Electrostatic Confinement Of Electrons Research Articles

Simulation of equilibrium and transport in advanced FRCS

The advanced beam-driven FRC is a field reversed configuration (FRC) with the addition of neutral beam (NB) injection, electrode biasing, and magnetic expander divertors. The resulting configuration has novel features that make it necessary to revisit many key results in FRC theory. Three of these features include (i) a large energetic ion population, (ii) in-principle capability to adjust the electric field and rotation profiles, and (iii) a combination of magnetic and electrostatic confinement of electrons in the SOL. In some fueling scenarios the electron density profile may exhibit a significant peak outside of the separatrix. To explore these features a hybrid fluid/kinetic equilibrium model has been used to reconstruct typical experimental profiles of the C-2W experiment. Results indicate that the energetic ions provide at least 50% of the total plasma pressure. These equilibrium profiles have been used as initial conditions for global, cross-separatrix, turbulent transport simulations using the 3D electrostatic particle-in-cell code ANC. Electrostatic fluctuations were found to nonlinearly saturate at an amplitude which is an order magnitude lower than that observed previously. The tokamak turbulence code GTC code has also been extended to handle FRC physics in the new GTC-X version, which has been used to perform simulations of turbulent transport in the SOL relevant to electrode biasing. It is found that equilibrium E × B flow shear significantly decreases ion temperature gradient saturation amplitude and ion heat transport. Also in the SOL, a 1D2V continuum code has been developed and applied to parallel electron heat transport. Results show the formation of pre-sheath potential and reduction of parallel electron heat loss close to the ideal ambipolar limit, a result which has been validated by experimental diagnostics. These transport modifications caused by the three novel configuration features help to explain the remarkable plasma performance of the C-2W experiment.

Graphene quantum blisters: A tunable system to confine charge carriers

Due to Klein tunneling, electrostatic confinement of electrons in graphene is not possible. This hinders the use of graphene for quantum dot applications. Only through quasi-bound states with finite lifetime has one achieved to confine charge carriers. Here, we propose that bilayer graphene with a local region of decoupled graphene layers is able to generate bound states under the application of an electrostatic gate. The discrete energy levels in such a quantum blister correspond to localized electron and hole states in the top and bottom layers. We find that this layer localization and the energy spectrum itself are tunable by a global electrostatic gate and that the latter also coincides with the electronic modes in a graphene disk. Curiously, states with energy close to the continuum exist primarily in the classically forbidden region outside the domain defining the blister. The results are robust against variations in size and shape of the blister which shows that it is a versatile system to achieve tunable electrostatic confinement in graphene.

Focused beams of fast neutral atoms in glow discharge plasma

Glow discharge with electrostatic confinement of electrons in a vacuum chamber allows plasma processing of conductive products in a wide pressure range of p = 0.01 – 5 Pa. To assist processing of a small dielectric product with a concentrated on its surface beam of fast neutral atoms, which do not cause charge effects, ions from the discharge plasma are accelerated towards the product and transformed into fast atoms. The beam is produced using a negatively biased cylindrical or a spherical grid immersed in the plasma. Ions accelerated by the grid turn into fast neutral atoms at p > 0.1 Pa due to charge exchange collisions with gas atoms in the space charge sheaths adjoining the grid. The atoms form a diverging neutral beam and a converging beam propagating from the grid in opposite directions. The beam propagating from the concave surface of a 0.24-m-wide cylindrical grid is focused on a target within a 10-mm-wide stripe, and the beam from the 0.24-m-diameter spherical grid is focused within a 10-mm-diameter circle. At the bias voltage U = 5 kV and p ∼ 0.1 Pa, the energy of fast argon atoms is distributed continuously from zero to eU ∼ 5 keV. The pressure increase to 1 Pa results in the tenfold growth of their equivalent current and a decrease in the mean energy by an order of magnitude, which substantially raises the efficiency of material etching. Sharpening by the beam of ceramic knife-blades proved that the new method for the generation of concentrated fast atom beams can be effectively used for the processing of dielectric materials in vacuum.

Surface hardening by means of plasma immersion ion implantation and nitriding in glow discharge with electrostatic confinement of electrons

At argon or nitrogen pressure 0.005–5 Pa a metal substrate is immersed in plasma of glow discharge filling a vacuum chamber and negatively biased to U = 1−10 kV. Emitted by substrate secondary electrons bombard the chamber walls and it results in growth of electron emission chamber walls and rise of gas ionization intensity inside chamber. At pressure p< 0.05 Pa nonuniformity of plasma density does not exceed ~10%. Measurement results of sheath width d between homogeneous plasma and a lengthy flat substrate were used to calculate dependence of the ion current density ji on its surface and ion-electron emission coefficient γi. The dependence allows evaluation of average dose of ion implantation on any complex-shaped product of the same material.

Non-self-sustained glow discharge with electrostatic confinement of electrons sustained by a fast neutral molecule beam

Experimental study of plasma produced at the nitrogen pressure 0.2–1 Pa in the chamber volume V ≈ 0.12 m3 as a result of injection into the chamber of a broad nitrogen molecule beam with 1–4 keV energy and 0.1–1 A equivalent current is carried out, and the study results are presented. Dependences of the plasma density distribution on the beam equivalent current Ib, energy Eb, and gas pressure p indicate a crucial role of fast molecules in gas ionization, and the probe characteristics reveal two groups of plasma electrons with the temperatures Te ∼ 0.4 eV and Te ∼ 16 eV. Immersion in plasma of an electrode isolated from the chamber and application to the electrode of a positive voltage U result in non-self-sustained discharge. When U changes from ∼0.5 to ∼1.5 V, the discharge current I rapidly rises to a certain value I*, and after that the rate of rise dI/dU drops by an order of magnitude. At U ∼ 10 V, the current I rises to I0 ≈ 1.5I*, and dI/dU once again drops by an order of magnitude. Current I0 specifies the number of electrons produced inside the chamber per second, and it grows up with Eb, Ib, and p. At U > 20 V, due to gas ionization by fast electrons emitted by the chamber and accelerated up to the energy ∼eU in the sheath between the plasma and the chamber walls, the current I rises again. When U grows up to ∼50 V, production of fast electrons with energies exceeding the ionization threshold begins inside the sheath, and the ionization intensity rises dramatically. At U > 150 V, contribution of fast electrons to gas ionization already exceeds the contribution of fast molecules, and the plasma density and its distribution homogeneity inside the chamber both grow up substantially. However, even in this case, the discharge is non-self-sustained, and only at U > 300 V it does not expire when the beam source is switched off.

Filling the vacuum chamber of a technological system with homogeneous plasma using a stationary glow discharge

Experimental study of a glow discharge with electrostatic confinement of electrons is carried out in the vacuum chamber volume V ≈ 0.12 m3 of a technological system “Bulat-6” in argon pressure range 0.005–5 Pa. The chamber is used as a hollow cathode of the discharge with the inner surface area S ≈ 1.5 m2. It is equipped with two feedthroughs, which make it possible to immerse in the discharge plasma interchangeable anodes with surface area S a ranging from ∼0.001 to ∼0.1 m2, as well as floating electrodes isolated from both the chamber and the anode. Dependences of the cathode fall U c = 0.4−3 kV on the pressure p at a constant discharge current in the range I = 0.2−2 A proved that aperture of the electron escape out of the electrostatic trap is equal to the sum S o = S a + S f of the anode surface S a and the floating electrode surface S f . The sum S o defines the lower limit p o of the pressure range, in which U c is independent of p. At p < p o the cathode fall U c grows up dramatically, when the pressure decreases, and the pressure p tends to the limit p ex, which is in fact the discharge extinction pressure. At p ≈ p ex electrons emitted by the cathode and the first generation of fast electrons produced in the cathode sheath spend almost all their energy up to 3 keV on heating the anode and the floating electrode up to 600–800°C and higher. In this case the gas in the chamber is being ionized by the next generations of electrons produced in the cathode sheath, their energy being one order of magnitude lower. When S a < (2m/M)1/2 S, where m is the electron mass and M is the ion mass, the anode may be additionally heated by plasma electrons accelerated by the anode fall of potential U a up to 0.5 kV.

Electrostatic confinement of electrons in graphene nanoribbons

Coulomb blockade is observed in a graphene nanoribbon device with a top gate. When two pn junctions are formed via the back gate and the local top gate, electrons are confined between the pn junctions which act as the barriers. When no pn junctions are induced by the gate voltages, electrons are still confined, as a result of strong disorder, but in a larger area. Measurements on five other devices with different dimensions yield consistent results.

Plasma immersion ion implantation based on glow discharge with electrostatic confinement of electrons

A new approach to plasma immersion ion implantation (PIII) is presented featuring glow discharge with electrostatic confinement of electrons being used for plasma generation. Its obvious advantage is a comparatively low cost price of related equipment capable to establish at gas pressures of 10 −5–10 −2 mbar in the working chamber volume of several liters up to dozens cubic meters dense and homogeneous plasma. Any vacuum chamber made of conductive material may be used as an electrostatic trap for plasma electrons and a DC power supply is well enough to maintain the discharge. Theoretical and experimental results reveal dependence of stationary glow discharge on electron losses and reduction of anode-to-chamber voltage under the influence of energetic electrons emitted from the surface of negatively biased substrate. Plasma density growth rate depends on amplitudes of high voltage pulses applied to the substrate and to the plasma. Two variants of PIII-systems are described. In the first system plasma ‘on duty’ is established with stationary discharge and negative high voltage pulses applied to the substrate, give rise to high current pulses in its circuit. In the second system two high voltage pulses of equal amplitude are being simultaneously applied to the substrate (negative pulse) and to the plasma (positive pulse). Energy of electrons bombarding the chamber walls and the anode of the second system is two times lower than the energy of singly charged ions bombarding the substrate. Practically, the second system ensures 1 order of magnitude reduction of X-ray yield, thus improving the safety.