Floating Potential Profile Research Articles

Potential Profile Measurements Inside a Gridded Cathode at High Potential in a Spherical Inertial Electrostatic Confinement Device

This paper proposes a Langmuir probe–based diagnostics for plasma parameters inside gridded cathodes at high bias potentials in inertial electrostatic confinement devices. As the first step for the proof of concept, floating potential profiles were measured in deuterium and helium plasmas in a glow-discharge mode. The measurements with fusion-relevant cathode voltages up to 55 kV were carried out successfully. The results revealed that the positive potential buildup at the center ranges from 5% to 8% of the applied bias voltage to the gridded cathode, which is found to be much smaller than those in earlier works under cathode voltages lower than 5 kV. It was also shown that the floating potential profile is different significantly between deuterium and helium discharge plasmas.

Detection of an electron beam in a high density plasma via an electrostatic probe

An electron beam is detected by a 1D floating potential probe array in a relatively high density (1012–1013 cm−3) and low temperature (∼5 eV) plasma of the Magnetic Reconnection Experiment. Clear perturbations in the floating potential profile by the electron beam are observed. Based on the floating potential profile and a current balance equation to the probe array tips, the effective width of the electron beam is determined, from which we determine the radial and toroidal beam current density profiles. After the profile of the electron beam is specified from the measured beam current, we demonstrate the consistency of the current balance equation and the location of the perturbation is also in agreement with field line mapping. No significant broadening of the electron beam is observed after the beam propagates for tens of centimeters through the high density plasma. These results prove that the field line mapping is, in principle, possible in high density plasmas.

Laboratory study of diffusion region with electron energization during high guide field reconnection

Floating potential profile was measured around the X-point during high guide field reconnection in UTST merging experiment where the ratio of guide field (Bg) to reconnecting magnetic field (Brec) is Bg/Brec>10. Floating potential measurement revealed that a quadrupole structure of electric potential is formed around the X-point during the fast reconnection phase due to the polarization by inductive electric field. Also, our floating potential measurement revealed the existence of parallel electric field in the vicinity of the X-point. While field-aligned components of inductive electric field (E∥ind) and electrostatic electric field (E∥es) cancel out with each other away from the X-point, E∥ind exceeds E∥es around the X-point, indicating the deviation from ideal MHD criterion within the region. The diffusion region extends in the outflow region and the scale length of region is an order of ion skin depth, which is quite different from the VTF experiment result. Based on the measured magnetic field and electric field profile, our particle trajectory analysis indicates that fast electrons with energies over 300 eV are produced within 1 μs around the X-point in the non-ideal MHD region. These results indicate that production of fast electrons or electron heating are expected to be observed in the vicinity of the X-point.

Spacecraft charging analysis with the implicit particle-in-cell code iPic3D

We present the first results on the analysis of spacecraft charging with the implicit particle-in-cell code iPic3D, designed for running on massively parallel supercomputers. The numerical algorithm is presented, highlighting the implementation of the electrostatic solver and the immersed boundary algorithm; the latter which creates the possibility to handle complex spacecraft geometries. As a first step in the verification process, a comparison is made between the floating potential obtained with iPic3D and with Orbital Motion Limited theory for a spherical particle in a uniform stationary plasma. Second, the numerical model is verified for a CubeSat benchmark by comparing simulation results with those of PTetra for space environment conditions with increasing levels of complexity. In particular, we consider spacecraft charging from plasma particle collection, photoelectron and secondary electron emission. The influence of a background magnetic field on the floating potential profile near the spacecraft is also considered. Although the numerical approaches in iPic3D and PTetra are rather different, good agreement is found between the two models, raising the level of confidence in both codes to predict and evaluate the complex plasma environment around spacecraft.

Studies of the cathode sheath of a low pressure hollow cathode discharge

In this work, a theoretical model for the cathode sheath using experimental data of a low pressure hollow cathode discharge (HCD) is derived considering it to be non-collisional. Secondary electrons emitted from the cathode surface are taken into account on the discharge model and their influence on the theoretical sheath potential profile is investigated. The plasma parameters and the floating potential profile along the discharge axis were inferred from the current-voltage characteristics of a single Langmuir probe positioned at the inter-cathode space of the HCD. For a low pressure HCD, typical values of the electron density and electron temperature are ne ≈ 1016 m−3 and Te = 4 eV, respectively. By using the probe data the floating potential profile was determined to verify the position of the plasma-cathode sheath interface and to promote a qualitative discussion between theoretical and experimental results.

The effect of the use of different electrode materials for edge-plasma biasing on plasma density and floating potential modifications

The effect of the use of different electrode materials for edge-plasma biasing on plasma density and floating-potential profile modifications has been recently investigated on the CASTOR tokamak. Dependences of biasing current, edge plasma density and floating potential on biasing voltage have been measured. Induced relaxation events and fast (10–50) kHz oscillations are shown and surface changes of the biasing electrodes are discussed in the paper.

Observation of bistable density transitions in magnetized plasma by voltage-biased electrode

Repeated transition phenomena with abrupt reductions and jumps of the electron density were observed (flip–flop pattern in bistable systems), by voltage biasing to an inserted electrode in the inner region of a rf- (radio frequency-) produced, cylindrical magnetized plasma. These global, self-excited, density transitions and back ones between two states were accompanied by changes of the floating potential profile and the bias current under various parameters. Control of the staying time probability in one of two states with hysteresis loops was attempted, changing the bias voltage, which showed the importance of a sheath region as well as fine structural patterns.