P. Diamond | University of California, San Diego (original) (raw)
Papers by P. Diamond
ABSTRACT Supernova remnant (SNR), as the major contributors to the galactic cosmic rays (CR), are... more ABSTRACT Supernova remnant (SNR), as the major contributors to the galactic cosmic rays (CR), are believed to maintain an average CR spectrum by diffusive shock acceleration (DSA) regardless of the way they release CRs into the interstellar medium (ISM). However, the interactions of the CRs with nearby gas clouds crucially depend on the release mechanism. Recently, such interactions have been actively studied by observations in the gamma-ray band to probe the SNR as a particle accelerator. Motivated by these observations, we call into question two aspects of a popular paradigm of the CR injection into the ISM, according to which they passively and isotropically diffuse in the prescribed magnetic fluctuations as test particles. First, we treat the escaping CR and the Alfv'en waves excited by them on an equal footing. Second, we adopt field aligned CR escape outside the source, where the waves become weak. An exact analytic self-similar solution for a CR ``cloud'' released by a dimmed accelerator will be presented. The observational consequences of this solution will be discussed.
Analytic solution for an ion-acoustic collisionless shock with reflected ions is obtained. Its re... more Analytic solution for an ion-acoustic collisionless shock with reflected ions is obtained. Its relation to classical non-reflecting solitons propagating at Mach numbers strictly limited by M < M * ≈ 1.6 (Boltzmann electrons) and M * ≈ 3.1 (trapped electrons), is quantified. Above M = M * the soliton begins to reflect upstream ions and turns into a shock. The shock has a double-structure consisting of two receding transitions. The first transition is the ion-acoustic shock itself formed in place of the soliton. The shock reflected ions progressively fill up an extended foot region ending with the second transition that propagates faster than the rear shock but slower than the most of reflected ions. A small fraction of these ions still remains trapped in the transition to maintain charge neutrality. Most of them pass through this front transition, and accelerate whereas their distribution becomes noteworthily monoenergetic. The obtained solution may thus have interesting implications for the laser-based ion accelerators. Applications to particle acceleration in geophysical and astrophysical shocks are discussed.
Cosmic rays (CRs) are thought to be accelerated in SNRs. The most favorable situation for proving... more Cosmic rays (CRs) are thought to be accelerated in SNRs. The most favorable situation for proving that the main, hadronic CR component is accelerated there is when they interact with dense gases, surrounding the SN shock, such as molecular clouds (MC). Here, a new mechanism of spectrum formation in partially ionized gases nearby SNRs is proposed. Using an analytic model of nonlinear diffusive shock acceleration, we calculate the spectra of protons and the resulting gamma -ray emission occuring when the SNR shock approaches a MC. We show that the spectrum develops a break in the TeV range and is suppressed in its GeV component. These modifications to the standard theory occur because of the proximity of the partially ionized MC-gas and because of particle and Alfven wave propagation effects inside the gas. The controversy about the recent CANGAROO observations of the SNR RXJ 1713.7-3946 is discussed in the light of the new spectra.
Nuclear Fusion, 2013
Understanding the L→H and H→L transitions is crucial to successful Iter operation. In this paper ... more Understanding the L→H and H→L transitions is crucial to successful Iter operation. In this paper we present novel theoretical and modelling study results on the spatio-temporal dynamics of the transition. We place spatial emphasis on the role of zonal flows and the micro→macro connection between dynamics and the power threshold (P T) dependencies. The model studied evolves five coupled fields in time and one space dimension, in simplified geometry. The content of this paper is a.) the model fundamentals and the space-time evolution during the L→I→H transition, b.) the physics origin of the well known ∇B-drift asymmetry in P T , c.) the role of heat avalanches in the intrinsic variability of the L→H transition, d.) the dynamics of the H→L back transition and the physics of hysteresis, e.) conclusion and discussion, with a special emphasis on the implications of transition dynamics for the L→H power threshold scalings.
Nuclear Fusion, 2013
The intermediate oscillatory phase during the L-H transition, termed I-phase, has been studied in... more The intermediate oscillatory phase during the L-H transition, termed I-phase, has been studied in the EAST superconducting tokamak by employing a newly developed dual gas puff imaging (GPI) system near the L-H transition power threshold. The experimental observations suggest that the oscillatory behavior appearing at the L-H transition could be induced by the synergistic effect of the two components of the sheared m,n=0 E´B flow, i.e., the turbulence-driven zonal flow (ZF) and the equilibrium flow (EF). The latter arises from the neoclassical equilibrium, and is, to leading order, balanced by the ion diamagnetic term in the radial force balance equation. A slow increase in the poloidal flow and its shear at the plasma edge are observed tens of milliseconds prior to the I-phase. During the I-phase, the turbulence level decays and recovers periodically. The turbulence recovery appears to originate from the vicinity of the separatrix with clear wave fronts propagating both outward into the far scrape-off layer and inward into the core plasma. The Reynolds work done by the turbulence on the ZFs has been directly measured using the GPI system in the experiments, providing a direct evidence of kinetic energy transfer from turbulence to ZFs, thus driving the ZFs at the plasma edge. The ZFs are damped shortly after turbulence suppression, due to the 2 loss of turbulent drive, which then leads to the subsequent recovery of the turbulence level, initiating the next dithering cycle, or followed by a final transition into the H-mode, as the EF shear is strong enough to maintain turbulence suppression, even without the assistance of the ZF shear. A new self-consistent zero-dimensional model, incorporating the evolution of the EF and ZF shear, as well as the parallel transport in the scrap-off layer, has been developed and successfully reproduced the L-I-H transition process with many features comparing favorably with the experimental observations.
Nature Communications, 2011
Recent observations of supernova remnant W44 by the Fermi spacecraft observatory support the idea... more Recent observations of supernova remnant W44 by the Fermi spacecraft observatory support the idea that the bulk of galactic cosmic rays is accelerated in such remnants by a Fermi mechanism, also known as diffusive shock acceleration. However, the W44 expands into weakly ionized dense gas, and so a significant revision of the mechanism is required. Here, we provide the necessary modifications and demonstrate that strong ion-neutral collisions in the remnant surrounding lead to the steepening of the energy spectrum of accelerated particles by exactly one power. The spectral break is caused by Alfven wave evanescence leading to the fractional particle losses. The gamma-ray spectrum generated in collisions of the accelerated protons with the ambient gas is calculated and successfully fitted to the Fermi observatory data. The parent proton spectrum is best represented by a classical test particle power law ∝E − 2 , steepening to E − 3 at E br ≈7 GeV due to deteriorated particle confinement.
We describe the formation of a shear layer from collisional drift turbulence in a cylindrical pla... more We describe the formation of a shear layer from collisional drift turbulence in a cylindrical plasma device. Broadband fluctuations combined with a chain of coherent structures are observed, turbulent particle transport across the shear layer is inhibited, and energy appears to be transferred from linearly unstable intermediate wave numbers into both larger and smaller spatial scales that are linearly stable.
A radially sheared azimuthal plasma fluid flow is observed in a cylindrical magnetized helicon pl... more A radially sheared azimuthal plasma fluid flow is observed in a cylindrical magnetized helicon plasma device with no external sources of momentum input and is sustained against collisional dissipation by the turbulent Reynolds stress. Measurements show that the cross-phase between turbulent velocity components determine the detailed shape of the Reynolds stress profile and the resulting time averaged shear layer profile. Recent work also shows the a-periodic formation of radially outward going plasma transport events which are born near the shear layer and which are associated with the slow evolution of the background plasma fluctuations. The results show a clear demonstration of turbulent-driven shear flows via momentum transport, and suggest that such shear flows may become unstable and thereby generate outward going radial transport events.
land-Analytic solution for an ion-acoustic collisionless shock with reflected ions is obtained. I... more land-Analytic solution for an ion-acoustic collisionless shock with reflected ions is obtained. Its relation to the well known ion-acoustic soliton solution limited by the critical Mach number M 1.6 is clarified. At this Mach number the soliton's electrostatic hump must reflect a sizable fraction of upstream ions. Considering the reflection efficiency first as a free parameter, we construct a new family of shock-like solutions in which the critical Mach number is increased up to M=2 for the Boltzmann electrons and up to M=7 for electrons adiabatically trapped behind the shock. The ions reflected off the shock ramp fill up an expanding precursor, terminated by a double layer type transition at its leading edge. By resolving then the latter transition we obtain the reflection rate appropriate for the given Mach number. The suggested exact solution for the shock transition is limited to a subclass of transitions with no overshoot. We discuss the possible strategies to relax this limitation. Possible applications for the magnetized plasmas in geophysical and astrophysical settings are also considered. The potential of the high-M solitons to generate strong beams of reflected ions is discussed.
Physical Review Letters, 1999
Results from 3D global gyrokinetic particle simulations of ion temperature gradient driven microt... more Results from 3D global gyrokinetic particle simulations of ion temperature gradient driven microturbulence in a toroidal plasma show that the ion thermal transport level in the interior region exhibits significant dependence on the ion-ion collision frequency even in regimes where the instabilities are collisionless. This is identified as arising from the Coulomb collisional damping of turbulence-generated zonal flows.
Physical Review Letters, 1995
It is shown that electron transport due to a generic low-frequency electrostatic turbulence in to... more It is shown that electron transport due to a generic low-frequency electrostatic turbulence in tokamak geometry results in the relaxation to a peaked, self-sustained plasma density profile no(r), rather than to a diffusion-induced Oat distribution. The relaxed density profile depends on the magnetic geometry and the distribution of turbulence. The associated inward pinch velocity V, = DV lnno results from the competition of the turbulent diffusion of trapped electrons over the poloidal magnetic flux coordinate P and the collisional relaxation toward a Maxwellian distribution function.
ABSTRACT Supernova remnant (SNR), as the major contributors to the galactic cosmic rays (CR), are... more ABSTRACT Supernova remnant (SNR), as the major contributors to the galactic cosmic rays (CR), are believed to maintain an average CR spectrum by diffusive shock acceleration (DSA) regardless of the way they release CRs into the interstellar medium (ISM). However, the interactions of the CRs with nearby gas clouds crucially depend on the release mechanism. Recently, such interactions have been actively studied by observations in the gamma-ray band to probe the SNR as a particle accelerator. Motivated by these observations, we call into question two aspects of a popular paradigm of the CR injection into the ISM, according to which they passively and isotropically diffuse in the prescribed magnetic fluctuations as test particles. First, we treat the escaping CR and the Alfv'en waves excited by them on an equal footing. Second, we adopt field aligned CR escape outside the source, where the waves become weak. An exact analytic self-similar solution for a CR ``cloud'' released by a dimmed accelerator will be presented. The observational consequences of this solution will be discussed.
Analytic solution for an ion-acoustic collisionless shock with reflected ions is obtained. Its re... more Analytic solution for an ion-acoustic collisionless shock with reflected ions is obtained. Its relation to classical non-reflecting solitons propagating at Mach numbers strictly limited by M < M * ≈ 1.6 (Boltzmann electrons) and M * ≈ 3.1 (trapped electrons), is quantified. Above M = M * the soliton begins to reflect upstream ions and turns into a shock. The shock has a double-structure consisting of two receding transitions. The first transition is the ion-acoustic shock itself formed in place of the soliton. The shock reflected ions progressively fill up an extended foot region ending with the second transition that propagates faster than the rear shock but slower than the most of reflected ions. A small fraction of these ions still remains trapped in the transition to maintain charge neutrality. Most of them pass through this front transition, and accelerate whereas their distribution becomes noteworthily monoenergetic. The obtained solution may thus have interesting implications for the laser-based ion accelerators. Applications to particle acceleration in geophysical and astrophysical shocks are discussed.
Cosmic rays (CRs) are thought to be accelerated in SNRs. The most favorable situation for proving... more Cosmic rays (CRs) are thought to be accelerated in SNRs. The most favorable situation for proving that the main, hadronic CR component is accelerated there is when they interact with dense gases, surrounding the SN shock, such as molecular clouds (MC). Here, a new mechanism of spectrum formation in partially ionized gases nearby SNRs is proposed. Using an analytic model of nonlinear diffusive shock acceleration, we calculate the spectra of protons and the resulting gamma -ray emission occuring when the SNR shock approaches a MC. We show that the spectrum develops a break in the TeV range and is suppressed in its GeV component. These modifications to the standard theory occur because of the proximity of the partially ionized MC-gas and because of particle and Alfven wave propagation effects inside the gas. The controversy about the recent CANGAROO observations of the SNR RXJ 1713.7-3946 is discussed in the light of the new spectra.
Nuclear Fusion, 2013
Understanding the L→H and H→L transitions is crucial to successful Iter operation. In this paper ... more Understanding the L→H and H→L transitions is crucial to successful Iter operation. In this paper we present novel theoretical and modelling study results on the spatio-temporal dynamics of the transition. We place spatial emphasis on the role of zonal flows and the micro→macro connection between dynamics and the power threshold (P T) dependencies. The model studied evolves five coupled fields in time and one space dimension, in simplified geometry. The content of this paper is a.) the model fundamentals and the space-time evolution during the L→I→H transition, b.) the physics origin of the well known ∇B-drift asymmetry in P T , c.) the role of heat avalanches in the intrinsic variability of the L→H transition, d.) the dynamics of the H→L back transition and the physics of hysteresis, e.) conclusion and discussion, with a special emphasis on the implications of transition dynamics for the L→H power threshold scalings.
Nuclear Fusion, 2013
The intermediate oscillatory phase during the L-H transition, termed I-phase, has been studied in... more The intermediate oscillatory phase during the L-H transition, termed I-phase, has been studied in the EAST superconducting tokamak by employing a newly developed dual gas puff imaging (GPI) system near the L-H transition power threshold. The experimental observations suggest that the oscillatory behavior appearing at the L-H transition could be induced by the synergistic effect of the two components of the sheared m,n=0 E´B flow, i.e., the turbulence-driven zonal flow (ZF) and the equilibrium flow (EF). The latter arises from the neoclassical equilibrium, and is, to leading order, balanced by the ion diamagnetic term in the radial force balance equation. A slow increase in the poloidal flow and its shear at the plasma edge are observed tens of milliseconds prior to the I-phase. During the I-phase, the turbulence level decays and recovers periodically. The turbulence recovery appears to originate from the vicinity of the separatrix with clear wave fronts propagating both outward into the far scrape-off layer and inward into the core plasma. The Reynolds work done by the turbulence on the ZFs has been directly measured using the GPI system in the experiments, providing a direct evidence of kinetic energy transfer from turbulence to ZFs, thus driving the ZFs at the plasma edge. The ZFs are damped shortly after turbulence suppression, due to the 2 loss of turbulent drive, which then leads to the subsequent recovery of the turbulence level, initiating the next dithering cycle, or followed by a final transition into the H-mode, as the EF shear is strong enough to maintain turbulence suppression, even without the assistance of the ZF shear. A new self-consistent zero-dimensional model, incorporating the evolution of the EF and ZF shear, as well as the parallel transport in the scrap-off layer, has been developed and successfully reproduced the L-I-H transition process with many features comparing favorably with the experimental observations.
Nature Communications, 2011
Recent observations of supernova remnant W44 by the Fermi spacecraft observatory support the idea... more Recent observations of supernova remnant W44 by the Fermi spacecraft observatory support the idea that the bulk of galactic cosmic rays is accelerated in such remnants by a Fermi mechanism, also known as diffusive shock acceleration. However, the W44 expands into weakly ionized dense gas, and so a significant revision of the mechanism is required. Here, we provide the necessary modifications and demonstrate that strong ion-neutral collisions in the remnant surrounding lead to the steepening of the energy spectrum of accelerated particles by exactly one power. The spectral break is caused by Alfven wave evanescence leading to the fractional particle losses. The gamma-ray spectrum generated in collisions of the accelerated protons with the ambient gas is calculated and successfully fitted to the Fermi observatory data. The parent proton spectrum is best represented by a classical test particle power law ∝E − 2 , steepening to E − 3 at E br ≈7 GeV due to deteriorated particle confinement.
We describe the formation of a shear layer from collisional drift turbulence in a cylindrical pla... more We describe the formation of a shear layer from collisional drift turbulence in a cylindrical plasma device. Broadband fluctuations combined with a chain of coherent structures are observed, turbulent particle transport across the shear layer is inhibited, and energy appears to be transferred from linearly unstable intermediate wave numbers into both larger and smaller spatial scales that are linearly stable.
A radially sheared azimuthal plasma fluid flow is observed in a cylindrical magnetized helicon pl... more A radially sheared azimuthal plasma fluid flow is observed in a cylindrical magnetized helicon plasma device with no external sources of momentum input and is sustained against collisional dissipation by the turbulent Reynolds stress. Measurements show that the cross-phase between turbulent velocity components determine the detailed shape of the Reynolds stress profile and the resulting time averaged shear layer profile. Recent work also shows the a-periodic formation of radially outward going plasma transport events which are born near the shear layer and which are associated with the slow evolution of the background plasma fluctuations. The results show a clear demonstration of turbulent-driven shear flows via momentum transport, and suggest that such shear flows may become unstable and thereby generate outward going radial transport events.
land-Analytic solution for an ion-acoustic collisionless shock with reflected ions is obtained. I... more land-Analytic solution for an ion-acoustic collisionless shock with reflected ions is obtained. Its relation to the well known ion-acoustic soliton solution limited by the critical Mach number M 1.6 is clarified. At this Mach number the soliton's electrostatic hump must reflect a sizable fraction of upstream ions. Considering the reflection efficiency first as a free parameter, we construct a new family of shock-like solutions in which the critical Mach number is increased up to M=2 for the Boltzmann electrons and up to M=7 for electrons adiabatically trapped behind the shock. The ions reflected off the shock ramp fill up an expanding precursor, terminated by a double layer type transition at its leading edge. By resolving then the latter transition we obtain the reflection rate appropriate for the given Mach number. The suggested exact solution for the shock transition is limited to a subclass of transitions with no overshoot. We discuss the possible strategies to relax this limitation. Possible applications for the magnetized plasmas in geophysical and astrophysical settings are also considered. The potential of the high-M solitons to generate strong beams of reflected ions is discussed.
Physical Review Letters, 1999
Results from 3D global gyrokinetic particle simulations of ion temperature gradient driven microt... more Results from 3D global gyrokinetic particle simulations of ion temperature gradient driven microturbulence in a toroidal plasma show that the ion thermal transport level in the interior region exhibits significant dependence on the ion-ion collision frequency even in regimes where the instabilities are collisionless. This is identified as arising from the Coulomb collisional damping of turbulence-generated zonal flows.
Physical Review Letters, 1995
It is shown that electron transport due to a generic low-frequency electrostatic turbulence in to... more It is shown that electron transport due to a generic low-frequency electrostatic turbulence in tokamak geometry results in the relaxation to a peaked, self-sustained plasma density profile no(r), rather than to a diffusion-induced Oat distribution. The relaxed density profile depends on the magnetic geometry and the distribution of turbulence. The associated inward pinch velocity V, = DV lnno results from the competition of the turbulent diffusion of trapped electrons over the poloidal magnetic flux coordinate P and the collisional relaxation toward a Maxwellian distribution function.