G. A. Emmert

Electric sheath and presheath in a collisionless, finite ion temperature plasma

The plasma‐sheath equation for a collisionless plasma with arbitrary ion temperature in plane geometry is formulated. Outside the sheath, this equation is approximated by the plasma equation, for which an analytic solution for the electrostatic potential is obtained. In addition, the ion distribution function, the wall potential, and the ion energy and particle flux into the sheath are explicitly calculated. The plasma‐sheath equation is also solved numerically with no approximation of the Debye length. The numerical results compare well with the analytical results when the Debye length is small.

Numerical simulation of plasma sheath expansion, with applications to plasma‐source ion implantation

In plasma‐source ion implantation a target is pulse biased to a high negative voltage, forming an expanding plasma sheath. A numerical simulation model for the evolution of the sheath has been developed and compared successfully with experimental results. The model is one dimensional (planar, cylindrical, or spherical). The time‐dependent, self‐consistent potential profile is calculated from Poisson’s equation coupled with collisionless fluid equations for the ions and a Boltzmann assumption for the electrons. In addition to the density and potential profile, the simulation yields the ion current to the surface and the energy spectrum of the ions hitting the surface.

Sheath and presheath in a collisionless plasma with a Maxwellian source

Bissell and Johnson [Phys. Fluids 30, 779 (1987)] have calculated the electrostatic potential variation in the sheath and presheath regions of a collisionless plasma in which the source of ions is assumed to be Maxwellian. To do this, they imposed the generalized Bohm criterion as a boundary condition. In this paper the plasma equation is solved numerically without imposing the Bohm criterion as a boundary condition. The results compare well with their results. In addition, the ion distribution function throughout the plasma region is calculated. Because of this particular source model, the ion distribution at the center of the plasma has a spiked non‐Maxwellian shape.

A collisional model of the plasma presheath

The problem of a collisional plasma flowing into a perfectly absorbing wall has been investigated using a kinetic approach. The plasma is assumed to have a nonzero ion temperature and a Boltzmann distribution of electrons. Ion collisions are included in the analysis through a Bhatnagar–Gross–Krook (BGK) collision term. An equation describing the electrostatic potential variation in the presheath region is derived. This equation is solved numerically for a range of collisionalities. In addition to the potential variation in the presheath, the ion distribution function, the wall potential, and the ion particle and energy fluxes into the sheath are also calculated. The calculation is repeated for three different cases. In the first case ion–neutral collisions are modeled by conserving only particles in the BGK operator. Ion–ion collisions are modeled by conserving particles and momentum in the second case, and by conserving particles, momentum, and energy in the third case. These results give insight into th...

Modeling of plasma flow downstream of an electron cyclotron resonance plasma source

We have simulated plasma flow along the divergent magnetic field of an electron cyclotron resonance (ECR) microwave plasma source stream. We assume that ions undergo charge exchange and elastic scattering collisions with the neutral gas in the downstream region. A Monte Carlo description for the ion dynamics, coupled with Boltzmann electrons, is used to develop an iterative scheme for solution of the Boltzmann equation and quasineutrality. We have studied the effect of the magnetic field and neutral gas pressure on the plasma and floating potentials, and on the ion energy incident on the sample.

Apollo - An advanced fuel fusion power reactor for the 21st century

A preconceptual design of a tokamak reactor fueled by a D-He-3 plasma is presented. A low aspect ratio (A=2-4) device is studied here but high aspect ratio devices (A > 6) may also be quite attractive. The Apollo D-He-3 tokamak capitalizes on recent advances in high field magnets (20 T) and utilizes rectennas to convert the synchrotron radiation directly to electricity. The overall efficiency ranges from 37 to 52% depending on whether the bremsstrahlung energy is utilized. The low neutron wall loading (0.1 MW/m/sup 2/) allows a permanent first wall to be designed and the low nuclear decay heat enables the reactor to be classed as inherently safe. The cost of electricity from Apollo is > 40% lower than electricity from a similar sized DT reactor.

TWO-DIMENSIONAL FLUID SIMULATION OF EXPANDING PLASMA SHEATHS

The transient sheath expansion around square and cross‐shaped targets is simulated numerically with a two‐dimensional fluid model. The angular distribution of the ions impinging on the target surface and the nonuniformity of the incident ion dose are calculated. The incident ion dose peaks near, but not at, the convex corner and has a minimum at the concave corner. The dip of the dose profile at the convex corner is shown to be caused by the product of a decreasing normal velocity profile and an increasing ion density profile along the target surface from the center to the corner.

Detection of highly enriched uranium using a pulsed D-D fusion source

Abstract This paper overviews the work that has been done to date towards the development of a compact, reliable means to detect Highly Enriched Uranium (HEU) and other fissile materials utilizing a pulsed Inertial Electrostatic Confinement (IEC) D-D fusion device. To date, the UW IEC device has achieved 115 kV pulses in excess of 2 ampere, with pulsed neutron rates of 1.8x109 n/s during a 0.5 ms pulse at 10 Hz. MCNP modeling indicates that detection of samples of U-235 as small as 10 grams is achievable at current neutron production rates, and initial pulsed and steady-state HEU detection experiments have verified these results.

Effect of collisions on ion dynamics in electron‐cyclotron‐resonance plasmas

A one‐dimensional kinetic code is used to study the effect of ion‐neutral (charge exchange and elastic scattering) and ion‐ion collisions on plasma flow in the downstream region of an electron‐cyclotron‐resonance plasma etching system. Ions are assumed to leave the source region at the Bohm velocity. Argon, nitrogen, and CF4 plasmas are simulated, assuming that the dominant ion species are Ar+, N2+, and CF3+, respectively. Results show that charge exchange and elastic scattering collisions play a significant role in reducing the electrostatic potential variation in the downstream region. For neutral gas pressures above ∼1 mTorr, the potential drop in the downstream region is small, which means that most of the energy with which ions hit the substrate surface is gained while crossing the substrate sheath region. Although the effect of ion‐ion collisions on the plasma potential profile and on the ion distribution function is weak, ion‐ion collisions are responsible for transferring energy from the parallel ...

Possibilities for breakeven and ignition of D-3He fusion fuel in a near term tokamak

The feasibility of D-3He reactor plasma conditions in a tokamak of the NET/INTOR class is investigated. It is found that, depending on the energy confinement scaling law, energy breakeven can be achieved in NET without significant modification of its design. Significant improvement in Q (ratio of fusion power to injected power) can be achieved by removing the tritium producing blanket and replacing the inboard neutron shield by a thinner shield optimized for the neutron spectrum in D-3He; this allows the plasma major radius and aspect ratio to be reduced and higher beta and Q-values (up to about 3) to be achieved. The implications of D-3He operation for neutron shielding, the heat loads on the first wall and the divertor as well as plasma refuelling are considered.