G. J. Liu

Review of recent theories and experiments for improving high-power microwave window breakdown thresholdsa)

Dielectric window breakdown is a serious challenge in high-power microwave (HPM) transmission and radiation. Breakdown at the vacuum/dielectric interface is triggered by multipactor and finally realized by plasma avalanche in the ambient desorbed or evaporated gas layer above the dielectric. Methods of improving breakdown thresholds are key challenges in HPM systems. First, the main theoretical and experimental progress is reviewed. Next, the mechanisms of multipactor suppression for periodic rectangular and triangular surface profiles by dynamic analysis and particle-in-cell simulations are surveyed. Improved HPM breakdown thresholds are demonstrated by proof-of-principle and multigigawatt experiments. The current theories and experiments of using dc magnetic field to resonantly accelerate electrons to suppress multipactor are also synthesized. These methods of periodic profiles and magnetic field may solve the key issues of HPM vacuum dielectric breakdown.

The influence of desorption gas to high power microwave window multipactor

Gas desorbed by electrons plays a key role in multipactor saturation and final plasma breakdown at the vacuum side of the window in high power microwave systems. A multipactor model involving electron-neutral collision and ionization is established. When desorption gas pressure reaches 1Torr, the electron impact energy apparently decreases, multipactor saturates at a lower surface charging field, and multipactor saturation meets easier, compared to vacuum. Experiments for different material windows of high power microwave, with the power of 1GW, frequency of 9.4GHz, and 20ns single and multiple pulses was conducted. It was discovered that gas pressure rose prominently in the horn, when breakdown occurred. The increase in local gas pressure during the 20ns pulse is estimated from the experiment to reach several Torr, a pressure that is consistent with the presented dynamic model.

Starting current of coaxial relative backward wave oscillator

This paper is devoted to study the starting current of the coaxial relativistic backward wave oscillator (CRBWO) using a simple physical model that employs the eigenmodes of the enclosed resonant cavity and the external quality factor of the open cavity Q. The agreement between the theoretical and simulation results is presented. It is found that CRBWO is suffering from the mode competition during the startup of the oscillation due to the wide interaction region over the range of the longitudinal wavenumbers.

Effect of non-uniform slow wave structure in a relativistic backward wave oscillator with a resonant reflector

This paper provides a fresh insight into the effect of non-uniform slow wave structure (SWS) used in a relativistic backward wave oscillator (RBWO) with a resonant reflector. Compared with the uniform SWS, the reflection coefficient of the non-uniform SWS is higher, leading to a lower modulating electric field in the resonant reflector and a larger distance to maximize the modulation current. Moreover, for both types of RBWOs, stronger standing-wave field takes place at the rear part of the SWS. In addition, besides Cerenkov effects, the energy conversion process in the RBWO strongly depends on transit time effects. Thus, the matching condition between the distributions of harmonic current and standing wave field provides a profound influence on the beam-wave interaction. In the non-uniform RBWO, the region with a stronger standing wave field corresponds to a higher fundamental harmonic current distribution. Particle-in-cell simulations show that with a diode voltage of 1.02 MV and beam current of 13.2 kA, a microwave power of 4 GW has been obtained, compared to that of 3 GW in the uniform RBWO.

TURBULENT PARTICLE TRANSPORT IN TOKAMAK PLASMAS WITH IMPURITIES

The turbulence driven by the ion temperature gradient, the mass shear flow parallel to the magnetic field, and the impurity ion density gradient in confined plasmas is studied in a sheared slab magnetic configuration. The turbulence drive from the temperature gradient and parallel shear flow of the majority ion component is shown to be enhanced by the shear flow and negative density gradient of the impurity ions. The particle diffusion induced by the turbulence is obtained within the framework of quasilinear fluid theory. Optimal transport parameters for an inward “pinch’’ of the majority ions and the outward flow of the impurity ions are determined. The corresponding effective diffusion coefficients that include the pinch effects are computed. Correlations with tokamak experimental observations such as an isotope scaling of plasma confinement time are discussed.

Analysis of ion temperature gradient modes in high β plasmas with sheared slab configuration model

A set of integral equations is developed to study drift instabilities in any β (plasma pressure/magnetic pressure) plasmas with the sheared slab magnetic configuration model. Both components of the perturbed vector potential, A∥ and A⊥, are considered in the equations, as well as the perturbation of the electrostatic potential φ. The magnetic gradient drift effects are taken into account. The ion temperature gradient modes are analyzed and found to be unstable in the high β regime. The stability of the high β modes is very sensitive to the mode frequency. The lower frequency modes are more difficult to be stabilized since the β effects cannot effectively change the frequency and then the particle-wave interaction in the lower frequency regime. The magnetic shear is shown to have strong stabilizing effects on the high β modes.

Effects of β and Te/Ti on the ion temperature gradient modes in anisothermal plasmas

Effects of β (plasma pressure/magnetic pressure) and Te/Ti (ratio of electron temperature to ion temperature) on the slab ion temperature gradient modes are studied locally for any β value in anisothermal hydrogenic plasmas with no impurities or fast ions. The stabilizing mechanism of finite β on the modes is investigated in detail. It is shown that increasing Te/Ti destabilizes the modes and that βi stabilizes the modes mainly by decreasing Te/Ti, while βe does so directly. βe is the dominant stabilizing factor for a fixed Te/Ti. The numerical results also indicate that the critical β(βc) for mode stabilization decreases as Te/Ti decreases. Ion heating favors the suppression of the ion temperature gradient modes in anisothermal plasmas for any β value.

Study of electromagnetic instabilities driven by ion temperature gradient and parallel sheared flows in high-β plasmas

The local electromagnetic modes driven by ion temperature gradient (ITG) and parallel sheared flows, including parallel velocity shear (PVS) of ions and plasma current, are studied for any β (plasma pressure/magnetic pressure) plasma. It is shown that a finite β not only weakens the driving mechanism directly but also reverses the effect of the current on the modes from weakly destabilizing to stabilizing. However, the local kinetic ITG–PVS modes are unstable in a high-β plasma with a parallel sheared ion flow (positive or negative). The plasma current and current gradient make nearly opposite contributions to the modes. As ηi (the parameter for ion temperature gradient) increases, the effects of the current and current gradient are weakened and then reversed.

Electromagnetic ion temperature gradient modes of tearing mode parity in high β sheared slab plasmas

The ion temperature gradient modes of tearing mode parity are investigated for arbitrary β (=plasma pressure/magnetic pressure) plasmas in a sheared slab. Under the low β limit, the results agree with Reynders’ conclusion that the lowest order (l=1) mode of tearing mode parity persists after the fundamental mode (l=0) of drift mode parity is completely stabilized by finite β [J. V. M. Reynders, Phys. Plasmas 1, 1953 (1994)]. However, when the effects of the magnetic gradient drift and the coupling to the compressional Alfven waves are included, the l=0 mode is more difficult to stabilize than the l=1 mode. It is also shown that the l=1 mode is much easier to stabilize by the magnetic shear than the l=0 mode. Generally, the l=1 mode grows faster in the low magnetic shear and low β regime while the l=0 mode is the dominant eigenmode of ion temperature gradient instability in high β plasmas.

Effects of flow shear on the ion temperature gradient modes in a high β plasma slab

The integral eigenmode equations derived previously for the study of drift instabilities in a sheared slab plasma with arbitrary β (plasma pressure/magnetic pressure) are extended. These equations are used to investigate the effects of flow shear on the ion temperature gradient driven (ITG) modes. It is found that the destabilizing effect of a parallel velocity shear, V0′, is weakened by the finite β effect, especially in case of weak magnetic shear. However, the perpendicular velocity shear, VE′, still effectively stabilizes these modes even in high β regions. A large enough VE′ can completely stabilize the ITG mode at arbitrary β. In addition, the effect of VE′ is highly enhanced in weak magnetic shear regions. When the parallel flow coexists with the perpendicular flow, the comprehensive flow effect depends on the relative sign of these velocity shears. The modes with higher growth rates may be stabilized by a smaller VE′ for V0′VE′>0.