Whistler modes in highly nonuniform magnetic fields. II. Propagation in three dimensions

Urrutia

2018

Self Cite

The properties of helicon modes in highly nonuniform magnetic fields are studied experimentally. The waves propagate in an essentially unbounded uniform laboratory plasma. Helicons with mode number m ¼ 1 are excited with a magnetic loop with dipole moment across the dc magnetic field. The wave fields are measured with a three-component magnetic probe movable in three orthogonal directions so as to resolve the spatial and temporal wave properties. The ambient magnetic field has the topology of a mirror or a cusp, produced by the superposition of a uniform axial field B 0 and the field of a current-carrying loop with the axis along B 0. The novel finding is the reflection of whistlers by a strong mirror magnetic field. The reflection arises when the magnetic field changes on a scale length shorter than the whistler wavelength. The simplest explanation for the reflection mechanism is the strong gradient of the refractive index which depends on the density and magnetic field. More detailed observations show that the incident wave splits when the k vector makes an angle larger than 90 with respect to B 0 which produces a parallel phase velocity component opposite to that of the incident wave. The reflection coefficient has been estimated to be close to unity. Interference between reflected and incident waves creates nodes in which the whistler mode becomes linearly polarized. When the magnetic field topology is that of a reversed field configuration (FRC), the incident wave is absorbed near the three-dimensional (3D) magnetic null point which prevents wave reflections. However, waves outside the separatrix are not absorbed and continue to propagate around the null point. When waves are excited inside the FRC, their polarization and helicon mode are reversed. Implications of these observations on research in space plasmas and helicon sources are pointed out.

Section: -12mentioning

confidence: 94%

“…It has been addressed in the present work and its preceding Papers I and II. 18,19 In the present paper, the nonuniform field topologies are an axially symmetric mirror field and a field reversed configuration (FRC), both with short gradient scale lengths.…”

Section: Introductionmentioning

confidence: 99%

Whistler modes in highly nonuniform magnetic fields. III. Propagation near mirror and cusp fields

Urrutia

2018

Self Cite

Whistler modes in highly nonuniform magnetic fields. I. Propagation in two-dimensions

Urrutia

2018

The propagation of whistler wave packets is studied in a large laboratory plasma with nonuniform ambient magnetic fields. The wave packets are excited by magnetic loop antennas and, for uniform fields, have the topology of helicon modes in unbounded plasmas. The waves propagate across the separatrix of X-type nulls and across circular field lines of magnetic islands. Classical wave refraction is not observed when the magnetic gradient scale length becomes shorter than the wavelength. Since the ambient magnetic field is generated by a line current, it is two dimensional and the wave magnetic field is measured and displayed in transverse planes. Using wave bursts, the space-time evolution of wave packets is observed. The group velocity has been measured. Waves can cross a separatrix and refract into nearly parallel whistler modes. The phase rotation of helicons is lost on a strongly curved field. In highly nonuniform magnetic fields, the phase velocity is determined from multipoint magnetic hodograms, the group velocity from the Poynting vector fields. The energy flow is highly field aligned while the phase flow can be highly oblique to the ambient magnetic field without encountering cyclotron resonance absorption. In the limiting case of circular field lines, counter propagating waves create azimuthal standing waves while still propagating radially. Standard concepts of plane wave refraction do not apply to whistler modes in highly nonuniform magnetic fields. Implications of these observations on research in space plasmas and helicon devices will be pointed out.

Whistler modes excited by magnetic antennas: A review

2019

A review of a deceptively simple topic is presented, i.e., the excitation of whistler modes by antennas. It includes the knowledge of antennas and of the waves and their coupling. This review will show how the research in the last few decades has advanced and become a refined and complex topic which covers nonlinear effects, instabilities, nonuniform fields, whistler modes with orbital angular momentum, wave field topologies, etc. This review is mainly focused on experimental work in laboratory plasmas, but the findings will be related to research on whistler waves in space plasmas, helicon plasma sources in the laboratory, and significant findings of other research groups. This review starts with antenna properties such as radiation patterns, radiation efficiencies, and the topology of the emitted wave packets. Next, the propagation of whistler modes in highly nonuniform ambient magnetic fields will be presented. Even in the linear regime, new phenomena have been discovered such as the reflection of whistler modes from strong magnetic field gradients or the eigenmodes of waves on circular magnetic field lines. Important nonlinear effects of whistler modes are presented. Whistler instabilities will be briefly reviewed although this is a broad topic by itself. Examples of magnetic reconnection in the Hall parameter regime will be shown. This review will also discuss the advances of wave diagnostics in dedicated laboratory plasmas, the difficulties of diagnostics in high power laboratory plasmas, and the limiting multipoint diagnostics in space plasmas.