Non-Axisymmetric Alfvén Wave Excited by a Helical Coupler in an Inhomogeneous Plasma

Amagishi, Yoshimitsu; Inutake, M.; Akitsu, Takashiro; Tsushima, A.

doi:10.1143/jjap.20.2171

Cited by 25 publications

(11 citation statements)

References 13 publications

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“…At these densities the Alfvén wavelength is of order 40 cm (f ¼ 390 kHz, f ci ¼ 950 kHz), but the plasma was not completely ionized (70%) and the collision rate v ei was higher than the wave frequency by a factor of ¼ 10 4 . In another experiment the compressional and shear waves were both observed using a Stix type coil 31 having a spectrum of wave numbers 32 with the plasma selecting parallel wavelengths that could fit in the machine. The waves were detected by a small (5 mm diameter, 100 turn) magnetic probe.…”

Section: -3mentioning

confidence: 99%

The many faces of shear Alfvén waves

et al. 2011

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One of the fundamental waves in magnetized plasmas is the shear Alfvén wave. This wave is responsible for rearranging current systems and, in fact all low frequency currents in magnetized plasmas are shear waves. It has become apparent that Alfvén waves are important in a wide variety of physical environments. Shear waves of various forms have been a topic of experimental research for more than fifteen years in the large plasma device (LAPD) at UCLA. The waves were first studied in both the kinetic and inertial regimes when excited by fluctuating currents with transverse dimension on the order of the collisionless skin depth. Theory and experiment on wave propagation in these regimes is presented, and the morphology of the wave is illustrated to be dependent on the generation mechanism. Three-dimensional currents associated with the waves have been mapped. The ion motion, which closes the current across the magnetic field, has been studied using laser induced fluorescence. The wave propagation in inhomogeneous magnetic fields and density gradients is presented as well as effects of collisions and reflections from boundaries. Reflections may result in Alfvénic field line resonances and in the right conditions maser action. The waves occur spontaneously on temperature and density gradients as hybrids with drift waves. These have been seen to affect cross-field heat and plasma transport. Although the waves are easily launched with antennas, they may also be generated by secondary processes, such as Cherenkov radiation. This is the case when intense shear Alfvén waves in a background magnetoplasma are produced by an exploding laser-produced plasma. Time varying magnetic flux ropes can be considered to be low frequency shear waves. Studies of the interaction of multiple ropes and the link between magnetic field line reconnection and rope dynamics are revealed. This manuscript gives us an overview of the major results from these experiments and provides a modern prospective for the earlier studies of shear Alfvén waves. V

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Section: -3mentioning

confidence: 99%

The many faces of shear Alfvén waves

et al. 2011

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“…The long parallel length (L $ 20 m) and reproducible plasma of the large plasma device (LAPD) 12 have made it an ideal facility for Alfvén wave experiments. The higher densities of helicon plasma sources, as with some previous facilities used for Alfvén wave studies, 13,14 can theoretically compensate for the much shorter achievable plasma lengths. 15,16 However, there are a number of challenges arising from the characteristics of helicon sources which need to be addressed.…”

Section: Introductionmentioning

confidence: 99%

Ducted kinetic Alfvén waves in plasma with steep density gradients

Houshmandyar

Scime

2011

Physics of Plasmas

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Given their high plasma density (n $ 10 13 cm À3 ), it is theoretically possible to excite Alfvén waves in a conventional, moderate length (L $ 2 m) helicon plasma source. However, helicon plasmas are decidedly inhomogeneous, having a steep radial density gradient, and typically have a significant background neutral pressure. The inhomogeneity introduces regions of kinetic and inertial Alfvén wave propagation. Ion-neutral and electron-neutral collisions alter the Alfvén wave dispersion characteristics. Here, we present the measurements of propagating kinetic Alfvén waves in helium helicon plasma. The measured wave dispersion is well fit with a kinetic model that includes the effects of ion-neutral damping and that assumes the high density plasma core defines the radial extent of the wave propagation region. The measured wave amplitude versus plasma radius is consistent with the pile up of wave magnetic energy at the boundary between the kinetic and inertial regime regions.

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“…The latter, which is a surface wave at frequencies approaching oci, is of considerable importance for heating purposes. (It is to be noted that the waves here vary as exp [-i(otkz + me)]; this gives the same m numbers as in KNOX et al (1975), but opposite those used by PAOLONI (1975)) Some of the other studies deal with the coupling of Alfven waves by helical couplers (HIPP et al, 1971;KNOX et al, 1975;HOFFMAN and SHOHET, 1978;AMAGISHI et al, 1981). Theoretically, estimates of the antenna loading impedance and the total power coupled into the various plasma-waveguide modes have been calculated.…”

mentioning

confidence: 86%

m=0,±1 low frequency fast Alfven modes of a plasma column loaded in a sheath helix

Ganguli¹,

Baskaran²

1987

Plasma Phys. Control. Fusion

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The dispersion relation for a cold, collisionless plasma column loaded in a sheath helix is derived. This is then solved numerically for the m = 0, 1 fast Alfven waves for different plasma radii and helix parameters; the frequency range is below the ion cyclotron frequency. The results are compared with those of the plasma-waveguide system. It is found that there is no m = 0 cut-off and instead there are two ( m = 0) waves which propagate down to low frequencies. While the m = + 1 mode retains its general behaviour, the m = -1 mode is altered significantly. Relevance of these results to the coupling of Alfven waves with helices is discussed. I N T R O D U C T I O N* A sheath helix may be considered as the limiting case of an N wire helix, where N -+ cc with the diameter of each wire being shrunk to zero. Such a helix would be azimuthally symmetric and SO would not have any coupled space harmonics.

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Non-Axisymmetric Alfvén Wave Excited by a Helical Coupler in an Inhomogeneous Plasma

Cited by 25 publications

References 13 publications

The many faces of shear Alfvén waves

The many faces of shear Alfvén waves

Ducted kinetic Alfvén waves in plasma with steep density gradients

m=0,±1 low frequency fast Alfven modes of a plasma column loaded in a sheath helix

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