arXiv subjects

Nathaniel J. Fisch

Publications and source records attributed to Nathaniel J. Fisch.

At least 55 records · Page 3Linked to original sources

arXiv2022

Wave propagation in rotating magnetised plasmas

Wave propagation properties in a medium are fundamentally affected when this medium is moving instead of at rest. In isotropic dielectric media rotation has two noteworthy contributions: one is a mechanically induced circular birefringence which materialises as a rotation of the polarisation, the other is image rotation which corresponds to a rotation of the transverse structure of a wave. Here we review the effect of rotation in a magnetised plasma. We first show that the mechanical effect of rotation on polarisation is in a magnetised plasma superimposed onto the classical Faraday rotation, and that failing to account for this new contribution could lead to errors in the interpretation of polarimetry data. We also demonstrate that image rotation is recovered in plasmas for a number of low-frequency magnetised plasma waves carrying orbital angular momentum, and that this phenomenon holds promise for the development of new rotation diagnostic tools in plasmas.

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

On the stabilisation of locked tearing modes in ITER and other large tokamaks

Tearing modes in tokamaks typically rotate while small and then lock at a fixed location when larger. Research on present-day devices has focused almost exclusively on stabilisation of rotating modes, as it has been considered imperative to avoid locked modes. However, in larger devices, stabilisation during the rotating phase is made difficult by fast locking at small island widths, and large broadening of the stabilising wave-driven current profile. In contrast, the smaller island width at locking not only mitigates the deleterious consequences of locked modes, but also permits their efficient stabilisation. On large devices, it thus becomes surprisingly advantageous to allow the mode to grow and lock naturally before stabilising it, challenging the mainstream strategy of neoclassical tearing mode stabilisation during the rotating phase. Calculations indicate that a locked island stabilisation strategy should be adopted in the ITER tokamak, with a large potential impact on the fusion gain and disruptivity.

physics.plasm-ph↗

arXiv2022

Particle Deceleration for Collective QED Signatures

Frequency upshifts have been proposed as a first experimental signature of collective effects in QED cascade generated electron-positron pair plasmas. Since the high effective masses of generated pairs will reduce any frequency change, stopped pairs at minimal Lorentz factor in the lab frame were thought to be the dominant contribution to the the laser upshift. However, we demonstrate that only considering stopped particles unduly neglects the contributions of particles re-accelerated in the laser propagation direction. Re-accelerated particles should, on a per particle basis, affect the laser more strongly, and over a much longer timescale. To maximize particle contributions to the laser upshift, we consider a Laguerre-Gaussian (LG) mode to better reflect generated pairs. The LG mode doesn't have an advantage in particle deceleration and re-acceleration when compared against a Gaussian beam, but the LG mode can maintain particle contributions for a longer duration, allowing for more pair density accumulation. Deceleration with a structured beam to keep pairs within the laser should create a larger upshift, thereby lowering the demands on the driving laser.

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

Generating optical supercontinuum and frequency comb in tenuous plasmas

There are several mechanisms by which the frequency spectrum of a laser broadens when it propagates at near-relativistic-intensity in tenuous plasma. Focusing on one dimensional effects, we identify two strong optical nonlinearities, namely, four-wave mixing (FWM) and forward Raman scattering (FRS), for creating octave-wide spectra. FWM dominates the interaction when the laser pulse is short and intense; Its combination with phase modulation produces a symmetrically broadened supercontinuum. FRS dominates when the laser pulse is long and relatively weak; It broadens the laser spectrum mainly towards lower frequencies and produces a frequency comb. The creation of the supercontinuum and frequency combs only frequency modulates, but does not compress, the laser pulse.

physics.plasm-ph↗

arXiv2021

Signature of Collective Plasma Effects in Beam-Driven QED Cascades

QED cascades play an important role in extreme astrophysical environments like magnetars. They can also be produced by passing a relativistic electron beam through an intense laser field. Signatures of collective pair plasma effects in these QED cascades are shown to appear, in exquisite detail, through plasma-induced frequency upshifts in the laser spectrum. Remarkably, these signatures can be detected even in small plasma volumes moving at relativistic speeds. Strong-field quantum and collective pair plasma effects can thus be explored with existing technology, provided that ultra-dense electron beams are colocated with multi-PW lasers.

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↗

arXiv2021

MP3 White Paper 2021 -- Research Opportunities Enabled by Co-locating Multi-Petawatt Lasers with Dense Ultra-Relativistic Electron Beams

Novel emergent phenomena are expected to occur under conditions exceeding the QED critical electric field, where the vacuum becomes unstable to electron-positron pair production. The required intensity to reach this regime, $\sim10^{29}\,\mathrm{Wcm^{-2}}$, cannot be achieved even with the most intense lasers now being planned/constructed without a sizeable Lorentz boost provided by interactions with ultrarelativistic particles. Seeded laser-laser collisions may access this strong-field QED regime at laser intensities as low as $\sim10^{24}\,\mathrm{Wcm^{-2}}$. Counterpropagating e-beam--laser interactions exceed the QED critical field at still lower intensities ($\sim10^{20}\,\mathrm{Wcm^{-2}}$ at $\sim10\,\mathrm{GeV}$). Novel emergent phenomena are predicted to occur in the "QED plasma regime", where strong-field quantum and collective plasma effects play off one another. Here the electron beam density becomes a decisive factor. Thus, the challenge is not just to exceed the QED critical field, but to do so with high quality, approaching solid-density electron beams. Even though laser wakefield accelerators (LWFA) represent a very promising research field, conventional accelerators still provide orders of magnitude higher charge densities at energies $\gtrsim10\,\mathrm{GeV}$. Co-location of extremely dense and highly energetic electron beams with a multi-petawatt laser system would therefore enable seminal research opportunities in high-field physics and laboratory astrophysics. This white paper elucidates the potential scientific impact of multi-beam capabilities that combine a multi-PW optical laser, high-energy/density electron beam, and high-intensity x rays and outlines how to achieve such capabilities by co-locating a 3-10 PW laser with a state-of-the-art linear accelerator.

physics.plasm-ph↗

arXiv2021

Modulation-Slippage Tradeoff in Resonant Four-Wave Upconversion

Following up on a proposal to use four-wave mixing in an underdense plasma at mildly relativistic laser intensities to produce vastly more energetic x-ray pulses [V. M. Malkin and N. J. Fisch, Phys. Rev. E, 101, 023211 (2020)], we perform the first numerical simulations in one dimension to demonstrate amplification of a short high frequency seed through four-wave mixing. We find that parasitic processes including phase modulation and spatial pulse slippage limit the amplification efficiency. We numerically explore the previously proposed "dual seed" configuration as a countermeasure against phase modulation. We show how this approach tends to be thwarted by longitudinal slippage. In the examples we considered, the best performance was in fact achieved through optimization of signal and pump parameters in a "single seed" configuration.

physics.plasm-ph↗

arXiv2020

Plasma physics in strong-field regimes: theories and simulations

In strong electromagnetic fields, unique plasma phenomena and applications emerge, whose description requires recently developed theories and simulations [Y. Shi, Ph.D. thesis, Princeton University (2018)]. In the classical regime, to quantify effects of strong magnetic fields on three-wave interactions, a convenient formula is derived by solving the fluid model to the second order in general geometry. As an application, magnetic resonances are exploited to mediate laser pulse compression, using which higher intensity pulses can be produced in wider frequency ranges, as confirmed by particle-in-cell simulations. In even stronger fields, relativistic-quantum effects become important, and a plasma model based on scalar quantum electrodynamics (QED) is developed, which unveils observable corrections to Faraday rotation and cyclotron absorption in strongly magnetized plasmas. Beyond the perturbative regime, lattice QED is extended as a numerical tool for plasma physics, using which the transition from wakefield acceleration to electron-positron pair production is captured when laser intensity exceeds the Schwinger threshold.

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

Perspectives on Physics of ExB Discharges Relevant to Plasma Propulsion and Similar Technologies

This paper provides perspectives on recent progress in the understanding of the physics of devices where the external magnetic field is applied perpendicularly to the discharge current. This configuration generates a strong electric field, which acts to accelerates ions. The many applications of this set up include generation of thrust for spacecraft propulsion and the separation of species in plasma mass separation devices. These ExB plasmas are subject to plasma-wall interaction effects as well as various micro and macro instabilities, and in many devices, we observe the emergence of anomalous transport. This perspective presents the current understanding of the physics of these phenomena, state-of-the-art computational results, identifies critical questions, and suggests directions for future research

physics.plasm-ph↗

arXiv2020

Calculating RF current condensation with self-consistent ray-tracing

By exploiting the nonlinear amplification of the power deposition of RF waves, current condensation promises new pathways to the stabilisation of magnetic islands. We present a numerical analysis of current condensation, coupling a geometrical optics treatment of wave propagation and damping to a thermal diffusion equation solver in the island. Taking into account the island geometry and relativistic damping, previous analytical theory can be made more precise and specific scenarios can be realistically predicted. With this more precise description, bifurcations and associated hysteresis effects could be obtained in an ITER-like scenario at realistic parameter values. Moreover, it is shown that dynamically varying the RF wave launching angles can lead to hysteresis and help to avoid the nonlinear shadowing effect.

physics.plasm-ph↗

arXiv2020

Optical phase conjugation in backward Raman amplification

Compression of an intense laser pulse using backward Raman amplification (BRA) in plasma, followed by vacuum focusing to a small spot size, can produce unprecedented ultrarelativistic laser intensities. The plasma density inhomogeneity during BRA, however, causes laser phase and amplitude distortions, limiting the pulse focusability. To solve the issue of distortion, we investigate the use of optical phase conjugation as the seed pulse for BRA. We show that the phase conjugated laser pulses can retain focusability in the nonlinear pump-depletion regime of BRA, but not so easily in the linear amplification regime. This somewhat counter-intuitive result is because the nonlinear pump-depletion regime features a shorter amplification distance, and hence less phase distortion due to wave-wave interaction, than the linear amplification regime.

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↗

arXiv2020

RF current condensation in the presence of turbulent enhanced transport

Sharp temperature gradients in a magnetically confined plasma can lead to turbulent motion of the plasma. This turbulence in turn enhances the transport of heat across magnetic field lines. The enhanced transport impacts the temperature differential that can be sustained in magnetic islands between the island centre and its periphery. It is shown here that, by limiting this temperature differential, this enhanced transport can have a profound influence on the extent to which the RF current condensation effect stabilises the island growth. Interestingly, because the heat transport is no longer simply linear in the temperature gradient, the RF current condensation effect also exhibits entirely new hysteresis phenomena.

physics.plasm-ph↗