Penetration and radial force balance in field-reversed configuration with large rotating magnetic field

Abstract

A field-reversed configuration ͑FRC͒ is formed by applying a rotating magnetic field ͑RMF͒ much larger than the axial magnetic field to a cylindrical glass vacuum chamber filled with 10 Pa argon gas without a preionization. The FRC with the plasma density 2.2ϫ 10 19 m −3 , the temperature 8.0 eV, the separatrix length 0.45 m, and the separatrix radius 0.035 m is sustained for the notably long period of 40 ms. It is observed that the antenna current which produces the RMF is reduced by about half after the FRC is formed. The interaction between the plasma and the antenna circuit increases the antenna resistance and changes the inductance of the antenna so that the circuit becomes nonresonant. The RMF is sufficiently large to fully penetrate to the center during the period and drive the current with a rigid rotor profile. The RMF is shown to play a major role in sustaining the plasma pressure. © 2008 American Institute of Physics. ͓DOI: 10.1063/1.2997341͔A rotating magnetic field ͑RMF͒ has been used for the purpose of the slow formation and the sustainment of a fieldreversed configuration ͑FRC͒. When the RMF frequency satisfies the conditions ci ഛ Ӷ ce and ei Ӷ ce ͑Ref. 1͒, where ci and ce are the cyclotron frequency of ions and electrons for RMF and ei the electron-ion collision frequency, the RMF drives plasma current and forms FRCs with temperature about 10-100 eV, and density about 10 where R the separatrix radius and ␦ = ͑2 / 0 ͒ 1/2 is the classical skin depth. The magnitude of the RMF required to penetrate into the plasma does not depend on the axial field.The effect of the RMF on the pressure balance equations was assumed negligible in the early theoretical investigations 10,11 and this assumption was verified by detailed measurements of the magnetic field, plasma density, and temperature.12 In a more recent work, Steinhauer 13showed that there is a correction RMF term in the pressure balance equations. We report here the first experimental evidence for the effect of the RMF on the radial pressure balance in an FRC sustained by an RMF. Figure 1 shows a schematic diagram of the experimental device. The cylindrical vacuum chamber is made of Pyrex glass and has an internal radius of 35 mm. The length of the discharge chamber is 600 mm. The vacuum is achieved to be less than 10 −4 Pa in 10 min by a rotary pump and a turbomolecular pump in a series. Argon gas is fed from the opposite side to the vacuum pumping system. The axial field is produced by six coils and the strength is fairly uniform along the central axis and 60 G at the maximum. Two RMF power supplies, of which sinusoidal output is 90°out of phase, deliver the current 300 A with the frequency 200 kHz and output power 26 kW for 40 ms. The current decreases to 200 A at t = 40 ms due to the limited capacity of the condenser. The two turn antennas are placed directly on the outer wall of the chamber in order to produce the largest RMF inside the chamber. The amplitude of the RMF in the vacuum is initially 120 G at the center and reduces to 80 G at t = 40...

An Extreme Ultraviolet Source for Photolithographic Applications Based on Rotamak Discharge

“…The amplitude of the RMF in vacuum is 225 G at the center for a 600 A RMF current. The antennas used in these experiments are considerably shorter than those used in earlier experiments 6) on the formation of a field-reversed configuration (FRC) in the Kansai Rotamak. We chose to use short antennas in these experiments in order to produce an almost spherical compact torus or rotamak configuration with a relatively small plasma volume.…”

“…In this paper, we report the measurements of EUV radiation from Xe plasma in the Kansai Rotamak. 6) The rotamak 7,8) is a compact torus maintained using a rotating magnetic field (RMF). It is particularly suitable as an EUV source 9) for photolithographic applications.…”

“…6) The rotamak 7,8) is a compact torus maintained using a rotating magnetic field (RMF). It is particularly suitable as an EUV source 9) for photolithographic applications. The rotamak discharges are remarkably reproducible 10) and can be maintained for as long as the RMF is applied.…”