Optimal design of helicon wave antenna and numerical investigation into power deposition on helicon physics prototype experiment

Abstract

<sec>Recently, helicon plasma sources have aroused the great interest particularly in plasma-material interaction under fusion conditions. In this paper, the helicon wave antenna in helicon physics prototype experiment (HPPX) is optimized. To reveal the effect of the radial density configuration on wave field and energy flow, Maxwell's equations for a radially nonuniform plasma with standard cold-plasma dielectric tensor are solved. Helicon wave coupling and power deposition are studied under different types of antennas, different antenna lengths and driving frequencies by using HELIC. Through the numerical simulation, the optimal antenna structure and size are obtained, that is, half helix antenna, which works at 13.56 MHz and has a length of 0.4 m, can generate nonaxisymmetric radio frequency energy coupling to excite higher electron density.</sec><sec>The influences of different static magnetic fields and axis plasma densities on power deposition are also analyzed. It is found that the absorbed power of the plasma to the helicon wave has different peak power points in a multiple static magnetic field and axial plasma densities, and the overall coupling trend increases with static magnetic field increasing, but decreases with axis plasma density increasing. According to the simulation results, the ionization mechanism of helicon plasma is discussed. In order to further study the coupling of helicon wave with plasma in HPPX, the induced electromagnetic field and current density distribution are given when the plasma discharges. Under parabolic density distribution, the field intensity of the induced electric field at the edge is large, while neither the induced magnetic field nor current density changes much along the radial direction, the energy is distributed evenly in the whole plasma. Under the Gaussian density distribution, the induced electric field intensity is higher at the edge, while the induced magnetic field and current density in the center are much higher than at the edge. </sec><sec>In this paper studied are the structure and size of helicon wave antenna, the influences of static magnetic field and axial plasma density on plasma power deposition and the distribution of induced electromagnetic field and current density during plasma discharge under different density distributions. This work will provide important theoretical basis for helicon wave antena design and relevant physical experiments on HPPX.</sec>

Influence of magnetic field on power deposition in high magnetic field helicon experiment

“…Due to the above conditions, the power on the antenna is equal to the absorbed power of plasma, so P ant = P abs . As for the absorbed power of plasma, the specific plasma power spectrum function S(k) is defined as [5] S…”

“…Here, 𝐽 plasma is the current density of plasma, which can be expressed in terms of the cold plasma tensor [5] J…”

“…The magnetic field density is uniform along the axial direction, and the density distribution of the particles is also uniform along the axial direction. The radial density can be determined by the following function: [5] n…”

“…[4] Due to the external magnetic field, the ionization mechanism of HWP is more than complex, and the wave-particle energy coupling mechanism has not been fully appreciated up to now. [5] Since Aigrain [6] first proposed the concept of the H wave and Boswell [7] first realized the discharge of HWP, Chen, [8] Blackwell, [9] Shamrai, [10] and others had been dedicated to studying the power deposition mechanism of HWP. Although the current hypothesis of the Trivelpiece-Gould wave (TG wave) energy deposition mechanism has gradually become the mainstream consensus, [11,12] the research on the influence of magnetic field on the power deposition needs further discussion.…”

Helicon plasma in a magnetic shuttle

“…The magnetic shuttle could provide the confinement of charged particles due to Lorentz force and the invariance of magnetic moment, and enables helicon plasma to capture the particle confinement physics relevant to fusion and space plasmas. The Helicon Physics Prototype eXperiment (HPPX [28]) constructed at Institute of Plasma Physics, Chinese Academy of Sciences, utilizes such a magnetic shuttle to produce high-density and large-volume helicon plasma and confine it thereafter. This work is devoted to analyzing the general physics of helicon plasma in a magnetic shuttle, and computing the wave physics and power absorption based on the real parameters of HPPX to provide guidance for ongoing experiments.…”