Observations of the interaction of a plasma stream with neutral gas: evidence of plasma loss through molecular-activated recombination

Abstract

We describe an experiment, the UMIST Linear System (ULS), in which a hydrogen plasma stream, guided by a longitudinal magnetic field, is injected through a diaphragm containing an orifice into a separately-pumped target chamber in which the neutral hydrogen pressure can be raised to a maximum of 8 mTorr. The stream is about 6 mm in diameter, has an electron temperature of up to 15 eV and an ion flux of 3 × 10 18 s −1 ; it is supersonic with Mach number up to M ≈ 3. We have studied both the passage of the stream through the orifice and the interaction of the supersonic plasma with neutral hydrogen in the target chamber. We find that transmission is incomplete even when the orifice diameter is five times that of the plasma; we attribute this to the presence of ion trajectories which extend well outside the visible plasma and are intercepted by the diaphragm. In the target chamber, the stream does not broaden, but the ion flux decreases approximately exponentially with distance, with a scale length of the order of the mean free path for momentum transfer in ion-neutral collisions, and much less than that expected for other processes, such as charge exchange or electron-ion recombination. Elastic collisions alone cannot decrease the flux, but would lead to a large accumulation of slow ions in thermal equilibrium with the neutral gas, which must be limited by some other loss process: collisional diffusion and electron-ion recombination are too slow, leading to a density approaching 10 20 m −3 . The observed density is of the order of 10 18 m −3 , requiring a process with a rate of 10-100 times faster. Calculated rates for molecular-activated recombination (MAR) of the slow ions are of this order, and the predicted density agrees with our observations to order of magnitude.

The interaction of a flowing plasma with a dipole magnetic field: measurements and modelling of a diamagnetic cavity relevant to spacecraft protection

“…In the absence of the downstream dipole field structure it would be expected that this plasma would propagate along the solenoidal confining field until it encountered the end target plate of the gas target chamber. Previous studies at low target chamber gas pressures similar to those used in these studies have indicated that any reductions in the peak density or temperature occur over characteristic axial scale lengths >1 m with the beam width (FWHM) remaining constant along the whole length of the target chamber [10]. This is consistent with the expected collisional scale lengths for these plasmas: the ion-ion mean free path is of the order of 10 m and the typical ion-neutral mean free path is approximately 25 cm, an order of magnitude greater than the ion Larmor radius.…”

“…The experimental studies were carried out on a linear plasma confinement device LinX, a modified version of the ULS device [10] previously used to study plasma-neutral interactions of relevance to tokamak divertors [11,12]. The plasma in this system consists of a supersonic plasma beam (Mach number>3) with peak densities and temperatures in the range 10 17 -10 19 and 5-7 eV, respectively.…”

“…Using the direct measurement of the upstream floating potential and evaluating the plasma potential using the second derivative of the Langmuir probe characteristic [15], we obtain α = 1.7, considerably less than that expected for an isotropic plasma. Supersonic ion flow will tend to reduce the value of α and a more detailed analysis [10,13,16,17] suggests a full expression for α:…”

Minimagnetospheres above the Lunar Surface and the Formation of Lunar Swirls

“…A scaled laboratory experiment has the potential to deliver physical insights and to offer observational signatures which can be examined, to establish whether they are consistent or not with the in-situ space data. The Plasma Wind Tunnel [25] used here shares the same phenomenological regime as the subset of analogous space parameters. These are: the plasma is collisionless, the bulk flow speed is supersonic and the electrons are magnetized but the ions are not.…”

Divertor conditions relevant for fusion reactors achieved with linear plasma generator

“…7 As such, the experiment combines the high ion flux capabilities of Pilot-PSI 8 with low background pressure and large beam diameter to reach the strongly coupled regime. It should be mentioned that differential pumping has been used before on linear plasma generators, [9][10][11][12] in which it was used to vary the neutral pressure in the target chamber by means of gas puff to study detached plasmas as a possible way to mitigate the power flux to the divertor. However, the electron density and fluxes in these machines were more than an order of magnitude lower as compared to the conditions at the FIG.…”