conservation of Rkt which is a basic assumption in computer codes is tested. Although the upshift of toroidal wavenumber kt at small major radius R is confirmed, Rkt is not well conserved. A mass density interferometer is demonstrated by employing the extraordinary fast wave.

Nonlinear reflection in a bandpass filter

It is shown that a nonlinear lumped‐circuit transmission line with a bandpass can transmit weak signals and reflect strong signals. Such nonlinear bandpass filters have an absolute limit on the power passed and may prove useful as circuit protectors.