Pressure dependence of an ion beam accelerating structure in an expanding helicon plasma

Abstract

We present measurements of the parallel ion velocity distribution function and electric field in an expanding helicon source plasma plume as a function of downstream gas pressure and radial and axial positions. The ion beam that appears spontaneously in the plume persists for all downstream pressures investigated, with the largest parallel ion beam velocities obtained for the lowest downstream pressures. However, the change in ion beam velocity exceeds what would be expected simply for a change in the collisionality of the system. Electric field measurements confirm that it is the magnitude of the potential structure responsible for accelerating the ion beam that changes with downstream pressure. Interestingly, the ion density radial profile is hollow close to the end of the plasma source for all pressures, but it is hollow at downstream distances far from the source only at the highest downstream neutral pressures.

Thermodynamics of a magnetically expanding plasma with isothermally behaving confined electrons

“…Normalized IEDFs are found to have two peaks exhibiting the well known high energy tail and can be fitted by two Gaussian functions, corresponding to the local plasma potential representing background ion group produced via a charge exchange process and the ion beam (high energy tail) resulting from the plasma potential structure. Although the mean free path of collision with neutral is shorter than the IEDF measurement position, it is expected that the effect on the deceleration will be very small [47]. Assuming that the MN consists of a mono-energetic ion beam, the average beam energy of the accelerated ions at each location (the central ∼29 cm and far regions ∼49 cm) can be estimated as 12.2±0.65 V and 13.3±0.38 V, respectively.…”

Ion beams in multi-species plasmas

“…At higher pressures, the electric field amplitude decreases and its spatial structure changes. 28 For multi-ion species plasmas, the work presented here arrives at a different conclusion than Biloiu and Scime. 20 The reason for this discrepancy is because Biloiu and Scime 20 kept the total flow rate equal to 10 sccm for the duration of their experiment.…”

“…Therefore, the electric field structure appears to change as a function of pressure, consistent with other work. 28 The density for an argon only plasma at a pressure of 0.1 mTorr is 4.3 Â 10 9 cm -3 . For xenon, previous measurements suggest the density is slightly higher.…”

Automatically Controlled Frequency-Tunable rf Plasma Thruster: Ion Beam and Thrust Measurements

“…Since the energy source of the potential drop is considered to be the electron energy as discussed in [21], the electron energy distribution significantly affects the ion acceleration energy. Actually, the increases in both the ion beam energy and the electron temperature with a decrease in the operating gas pressure have been detected so far [16,22,23].…”

“…Charles and Boswell have discovered a formation of a current-free electric double layer, which has a potential drop over a narrow region of about a few tens of Debye length, in the magnetically expanding radiofrequency (rf) plasmas [6]. A number of subsequent experiments have also shown the similar structure or a broader (but still narrower than the scale of the magnetic field gradient) potential drop over about 10 cm in the expanding magnetic fields [7][8][9][10][11][12][13][14][15][16]. Measurements of ion energy distributions by using a retarding field energy analyzer and a laser-induced fluorescence method have shown a generation of a supersonic ion beam at the low-potential side, where the number of the beam ions decays along the axis due to a charge exchange process [7].…”