Helicon waves in uniform plasmas. II. High m numbers

Journal Article · Tue Sep 15 04:00:00 EDT 2015 · Physics of Plasmas

DOI:https://doi.org/10.1063/1.4930106· OSTI ID:22490134

Stenzel, R. L.; Urrutia, J. M. ^[1]

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1. Department of Physics and Astronomy, University of California, Los Angeles, California 90095-1547 (United States)

Helicons are whistler modes with azimuthal wave numbers. They have been studied in solids and plasmas where boundaries play a role. The present work shows that very similar modes exist in unbounded gaseous plasmas. Instead of boundaries, the antenna properties determine the topology of the wave packets. The simplest antenna is a magnetic loop which excites m = 0 or m = 1 helicons depending on whether the dipole moment is aligned parallel or perpendicular to the ambient background magnetic field B{sub 0}. While these low order helicons have been described by J. M. Urrutia and R. L. Stenzel [“Helicon modes in uniform plasmas. I. Low m modes,” Phys. Plasmas 22, 092111 (2015)], the present work focuses on high order modes up to m = 8. These are excited by antenna arrays forming magnetic multipoles. Their wave magnetic field has been measured in space and time in a large and uniform laboratory plasma free of boundary effects. The observed wave topology exhibits m pairs of unique field line spirals which may have inspired the name “helicon” to this mode. All field lines converge into these nested spirals which propagate like corkscrews along B{sub 0}. The field lines near the axis of helicons are perpendicular to B{sub 0} and circularly polarized as in parallel whistlers. Helical antennas couple to these transverse fields but not to the spiral fields of helicons. Using a circular antenna array of phased m = 0 loops, right or left rotating or non-rotating multipole antenna fields are generated. They excite m < 0 and m > 0 modes, showing that the plasma supports both modes equally well. The poor excitation of m < 0 modes is a characteristic of loops with dipole moment across B{sub 0}. The radiation efficiency of multipole antennas has been found to decrease with m.

• OSTI ID: 22490134
• Journal Information: Physics of Plasmas, Journal Name: Physics of Plasmas Journal Issue: 9 Vol. 22; ISSN PHPAEN; ISSN 1070-664X
• Country of Publication: United States
• Language: English

Related Subjects

70 PLASMA PHYSICS AND FUSION TECHNOLOGY
ANTENNAS
DIPOLE MOMENTS
EXCITATION
HELICON WAVES
MAGNETIC FIELDS
MULTIPOLES
PLASMA
SOLIDS
SPACE DEPENDENCE
TIME DEPENDENCE
TOPOLOGY
WAVE PACKETS
WHISTLER INSTABILITY
WHISTLERS