Observations of single-pass ion cyclotron heating in a trans-sonic flowing plasma

Abstract

The VAriable Specific Impulse Magnetoplasma Rocket (VASIMR®) is a high power electric spacecraft propulsion system, capable of Isp/thrust modulation at constant power [F. R. Chang Díaz et al., Proceedings of the 39th AIAA Aerospace Sciences Meeting and Exhibit, Reno, NV, 8–11 Jan. 2001]. The VASIMR® uses a helicon discharge to generate plasma. This plasma is energized by an rf booster stage that uses left hand polarized slow mode waves launched from the high field side of the ion cyclotron resonance. In the experiments reported in this paper, the booster uses 2–4 MHz waves with up to 50 kW of power. This process is similar to the ion cyclotron heating (ICH) in tokamaks, but in the VASIMR® the ions only pass through the resonance region once. The rapid absorption of ion cyclotron waves has been predicted in recent theoretical studies. These theoretical predictions have been supported with several independent measurements in this paper. The single-pass ICH produced a substantial increase in ion velocity. Pitch angle distribution studies showed that this increase took place in the resonance region where the ion cyclotron frequency was roughly equal to the frequency on the injected rf waves. Downstream of the resonance region the perpendicular velocity boost should be converted to axial flow velocity through the conservation of the first adiabatic invariant as the magnetic field decreases in the exhaust region of the VASIMR®. This paper will review all of the single-pass ICH ion acceleration data obtained using deuterium in the first VASIMR® physics demonstrator machine, the VX-50. During these experiments, the available power to the helicon ionization stage increased from 3 to 20+ kW. The increased plasma density produced increased plasma loading of the ICH coupler. Starting with an initial demonstration of single-pass ion cyclotron acceleration, the experiments demonstrate significant improvements in coupler efficiency and in ion heating efficiency. In deuterium plasma, ≥80% efficient absorption of 20 kW of ICH input power was achieved. No clear evidence for power limiting instabilities in the exhaust beam has been observed.

The role of second‐order radial density gradient for helicon power absorption

“…Because the TG mode sits around plasma edge, and global power is conserved, the power redistribution may also imply an overwhelming relation between TG mode and helicon mode. While quantitative confirmation of this overwhelming competition due to SRDG requires further research, extension many also include the axial variation of static magnetic field, which is believed to be important for the transition from TG mode to helicon mode, through so‐called “magnetic beach heating” effects . This finding can be very important experimentally in that the radial power absorption can be controlled by adjusting the radial location of and is of practical interest for applying a helicon source to plasma material processing for example, which requires a certain distribution of heat flux.…”

Performance studies of the VASIMR® VX-200

“…Useful thrust is produced as the plasma accelerates in an expanding magnetic field, a process described by conservation of the first adiabatic invariant as the magnetic field strength decreases in the exhaust region of the VASIMR ® . 9,10,11 This paper will T Figure 1. Cartoon block diagram of the VASIMR™ system, illustrating the basic physics.…”

Numerical Modeling of Electrodeless Electric Thruster by Ion Cyclotron Resonance/Ponderomotive Acceleration

“…While a well-known application of the ICR to electric thrusters is the Variable Specific Impulse Magnetoplasma Rocket (VASIMR) [2], we have focused on the ponderomotive effect on ion acceleration, although the PA and ICR author's e-mail: otsuka@esst.kyushu-u.ac.jp a) present address: Photon Pioneers Center in Osaka University, Suita, Osaka 565-0871, Japan * ) This article is based on the presentation at the 22nd International Toki Conference (ITC22).…”

“…To overcome this problem, "electrodeless" electric thrusters have been extensively studied worldwide in various projects [1][2][3]. In an electrodeless electric thruster, plasmas are accelerated by external electromagnetic fields, and they are not in contact with the electrodes, which are placed outside the plasma region (see Fig.…”

“…On the other hand, the VASIMR experiments have successfully demonstrated the large acceleration of ions within the parameters necessary to minimize ion wall loss. Namely, in the VASIMR experiments [2], the ion gyroradius is reduced in the following two ways: 1) choosing a light propellant species such as helium (He) or deuterium, and 2) using an intense background magnetic field on the order of 0.25 T, so that the ion wall loss effect can be minimized. However, the latter method leads to an increased weight of the propulsion system, which is a disadvantage in the development of a lightweight thruster.…”