Plasma production using radiofrequency fields near or below the ion cyclotron range of frequencies

Abstract

The use of radiofrequency (RF) fields in the megahertz range to produce the initial plasmas for nuclear fusion experiments can provide a broader operating regime for many stellarator devices and can result in a saving of volt-seconds in tokamaks. This process has been used for many years in various devices at the Institute of Physics and Technology at Khar'kov. The initial breakdown of neutral gas by non-resonant RF near-fields can be understood in terms of RF electric fields parallel to the static magnetic field and a ponderomotive trap with a long scale length parallel to the static magnetic field. If the parallel fields are not too strong for a given antenna geometry, the concept of a ponderomotive trap is valid and the trapped electrons can oscillate in the near-field region for many RF periods. Efficient breakdown can begin if the oscillation energy of these trapped electrons is sufficient to ionize the gas. Good agreement between the experimental observations and an analytic model for the early phase of the breakdown process is presented.

Discharge initiation by ICRF antenna in IShTAR

“…To understand this occurrence we reinvestigate the frequency and pressure dependency for trapping electrons in ponderomotive well to initiate the electron density build-up. The first approximation of the plasma initiation by ICRF antenna was introduced by Lyssoivan and Carter [7]. They derived conditions for the electric field and frequency to accelerate electrons above the ionization potential in the RF field.…”

ICRF Wall Conditioning: Present Status and Developments for Future Superconducting Fusion Machines

“…The initiation of ICRF discharge in a toroidal magnetic field B results from the absorption of RF energy mainly by electrons [2,4,[21][22]. The RF z Ẽ -field (parallel to the B T B -field) is considered to be responsible for this process.…”

“…Plasma density build-up and its homogeneity: The plasma production process has been studied in present-day tokamaks and stellarators under various conditions [1][2][3][4][5][6][7][8][9][10][11][12][13][14][15][20][21][22][23]. The ICRF plasmas with typical density n e ≈3×10…”

“…An analysis of the parallel equation of motion for electrons in the presence of antenna-near locally generated z Ẽ -field revealed that the electrons perform complex motions: linear fast oscillations under the action of the Lorentz force and non-linear slow oscillations under the action of the RF ponderomotive force [21][22]. If the oscillation energy of the electrons exceeds the ionization potential for molecules (atoms), …”

Plasma and antenna coupling characterization in ICRF-wall conditioning experiments

“…According to the theory of ICRF plasma production [4,5] with poloidal ICRH antennas, the antenna E z -component (along the toroidal magnetic field B T ), induced by the RF voltage difference between the antenna box and strap, initiate working gas breakdown and ionization near the antenna [6]. In the typical ICRF band (∼20-60 MHz), the electromagnetic waves (TM cylindrical modes) cannot propagate along the vacuum torus of present-day fusion machines due to small cross-section size: Ä 2 z = ω 2 /c 2 − Ä 2 ⊥ < 0 (k z is the parallel wave-vector).…”