This is a comparable to work now underway on the polywell developed by Bussard and now been continued. The geometry is attractive and promising. I would like to see several efforts underway so that it can attract as many fresh minds as possible. It simply comes so close that you think we are one good idea away from success and that once mastered we will have the scalable fusion reactor of the future.
It is almost like the Tokomak except we should achieve proof of concept a lot sooner and easier.
This is a bit technical but worth the effort. The art work is also useful.
Inertial-Electrostatic-Confinement Fusion Device
J. Park, S.M. Stange (P-24), R.A. Nebel (T-15), K.M. Subramanian (University of Wisconsin
Excerpted from LA-14202-PR
Excerpted from LA-14202-PR
Introduction
Inertial-electrostatic-confinement (IEC) systems provide an economical and technologically straightforward means to produce fusion reactions in a table-top device.1,2 IEC devices confine a plasma in a potential well created by electrostatic fields or a combination of electrostatic and magnetic fields. The fields can be produced either by grids or by virtual cathodes, typically in spherical or cylindrical geometry. The fields accelerate ions towards the center of the device, where fusion reactions can occur (Figure 1). The technological simplicity of the IEC system was the basis for its early success—it produced a steady-state neutron yield of 2 × 1010 neutrons/s in the late 1960s.3
Applications
One of the most promising applications for an IEC-based neutron source is the active nuclear assay of highly enriched uranium and high explosives (HE), such as landmines. High-energy neutrons (e.g., 14.1 MeV neutrons from deuterium-tritium [DT] fusion reactions) have the ability to penetrate shielded materials very effectively. For example, Monte Carlo neutron and photon (MCNP) transport-code calculations indicate that 14.1 MeV neutrons can penetrate soil as deep as 1 m and detect HE, such as landmines. We are currently working with a private company to develop a compact and economical intense neutron source based on the IEC system. The enhanced-detection capability of the IEC-based neutron source, compared to natural radiation sources, could provide cutting-edge technology for homeland defense and humanitarian causes.
Periodically Oscillating Plasma Sphere
Though useful for practical neutron sources, the existing IEC fusion devices suffer low fusion yields, ~ 0.01% of input power. This is because the Coulomb-collision cross section is much greater than the fusion-collision cross section by several orders of magnitude. The ion beams in the IEC device rapidly lose the energy by Coulomb collisions before producing fusion reactions, leading to a net loss in energy.
A new electrostatic plasma equilibrium that should mitigate this problem has been proposed by LANL theorists4 and recently confirmed experimentally.5 This concept requires uniform electron injection into the central region of a spherical device to produce harmonic oscillator potential. An ion cloud (referred to as the Periodically Oscillating Plasma Sphere, or POPS) in such an environment will undergo harmonic oscillation with an oscillation frequency independent of amplitude. Tuning the external radio-frequency (rf) electric fields to this naturally occurring mode allows the ion motions to be phase-locked. This simultaneously produces very high densities and temperatures during the collapse phase of the oscillation when all the ions converge into the center. Solutions to POPS oscillation have the remarkable property that they maintain equilibrium distribution of the ions at all times. This would eliminate any power loss due to Coulomb collisions and would greatly increase the neutron yield up to more than 100%, resulting in a net energy gain for fusion-power generation.
In a practical embodiment, the POPS system would use a massively modular system to achieve high-mass-power density as shown in the conceptual drawing in Figure 2. Such a device would contain thousands of tiny spherical IEC reactors within a single reactor vessel to produce a large amount of fusion power (i.e., ~ 100–1000 MW). A modular IEC device would have very high-mass-power density, comparable to a light-water reactor, while maintaining conventional wall loads (~ 1 MW/m2) and being economically competitive with other sources of power.
First Experimental Confirmation POPS Oscillation
The POPS oscillation has been experimentally measured for the first time, confirming the scientific basis for a POPS-based fusion device. The harmonic potential well is created by electron injection.6 Ions in the potential well undergo harmonic oscillation. By applying rf fluctuation to the grid voltage, we were able to phase-lock the POPS oscillation and to measure the resonance behavior of the ions. Mathematically, ion dynamics during the driven POPS oscillation are equivalent to the driven harmonic oscillatior as described by the Mathieu equations. The ions can gain a large amount of energy from a small external perturbation when the driving frequency is equal to the resonance frequency. The ion orbits become unstable, and ion loss from the potential well is enhanced. In the experimental setting, the enhanced ion loss compensates the background ionization and extends the lifetime of the potential well. On the other hand, rf fluctuation outside the POPS resonance frequency makes little impact to the ion loss. This resonance behavior of ion dynamics is shown in Figure 3, where the temporal variation of the plasma response is measured for various rf frequencies. Without rf fluctuation, the lifetime of the potential well is very short, ~ 0.5 ms, due to significant background ionization. By applying small rf fluctuation (~ 4 V amplitude compared to a direct-current [dc] bias voltage of 250 V) at POPS frequency, the lifetime increases greatly to ~ 2.5 ms. In comparison, rf fluctuation outside the resonance frequency changes the lifetime only slightly.
The frequency at which the POPS oscillation is found scales as fPOPS = (√2/π) *(Vwell/r2wellMion)0.5. In using a harmonic-oscillator analogy, the ion mass provides the inertia, whereas the curvature of potential well is equal to the coefficient of the restoring force. Because this was the first time that the POPS oscillation has ever been experimentally observed, extensive efforts were made to verify the POPS frequency scaling as a function of the well depth and the ion mass. As shown in Figure 4, excellent agreement was obtained between the experiments and the theory, confirming that the observed resonance is the ion mode associated with the POPS oscillation. The potential well depth was controlled by varying the dc component of the inner-grid bias, whereas the well radius is fixed by the inner-grid dimension. Note that the well radius was estimated as rwell = rgrid + λDeff, where λDeff is the effective Debye length to account for the Debye shielding. We also varied the fill gas, using three different ion species, H2+, He+, and Ne+ to investigate the POPS frequency scaling.
Particle Simulation of POPS Plasma Compression
One of the most significant issues facing a fusion device based on POPS is the plasma compression, which determines the achievable fusion rates. In the case of deuterium-deuterium (DD) fuel, a radial plasma compression of 25 is sufficient for active nuclear assay, whereas the neutron tomography would require a compression of 100. In comparison, a practical fusion-power plant would require a compression of 2000 for DD fuel but less than 100 for DT fuel. One factor that greatly affects the compression ratio is the extent of space-charge neutralization. Inadequate space-charge neutralization can cause self-repulsion of the ion cloud during the collapse phase, limiting the compression.
A gridless particle code of one dimension in space and two dimensions in velocity space has been developed to investigate the space-charge neutralization during POPS compression.7 Figure 5 shows the radial profiles of ion density and plasma potential during POPS compression. The results in the left are from the expansion phase of POPS oscillation. The ion density profile is Gaussian in space, and the plasma potential profile matches the required harmonic oscillator potential for ions, produced by constant electron injection. In the middle, the ion density and the plasma potential during the collapsed phase of POPS oscillation are shown. A large distortion of plasma potential is due to the insufficient space-charge neutralization and ion self-repulsion during the POPS compression. This has limited the radial plasma compression to only 6.3. In comparison, the results on the right come from the case where we modulate the initial velocity distribution of injected electrons as a function of time to improve the space-charge neutralization. This simple remedy helped to improve the space-charge neutralization in the core during the collapse phase. A radial plasma compression of 19 has been obtained, resulting in the ion-density enhancement of ~ 10,000 in the core as compared to the expansion phase. Currently, we are investigating a method, proposed by Louis Chacon (Plasma Theory Group, T-15), to correctly modify the injected electron distribution to eliminate the space-charge neutralization problem and to improve the plasma compression.
Conclusion
The IEC Team in our Plasma Physics Group (P-24) and T-15 is working on developing practical fusion devices based on an IEC scheme. The recent experimental confirmation of the POPS oscillation and successful plasma compression in a particle simulation has provided solid scientific foundation for further exploration of this promising fusion device concept. This exploration will include direct experimental measurement of plasma compression and fully two-dimensional particle simulations of POPS dynamics. Successful plasma compression of at least 50 will be followed by a demonstration of nuclear fusion reactions using POPS.
References
1. W.C. Elmore, J.L. Tuck, and K.M. Watson, “On the inertial-electrostatic confinement of a plasma,” Physics of Fluids 2, 239 (1959).
2. P.T. Farnsworth, “Electric Discharge Device for Producing Interactions Between Nucleii,” U.S. Patent No. 3,358,402, issued June 28, 1966, initially filed May 5, 1956, reviewed Oct. 18, 1960, filed Jan. 11, 1962.
3. R.L. Hirsch, “Experimental studies of a deep, negative, electrostatic potential well in spherical geometry,”Physics of Fluids 11, 2486 (1968).
4. R.A. Nebel and D.C. Barnes, “The periodically oscillating plasma sphere,” Fusion Technology 38, 28 (1998).
5. J. Park et al., “First experimental confirmation of periodically oscillating plasma sphere (POPS) oscillation,” submitted to Physical Review Letters.
6. J. Park et al., “Experimental studies of electrostatic confinement on the INS-e device,” Physics of Plasmas 10, 3841–3849 (2003).
7. R.A. Nebel et al., “Theoretical and experimental studies of kinetic equilibrium and stability in the virtual cathode of the intense neutron source (INS-e) device,” submitted to Physics of Plasmas.
Acknowledgment
This work is supported by the DOE Office of Science/Fusion Energy Sciences Innovative Confinement Concepts Program. The authors gratefully acknowledge Carter Munson (P-24), Martin Taccetti (Hydrodynamics and X-ray Physics Group, P-22), Dan Barnes (Coronado Consulting), Martin Schauer (Neutron Science and Technology Group, P-23), and John Santarius (University of Wisconsin) for many invaluable discussions on this project; Dave Beddingfield (Safeguards Science and Technology, N-1) for conducting MCNP calculations; Tom Intrator (P-24) for providing us with thoriated tungsten wire; and N-1 and the Nuclear Nonproliferation Division management for providing the facility for this experiment.
For further information, contact Jaeyoung Park
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