arXiv subjects

Nathaniel J. Fisch

Publications and source records attributed to Nathaniel J. Fisch.

At least 19 recordsLinked to original sources

arXiv2026

Thresholdless IBW Emission and Alpha-to-Thermal-Ion Energy Channeling via Pole Resonance of Fusion-Product Ions

Fusion reactions can release a substantial fraction, and in aneutronic reactions nearly all, of their energy as the kinetic energy of fusion-product ions. In magnetized plasmas, these energetic ions can form ring distributions in velocity space. Such distributions can drive Dory-Guest-Harris (DGH) electrostatic instabilities, but these self-instabilities require a finite energetic-ion density and can be stabilized by a thermal background. We show that the same background can instead enable a distinct instability mechanism: a cyclotron pole of the minority energetic-ion susceptibility destabilizes a stable ion Bernstein wave (IBW) eigenmode. When the energetic-ion harmonic is distinct from the thermal-ion harmonics, exact root-pole resonance is thresholdless in the ideal collisionless limit. A species-resolved power balance shows that the energetic ions supply the free energy while the thermal plasma receives it. In the LAPD proton--alpha example, $99.96\%$ of the alpha-particle power loss enters the coherent proton response. This self-excited, ion-directed transfer provides a possible linear building block for alpha-particle energy channeling in proton-Boron11 fusion.

physics.plasm-ph↗

arXiv2026

Electrons quench ion tails during evaporative cooling in hot ion mode plasmas

It is shown how evaporative cooling in hot plasma is essentially different from evaporative cooling in other media, such as neutral gases or Bose-Einstein condensates. The fundamental difference in plasmas arises both from the large mass ratio between electrons and ions in fully ionized plasma and the unusually sensitive dependence of plasma collisionality on speed. Thus, a hot ion mode plasma ($T_e < T_i$) is shown to support a distinctive evaporative cooling regime, where a strong reduction in the ion evaporation rate appears. This new regime may have application to approaches to economical nuclear fusion, where the ion tail plays an outsized role in the fusion reaction rate.

physics.plasm-ph↗

arXiv2026

Fusion-power amplification by compressive hydrodynamic fluctuations

Compressive fluctuations in hot plasma, including acoustic waves and compressible turbulence, increase the rate of fusion reactions. This power amplification comprises hydrodynamic, ``two-temperature,'' and kinetic components, the first resulting from the clumping of hot ions in the peaks of the fluctuations, the second from the unequal heating of ions and electrons as fluctuations dissipate, and the third from the long mean free paths of fast ions near the Gamow peak, which allow these ions to stream across gradients in fluctuating hydrodynamic fields before colliding. In many cases, the increase in fusion power produced by waves exceeds that produced if the wave energy were instead used for heating. Response functions describing the modification to fusion power by compressive fluctuations are obtained in magnetized and unmagnetized fusion plasmas. Comparison to the related shear flow reactivity enhancement effect, a kinetic mechanism that increases fusion power in some divergence-free flows, illustrates a fundamental distinction between compressible and solenoidal turbulence in fusion plasmas.

physics.plasm-ph↗

arXiv2026

Single-Shot Intensity-Correlation Diffractive X-ray Imaging of ICF Plasmas

X-ray radiography of inertial confinement fusion plasmas is currently limited to several-micron resolution by geometric blur, diffraction, and photon-throughput tradeoffs. We propose single-shot intensity-correlation diffractive imaging (IDI) as a lensless route to submicron plasma turbulence measurements under low-self-emission conditions. Rather than relying on physical apertures, IDI reconstructs plasma morphology by Fourier transforming the spatial correlations of chaotic far-field speckles via the Hanbury Brown-Twiss effect. The Fourier phase is retrieved by applying bispectral closure-phase constraints derived from third-order intensity correlations. We demonstrate this submicron capability in a numerical simulation using a $50~\mathrm{keV}$ x-ray probe scattered by a spiral plasma structure.

physics.plasm-ph↗

arXiv2026

Asymptotic behavior of the shear flow reactivity enhancement effect

Fusion reactivity is enhanced in the vicinity of strongly sheared flow due to the tendency of fast ions near the Gamow peak to travel long distances between collisions, thereby sometimes crossing gradients in the background flow and attaining a velocity boost relative to the thermal background. This ``shear flow reactivity enhancement effect'' (SFRE) allows turbulent kinetic energy on fine spatial scales to contribute to fusion reactivity before thermalizing, which, remarkably, enables ignition of some inertial confinement fusion (ICF) hot spots under conditions where fully thermalized plasma would fail to ignite. The size of the SFRE is a consequence of the dramatic scale separations distinguishing thermal ions, which govern fluid quantities, and fast ions, which govern fusion reactivity. It is demonstrated in this work that, as the Gamow energy increases relative to the thermal energy, the SFRE in unmagnetized plasma becomes asymptotically large compared to hydrodynamic effects such as viscous dissipation. An asymptotic formula is derived in this limit, quantifying the SFRE for reactants of disparate masses and charge states.

physics.plasm-ph↗

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↗

arXiv2026

Detecting Solenoidal Plasma Turbulence via Laser Polarization Rotation

Recent theoretical studies suggest that solenoidal turbulence can significantly enhance fusion reactivity, yet no standard diagnostic exists to directly measure these solenoidal flows in high-energy-density plasmas, nor to distinguish between solenoidal and compressional turbulence. We propose a method that directly diagnoses the energy and spatial structure of this rotational turbulence using the cross-polarization scattering of a probe laser. By coupling to the plasma vorticity, the scattering generates a cross-polarized signal proportional to the turbulent vorticity, effectively acting as a calorimeter for shear flows. We identify a diffractive scattering signature analogous to ``Debye-Scherrer ring'' that reveals the eddy size distribution. We show that this technique is applicable to National Ignition Facility (NIF) implosion conditions and other high-energy-density scenarios.

physics.plasm-ph↗

arXiv2026

Long-Pulse Fast Ignition in MagLIF

The fast ignition paradigm for inertial confinement fusion (ICF) allows for extremely high gains but requires fuel to be heated very quickly to outpace hotspot disassembly and energy losses. This demands lasers with high power and intensity, posing engineering challenges that have called into question the fundamental practicality of fast ignition. Magnetized liner inertial fusion (MagLIF) circumvents these problems through its large-aspect-ratio cylindrical geometry and strong axial magnetic fields that allow for ignition at lower areal densities. Furthermore, MagLIF's large aspect ratio and higher yields relax other constraints on energy deposition and repetition rate while its axial magnetic fields can be used to collimate ignitor electrons and thereby increase allowed standoff distance and save on ignitor energy. This tremendous overall relaxation of the engineering constraints that have historically limited the practicality of fast ignition suggests that the paradigm may be considerably more viable in a MagLIF context.

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

An ignition criterion for inertial fusion boosted by microturbulence

Turbulence enhances fusion reactivity, enabling ignition at lower temperature. A modified Lawson-like ignition criterion is derived for inertially confined plasmas harboring turbulent kinetic energy. Remarkably, if small-scale turbulence is driven in the hot spot while avoiding mixing at the boundary, less energy is required to ignite a target. The optimal length scale for hot-spot turbulence is quantified, typically lying in the micron range.

physics.plasm-ph↗

arXiv2025

Ultra-strong Quantum Squeezing Mediated by Plasma Waves

Quantum squeezed states enable precision measurements beyond the standard quantum limit, but conventional solid-state media fundamentally limit pump intensities to the ionization threshold. We demonstrate that plasma waves can mediate ultra-strong two-mode squeezing through stimulated Raman scattering, achieving up to ultrastrong squeezing using $10^{16}{Wcm^{-2}}$ pump lasers. Employing two copropagating pump beams with frequency difference matching twice the plasma frequency, we generate quantum-correlated photon pairs through phonon-mediated four-wave mixing. The process exhibits remarkable thermal noise tolerance, allowing strong squeezing even with large thermal phonon numbers. This plasma-based approach produces squeezed states with ultrahigh photon numbers, opening new possibilities for strong-field applications across optical to X-ray wavelengths.

physics.plasm-ph↗

arXiv2025

Analytical models for the enhancement of fusion reactivity by turbulence

The reactivity of fusion plasma depends not only on its local density and temperature but also, through a recently identified kinetic effect, on the relative velocities of nearby fluid elements. Turbulence on fine spatial scales therefore enhances fusion reactivity. The enhancement is quantified here for general subsonic turbulent flows. Leveraging this effect in the design of inertial confinement fusion (ICF) experiments could enable substantial energy savings.

physics.plasm-ph↗

arXiv2025

Enhancement to Fusion Reactivity in Sheared Flows

Sheared flow increases the reactivity of fusion plasma. In unmagnetized plasma with flow gradients comparable to the mean free path of reacting ions, fusion reactivity can be more than doubled. The effect is of particular relevance to inertial confinement fusion (ICF), where it allows implosion kinetic energy to contribute to the fusion burn even before thermalizing. In fast ignition, this allows remarkable energy savings because colder fuel stops alpha particles more quickly, enabling ignition in a smaller hot spot.

physics.plasm-ph↗

arXiv2025

Image rotation in plasmas

Because of the speed of light compared to material motion, the dragging of light is difficult to observe under laboratory conditions. Here we report on the first observation of image rotation, i. e. a dragging by the medium of the wave's transverse structure, of Alfvén waves in plasmas. Exploiting the naturally slow group velocity of these waves, significant wave rotation is achieved for modest angular frequency. Control over the rotation of the wave's structure is demonstrated through the plasma rotation imposed by biased electrodes. Remarkably, experimental results are well reproduced by light dragging theory derived for isotropic media, even if magnetized plasmas are anisotropic. In addition to offering new insights into the fundamental issue of angular momentum coupling between waves and media, these findings also open possibilities for new remote rotation sensing tools.

physics.plasm-ph↗

arXiv2025

Spatiotemporal plasma hologram

We present the first experimental realization of a four-dimensional (4D) plasma hologram capable of recording and reconstructing the full spatiotemporal information of intense laser pulses. The holographic encoding is achieved through the interference of a long object pulse and a counter-propagating short reference pulse, generating an ionized plasma grating that captures both spatial and temporal characteristics of the laser field. A first-order diffractive probe enables the retrieval of encoded information, successfully reconstructing the spatiotemporal profiles of Gaussian and Laguerre-Gaussian beams. The experiment demonstrates the ability to encode artificial information into the laser pulse via spectral modulation and retrieve it through plasma grating diffraction, high-lighting potential applications in ultraintense optical data processing. Key innovations include a single-shot, background-free method for direct far-field spatiotemporal measurement and the obser-vation of laser focus propagation dynamics in plasma. The plasma grating exhibits a stable lifetime of 30-40 ps and supports high repetition rates, suggesting usage for high-speed optical switches and plasmatic analog memory. These advancements establish plasma holography as a robust platform for ultrafast laser manipulation, with implications for secure optical communication, analog computing,and precision spatiotemporal control of high-intensity lasers.

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

Amplification of turbulence through multiple planar shocks

We study the amplification of isotropic, incompressible turbulence through multiple planar, collisional shocks, using analytical linear theory. There are two limiting cases we explore. The first assumes shocks occur rapidly in time such that the turbulence does not evolve between shocks. Whereas the second case allows enough time for turbulence to isotropize between each shock. For the latter case, through a quasi-equation-of-state, we show that the weak multi-shock limit is agnostic to the distinction between thermal and vortical turbulent pressures, like an isotropic volumetric compression. When turbulence does not return to isotropy between shocks, the generated anisotropy -- itself a function of shock strength -- can feedback on amplification by further shocks, altering choices for maximal or minimal amplification. In addition for this case, we find that amplification is sensitive to the shock ordering. We map how choices of shock strength can impact these amplification differences due to ordering, finding, for example, shock pairs which lead to identical mean post-shock fields (density, temperature, pressure) but maximally distinct turbulent amplification.

astro-ph.GA↗

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↗