arXiv subjects

Ian E. Ochs

Publications and source records attributed to Ian E. Ochs.

At least 19 recordsLinked to original sources

arXiv2026

Enhancement of Ponderomotive End Plugs with Low-Mass Dopants

During conventional confinement of a linear plasma using ponderomotive end plugs, a repulsive potential is created at the axial ends by employing a perpendicularly-polarized wave with a wave frequency greater than the ion gyrofrequency. This potential is then partially cancelled out by an ambipolar potential that arises to equilibrate electron and ion densities along the field line. However, recent work on centrifugally-confined plasmas has shown that the appropriate choice of a dopant in the end region can dramatically change the structure of the ambipolar potential. For the ponderomotive potential, this ambipolar shaping can be even more powerful, since a lightweight dopant can see a different sign of ponderomotive potential relative to the confined species. As a result, the dopant can reverse the sign of the ambipolar potential, causing it to dramatically enhance the end plug ponderomotive confinement.

physics.plasm-ph↗

arXiv2025

Approximating the Particle Distribution in Rotating and Tandem Mirror Traps

Steady state distribution functions can be used to calculate stability conditions for modes, radiation energy losses, and particle loss rates. Heuristic analytic approximations to these distributions can capture key behaviors of the true distributions such as the relative speeds of different transport processes while possessing computational advantages over their numerical counterparts. In this paper, we motivate and present a closed-form analytic model for a distribution of particles in a centrifugal or tandem mirror. We find that our model outperforms other known models in approximating numerical steady-state simulations outside of a narrow range of low confining potentials. We demonstrate the model's suitability in the high confining potential regime for applications such as loss cone stability thresholds, fusion yields, and available energy.

physics.plasm-ph↗

arXiv2025

Energy Spectrum of Lost Alpha Particles in Magnetic Mirror Confinement

In a magnetic mirror fusion reactor, capturing the energy of fusion-produced alpha particles is essential to sustaining the reaction. However, since alpha particles are born at energies much higher than the confining potential, a substantial fraction are lost due to pitch-angle scattering before they can transfer their energy to the plasma via drag. The energy of lost alpha particles can still be captured through direct conversion, but designing an effective mechanism requires a description of the energies and times at which they become deconfined. Here we present analytical solutions for the loss velocity, energy, and time distributions of alpha particles in a magnetic mirror. After obtaining the Fokker-Planck collision operator, we asymptotically solve for the eigenfunctions of the Legendre operator to reveal a closed-form solution. Our framework applies to any high-energy species, for any applied potential and mirror ratio R > 1, making this work broadly applicable to mirror devices.

physics.plasm-ph↗

arXiv2025

Preventing ash from poisoning proton-boron 11 fusion plasmas

Proton-Boron 11 (pB11) fusion is safe and clean, but also difficult to harness for breakeven power production. Particularly deleterious are fusion-born alpha particles, which massively increase both plasma pressure and bremsstrahlung losses unless they are pulled promptly from the plasma. We show that even if one cannot extract the alphas quickly, one can still achieve net power production, by separating the plasma into two regions: a fusion region, accessible to all species, and an alpha storage region, accessible only to alphas and electrons. This new demixing strategy could make pB11 fusion much easier to achieve.

physics.plasm-ph↗

arXiv2024

Synchrotron-driven instabilities in relativistic plasmas of arbitrary opacity

Recent work has shown that synchrotron emission from relativistic plasmas leads the electron distribution to form an anisotropic ring in momentum space, which can be unstable to both kinetic and hydrodynamic instabilities. Fundamental to these works was the assumption that the plasma was optically thin, allowing all emitted radiation to escape. Here, we examine the behavior of these instabilities as the plasma becomes more optically thick. To do this, we extend a recently-developed Fokker-Planck operator for synchrotron emission and absorption in mildly relativistic plasmas to fully relativistic plasmas. For a given set of plasma parameters, photons emitted by higher-energy electrons tend to be higher frequency, and thus more easily escape the plasma. As a result, the ratio of the photon emission rate (radiative drag) to absorption rate (radiative diffusion) for a given electron is extremely energy-dependent. Given this behavior, we determine the critical parameters that control the opacity, and show how the plasma gradually transitions to become more isotropic and stable at higher opacity.

physics.plasm-ph↗

arXiv2024

Electron Tail Suppression and Effective Collisionality due to Synchrotron Emission and Absorption in Mildly Relativistic Plasmas

Synchrotron radiation losses are a significant cause of concern for high-temperature aneutronic fusion reactions such as proton-Boron 11. The fact that radiation losses occur primarily in the high-energy tail, where the radiation itself has a substantial impact on the electron distribution, necessitates a self-consistent approach to modeling the diffusion and drag induced by synchrotron absorption and emission. Furthermore, an accurate model must account for the fact that the radiation emission spectrum is momentum-dependent, and the plasma opacity is frequency-dependent. Here, we present a simple Fokker-Planck operator, built on a newly-solved-for blackbody synchrotron diffusion operator, which captures all relevant features of the synchrotron radiation. Focusing on magnetic mirror fusion plasmas, we show that significant suppression of the electron distribution occurs for relativistic values of the perpendicular electron momentum, which therefore emit much less radiation than predicted under the assumption of a Maxwell-Juttner distribution.

physics.plasm-ph↗

arXiv2023

The Critical Role of Isopotential Surfaces for Magnetostatic Ponderomotive Forces

By producing localized wave regions at the ends of an open-field-line magnetic confinement system, ponderomotive walls can be used to differentially confine different species in the plasma. Furthermore, if the plasma is rotating, this wall can be magnetostatic in the lab frame, resulting in simpler engineering and better power flow. However, recent work on such magnetostatic walls have shown qualitatively different potentials than those found in the earlier, non-rotating theory. Here, using a simple slab model of a ponderomotive wall, we resolve this discrepancy. We show that the form of the ponderomotive potential in the co-moving plasma frame depends on the assumption made about the electrostatic potential in the lab frame. If the lab-frame potential is unperturbed by the magnetic oscillation, one finds a parallel-polarized wave in the co-moving frame, while if each field line remains equipotential throughout the perturbation region, one finds a perpendicularly-polarized wave. This in turn dramatically changes the averaged ponderomotive force experienced by a charged particle along the field line: not only its scaling, but also its direction.

physics.plasm-ph↗

arXiv2023

Lowering the reactor breakeven requirements for proton-Boron 11 fusion

Recently, it has been shown that altering the natural collisional power flow of the proton-Boron 11 (pB11) fusion reaction can significantly reduce the Lawson product of ion density and confinement time required to achieve ignition. However, these products are still onerous - on the order of $7 \times 10^{15}$ cm$^{-3}$s under the most optimistic scenarios. Fortunately, a breakeven fusion power plant does not require an igniting plasma, but rather a reactor that produces more electrical power than it consumes. Here, we extend the existing 0D power balance analysis to check the conditions on power plant breakeven. We find that even for the base thermonuclear reaction, modern high-efficiency thermal engines should reduce the Lawson product to $1.2 \times 10^{15}$ cm$^{-3}$s. We then explore the impact of several potential improvements, including fast proton heating, alpha power capture, direct conversion, and efficient heating. We find that such improvements could reduce the required Lawson product by a further order of magnitude, bringing aneutronic fusion to within target ITER design parameters.

physics.plasm-ph↗

arXiv2023

Observation of fast current redistribution in an imploding plasma column

Spectroscopic measurements of the magnetic field evolution in a Z-pinch throughout stagnation and with particularly high spatial resolution reveal a sudden current redistribution from the stagnating plasma (SP) to a low density plasma (LDP) at larger radii, while the SP continues to implode. Based on the plasma parameters it is shown that the current is transferred to an increasing-conductance LDP outside the stagnation, a process likely to be induced by the increasing impedance of the SP. Since an LDP often exists around imploding plasmas and in various pulsed-power systems, such a fast current redistribution may dramatically affect the behavior and achievable parameters in these systems.

physics.plasm-ph↗

arXiv2022

Improving the Feasibility of Economical Proton-Boron 11 Fusion via Alpha Channeling with a Hybrid Fast and Thermal Proton Scheme

The proton-Boron 11 (p-B11) fusion reaction is much harder to harness for commercial power than the easiest fusion reaction, namely the deuterium and tritium (DT) reaction. The p-B11 reaction requires much higher temperatures, and, even at those higher temperatures, the cross section is much smaller. However, as opposed to tritium, the reactants are both abundant and non-radioactive. It is also an aneutronic reaction, thus avoiding radioactivity-inducing neutrons. Economical fusion can only result, however, if the plasma is nearly ignited; in other words if the fusion power is at least nearly equal to the power lost due to radiation and thermal conduction. Because the required temperatures are so high, ignition is thought barely possible for p-B11, with fusion power exceeding the bremsstrahlung power by only around 3\%. We show that there is a high upside to changing the natural flow of power in the reactor, putting more power into protons, and less into the electrons. This redirection can be done using waves, which tap the alpha particle power and redirect it into protons through alpha channeling. Using a simple power balance model, we show that such channeling could reduce the required energy confinement time for ignition by a factor of 2.6 when energy is channeled into thermal protons, and a factor of 6.9 when channeled into fast protons near the peak of the reactivity. Thus, alpha channeling could dramatically improve the feasibility of economical p-B11 fusion energy.

physics.plasm-ph↗

arXiv2022

Fueling limits in a cylindrical viscosity-limited reactor

Recently, a method to achieve a "natural hot-ion mode" was suggested, by utilizing ion viscous heating in a rotating plasma with a fixed boundary. We explore the steady-state solution to the Braginskii equations and find the parameter regime in which a significant temperature difference between ions and electrons can be sustained in a driven steady state. The threshold for this effect occurs at $ρ_i\gtrsim0.1R$. An analytic, leading order low flow solution is obtained, and a numerical, moderate Mach number $M\lesssim2$ is investigated. The limitation is found to be at moderate Mach numbers.

physics.plasm-ph↗

arXiv2022

Momentum Conservation in Current Drive and Alpha-Channeling-Mediated Rotation Drive

Alpha channeling uses waves to extract hot ash from a fusion plasma, while transferring energy from the ash to the wave. Intriguingly, it has been proposed that the extraction of this charged ash could create a radial electric field, efficiently driving ExB rotation. However, existing theories ignore the response of the nonresonant particles, which play a critical role in enforcing momentum conservation in quasilinear theory. Because cross-field charge transport and momentum conservation are fundamentally linked, this non-consistency throws the whole effect into question. Here, we review recent developments that have largely resolved this question of rotation drive by alpha channeling. We build a simple, general, self-consistent quasilinear theory for electrostatic waves, applicable to classic examples such as the bump-on-tail instability. As an immediate consequence, we show how waves can drive currents in the absence of momentum injection even in a collisionless plasma. To apply this theory to the problem of ash extraction and rotation drive, we develop the first linear theory able to capture the alpha channeling process. The resulting momentum-conserving linear-quasilinear theory reveals a fundamental difference between the reaction of nonresonant particles to plane waves that grow in time, versus steady-state waves that have nonuniform spatial structure, allowing rotation drive in the latter case while precluding it in the former. This difference can be understood through two conservation laws, which demonstrate the local and global momentum conservation of the theory. Finally, we show how the oscillation-center theories often obscure the time-dependent nonresonant recoil, but ultimately lead to similar results.

physics.plasm-ph↗

arXiv2021

Finite-Difference Multiple Fluid Solution for Source-Driven Rotation in Highly Magnetized Linear Plasma Device

The rotation profile of a magnetized plasma cylinder composed of multiple fluids is investigated analytically, expanding on previous results. The analytic steady-state solution is used as a benchmark for a time-dependent multiple-fluid finite-difference code, MITNS: Multiple-Ion Transport Numerical Solver. Magnetic field evolution is taken into account, both analytically and numerically. Its details are shown to be of importance when particles are allowed out of the domain. MITNS reproduces the asymptotic expansion results for a small parameter $δ\lll1$. For $\sqrt{m_e/m_i} \sim δ\ll 1$, a slightly different regime, dominated by viscosity-induced transport of ions, is found numerically and analytically. This validation supports the use of this code for more complex time-dependent calculations in the future.

physics.plasm-ph↗

arXiv2020

Nonresonant Diffusion in Alpha Channeling

The gradient of fusion-born alpha particles that arises in a fusion reactor can be exploited to amplify waves, which cool the alpha particles while diffusively extracting them from the reactor. The corresponding extraction of the resonant alpha particle charge has been suggested as a mechanism to drive rotation. By deriving a coupled linear-quasilinear theory of alpha channeling, we show that, for a time-growing wave with a purely poloidal wavevector, a current in the nonresonant ions cancels the resonant alpha particle current, preventing the rotation drive but fueling the fusion reaction.

physics.plasm-ph↗

arXiv2020

Magnetogenesis by Wave-Driven Momentum Exchange

When multiple species interact with an electrostatic ion acoustic wave, they can exchange momentum, despite the lack of momentum in the field itself. The resulting force on the electrons can have a curl, and thus give rise to compensating electric fields with curl on magnetohydrodynamic timescales. As a result, a magnetic field can be generated. Surprisingly, in some astrophysical settings, this mechanism can seed magnetic fields with growth rates even larger than through the traditional Biermann battery.

physics.plasm-ph↗

arXiv2020

Momentum-Exchange Current Drive by Electrostatic Waves in an Unmagnetized Collisionless Plasma

For an electrostatic wave interacting with a single species in a collisionless plasma, momentum conservation implies current conservation. However, when multiple species interact with the wave, they can exchange momentum, leading to current drive. A simple, general formula for this driven current is derived. As examples, we show how currents can be driven for Langmuir waves in electron-positron-ion plasmas, and for ion-acoustic waves in electron-ion plasmas.

physics.plasm-ph↗

arXiv2019

Current channel evolution in ideal Z pinch for general velocity profiles

Recent diagnostic advances in gas-puff Z pinches at the Weizmann Institute for the first time allow the reconstruction of the current flow as a function of time and radius. These experiments show an unexpected radially-outward motion of the current channel, as the plasma moves radially-inward [C. Stollberg, Ph.D thesis, Weizmann Institute, 2019]. In this paper, a mechanism that could explain this current evolution is described. We examine the impact of advection on the distribution of current in a cylindrically symmetric plasma. In the case of metric compression, with |v_r| proportional to r, the current enclosed between each plasma fluid element and the axis is conserved, and so the current profile maintains its shape. We show that for more general velocity profiles, this simple behavior quickly breaks down, allowing for non-conservation of current in a compressing conductor, rapid redistribution of the current density, and even for the formation of reverse currents. In particular, a specific inward radial velocity profile is shown to result in radially-outward motion of the current channel, recovering the surprising current evolution discovered at the Weizmann Institute.

physics.plasm-ph↗

arXiv2018

Favorable Collisional Demixing of Ash and Fuel in Magnetized Inertial Fusion

Magnetized inertial fusion experiments are approaching regimes where the radial transport is dominated by collisions between magnetized ions, providing an opportunity to exploit effects usually associated with steady-state magnetic fusion. In particular, the low-density hotspot characteristic of magnetized liner inertial fusion results in diamagnetic and thermal frictions which can demix thermalized ash from fuel, accelerating the fusion reaction. For reactor regimes in which there is substantial burnup of the fuel, increases in the fusion energy yield on the order of 5% are possible.

physics.plasm-ph↗