Gridded Inertial Electrostatic Confinement Research Articles

Measurements and modeling of ion divergence from a gridded inertial electrostatic confinement device using laser induced fluorescence

Inertial electrostatic confinement (IEC) is a method of confining and heating a plasma at benchtop scales to sufficient energies for nuclear fusion to occur. Ion velocity and flow direction were measured in an IEC discharge using laser induced fluorescence (LIF) on argon ions. A cathode of two parallel rings, with a common axis of symmetry, resulted in predominant discharge beams, otherwise known as microchannels, along this axis. The device was operated in the abnormal glow discharge regime where both current and voltage increase monotonically, replicating a conventional high voltage IEC device. It was found that argon ions accelerated and flowed outward from the midpoint between the rings along the axis; we have labeled this ion motion as being divergent. The predominant direction of ion flow in the discharge is opposite to the conventional ion focus model, where the discharge at the cathode center is assumed to be the result of ion flow toward it from outside of the cathode. An ion sheath model is shown to produce a virtual anode at the axial midpoint between the rings. The model also shows that ions within the virtual anode are accelerated outward with a spatial velocity profile that replicates those measured using LIF.

Fusion energy in an inertial electrostatic confinement device using a magnetically shielded grid

Theory for a gridded inertial electrostatic confinement (IEC) fusion system is presented, which shows a net energy gain is possible if the grid is magnetically shielded from ion impact. A simplified grid geometry is studied, consisting of two negatively biased coaxial current-carrying rings, oriented such that their opposing magnetic fields produce a spindle cusp. Our analysis indicates that better than break-even performance is possible even in a deuterium-deuterium system at bench-top scales. The proposed device has the unusual property that it can avoid both the cusp losses of traditional magnetic fusion systems and the grid losses of traditional IEC configurations.

Progress in the Understanding of Gridded Inertial Electrostatic Confinement Devices at the University of Wisconsin

For nearly two decades, as many as 4 Inertial Electrostatic Confinement (IEC) devices have been operated simultaneously at the University of Wisconsin-Madison. Over that time period we have learned that the early perceptions of how IEC devices operate are quite different from the actual performance in the Laboratory. Over the past 2 years we have gained even more understanding of IEC physics and technology. Experimental measurements and theoretical improvements have better characterized both the negative ions that contribute up to ~10% of the fusion rate in some cases and the neutral energy distributions in IEC devices at moderate pressure (0.07-0.7 Pa ≈ 0.5-5 mTorr). We also now understand more of why operation with helium plasmas has such a detrimental effect on high voltage performance of the traditional tungsten alloy grid wires. Most of the previous IEC work had been confined to < 100 kV with short operation times up to 150 kV. We have recently expanded our operating regime to ≈ 200 kV anode-cathode potential difference, which is, to our knowledge, the highest-voltage IEC operation reported in the worldwide IEC literature. Several design modifications were required to achieve steady state operation at these high voltages and some are described in this article.

Nonlinear saturation of the ion-electron Buneman instability in a spherical positively pulsed gridded inertial electrostatic confinement device

A pulsed, positively biased gridded inertial electrostatic confinement device has been investigated experimentally, using Doppler broadened spectra and current and voltage traces as primary diagnostics. In the high current and energy regime explored in this paper resulting from the removal of the series ballast resistance from the external biasing circuit, large amplitude oscillations in the plasma current and potential were observed within 100 ns of the discharge onset. These oscillations are attributed to the nonlinear and saturated Buneman instability, characterised by a locked oscillation frequency as a function of increasing anode potential. The saturated Buneman instability is known to exhibit ion mass independent behaviour and cause electron trapping, resulting in a transient spatio-temporal virtual cathode and ponderomotive ion confinement, as evidenced by broadened spectra when operated at high currents.

Planar geometry inertial electrostatic confinement fusion device

In the classic gridded inertial electrostatic confinement (IEC) fusion reactor, ion bombardment of the grid leads to heating, thermionic electron emission, significant power loss, and ultimately melting of the grid. Gridless IEC devices have sought to overcome these limitations. Klein reported a gridless device in which ions are circulated as a linear beam in an electrostatic analogue of an optical resonator. To overcome limits of stored ions due to space charge effects at the turning regions, the device employed multiple overlapping traps. The work reported here seeks to further increase the turning region space in a gridless trap by employing a planar geometry. Ion trapping in the planar device was examined by simulating trajectories of 2H+ ions with SIMION 8.1 software. Simulations were carried out using multiple potentials as in Klein's device and for a single potential trap as a planar analogue of the anharmonic ion trap. Scattering by background gas was simulated using a hard sphere collision model, and the results suggested the device will require operation at low pressure with a separate ion source.

New Insight into Gridded Inertial Electrostatic Confinement (IEC) Fusion Devices

Gridded inertial electrostatic confinement (IEC) devices use a 10-200 kV voltage difference to accelerate ions through a 0.1-10 mTorr background gas in a spherical or cylindrical geometry. The detailed investigation of a gridded IEC device using DD fuel has resulted in several surprises that have greatly altered our perception of how these systems operate. It was found that there are at least 4 major misconceptions that have been in place for over 15 years on how such IEC systems operate. These misconceptions range all the way from what energetic ion is causing the majority of fusions, to the energy and charge state of the reacting ions. Experimental results will illustrate some of the surprising reactions that are taking place in DD gridded system.

Effects of the Cathode Grid Wires on Fusion Proton Measurements in Inertial-Electrostatic Confinement Devices

Gridded inertial-electrostatic confinement (IEC) devices interest fusion researchers owing to their ability to burn advanced fusion fuels and have many near-term applications. In these devices, a high voltage (10-180 kV) accelerates ions radially between nearly transparent electrodes in spherical or cylindrical geometry. In this paper, we report experiments that study fusion reactions within the microchannels formed between the wires of the nearly transparent IEC cathode grid. Fusion proton counts were measured while sweeping the microchannels across a proton detector by rotating the central cathode grid with respect to the detector. The observed proton counts increased or decreased in correspondence with the grid wire orientation with respect to the proton detector. The fusion reactions were thus inferred to be nonuniform around the central grid and primarily occurring in channels formed by the grid wire gaps. We interpret this effect as an indication that most ions and charge-exchanged neutrals traverse radially along these microchannels. The grid wires will also shadow fusion reactions taking place at radii smaller than the cathode radius. Both the microchannels and the grid-wire shadowing causes the proton counts to vary, in measurements presented herein, by as much as 45%. We explore whether these effects could have played a role in previous research that reported potential-well structures.

Consolidated electron emission effects in an IEC device

Gridded inertial electrostatic confinement (IEC) devices are of interest to the research community for their multiple near-term applications. The number of applications of an IEC device increases with increasing fusion reaction rate. However, all attempts to improve the fusion reactivity of the IEC device have resulted in a linear or less than linear response with the power supply current. This work is geared toward determining the reasons for the observed response of the IEC device. Such an understanding would help formulate new ways to improve the efficiency of the device.Experiments were conducted with single loop grids built from different materials (Re and W25%Re) to study the electron emission from the cathode in an IEC device. A single loop grid produces a (∼line) cylindrical fusion source and was used to study the electron emission from cathode. Electron emission from the cathode increases non-linearly due to the presence of multiple sources (secondary electron emission, field emission and photoemission), as a result of which the ion current increases in a less than linear fashion with the power supply current. The ion recirculation current equation has been updated to accommodate various electron contributions. Several techniques to mitigate the electron emission from the cathode are suggested in this paper.

Study of Fuel Ratios on the Fusion Reactivity in an Inertial Electrostatic Confinement Device Using a Residual Gas Analyzer

Gridded Inertial Electrostatic confinement (IEC) devices are of interest due to their flexibility in burning advanced fuels, their tuning ability of the applied voltage to the reaction cross-section. Although this device is not suitable for power production in its present form, it does have several near term applications. The number of applications of this device increases with increasing fusion reactivity. These devices are simple to operate but are inherently complicated to understand and an effort to incrementally understand the device to improve its operational efficiency is underway at University of Wisconsin, Madison. Of all the parameters under study we are focusing on the effects of flow rate and flow ratio on the fusion reactivity in the present paper. Experiments were conducted to understand the influence of fuel flow ratio on the fusion reactions. The residual gas analyzer (RGA) was used to study the impurity concentration as the flow ratio was changed. It was observed that the higher flow rate resulted in reduced impurity levels and hence an increase in fusion rate. Several different species of gases were detected, some of these molecules formed inside the RGA analyzer. The flow ratio scan revealed that the optimum mixture of D2 with 3He to be D2:3He::1:2 for maximum D–3He fusion rate.

Experimental Observation of a Periodically Oscillating Plasma Sphere in a Gridded Inertial Electrostatic Confinement Device

The periodically oscillating plasma sphere (POPS) [D. C. Barnes and R. A. Nebel, Phys. Plasmas 5, 2498 (1998).] oscillation has been observed in a gridded inertial electrostatic confinement device. In these experiments, ions in the virtual cathode exhibit resonant behavior when driven at the POPS frequency. Excellent agreement between the observed POPS resonance frequency and theoretical predictions has been observed for a wide range of potential well depths and for three different ion species. The results provide the first experimental validation of the POPS concept proposed by Barnes and Nebel [R. A. Nebel and D. C. Barnes, Fusion Technol. 34, 28 (1998).].

Periodically oscillating plasma sphere

The periodically oscillating plasma sphere, or POPS, is a novel fusion concept first proposed by D. C. Barnes and R. A. Nebel [Fusion Technol. 38, 28 (1998)]. POPS utilizes the self-similar collapse of an oscillating ion cloud in a spherical harmonic oscillator potential well formed by electron injection. Once the ions have been phase-locked, their coherent motion simultaneously produces very high densities and temperatures during the collapse phase of the oscillation. A requirement for POPS is that the electron injection produces a stable harmonic oscillator potential. This has been demonstrated in a gridded inertial electrostatic confinement device and verified by particle simulation. Also, the POPS oscillation has been confirmed experimentally through observation that the ions in the potential well exhibit resonance behavior when driven at the POPS frequency. Excellent agreement between the observed POPS frequencies and the theoretical predictions has been observed for a wide range of potential well depths and three different ion species. Practical applications of POPS require large plasma compressions. These large compressions have been observed in particle simulations, although space charge neutralization remains a major issue.

Neutron Production and Ionization Efficiency in A Gridded IEC Device at High Currents

Experiments at the University of Illinois at Urbana-Champaign (UIUC) are exploring high current operation in a gridded, Inertial Electrostatic Confinement (IEC) device. Until recently all IEC operation has been done at relatively low currents. Calculations indicate that much higher voltages and higher currents are needed to form deep potential wells as required ultimately for reactor applications. Recent experiments have achieved 8×108n/s at peak of 100 microsecond pulses at a cathode-grid potential of 50 kV and 17 amps of current (vs. kA currents projected for a power reactor).

The Periodically Oscillating Plasma Sphere

A new method of operating an inertial electrostatic confinement (IEC) device is proposed, and its performance is evaluated. The scheme involves an oscillating thermal cloud of ions immersed in a bath of electrons that form a harmonic oscillator potential. The scheme is called the periodically oscillating plasma sphere, and it appears to solve many of the problems that may limit other IEC systems to low gain. A set of self-similar solutions to the ion fluid equations is presented, and plasma performance is evaluated. Results indicate that performance enhancement of gridded IEC systems such as the Los Alamos intense neutron source device is possible as well as high-performance operation for low-loss systems such as the Penning trap experiment. Finally, a conceptual idea for a massively modular Penning trap reactor is also presented.