Wide Range Of Plasma Densities Research Articles

Recovery of hydrogen plasma at the sub-nanosecond timescale in a plasma-wakefield accelerator

Plasma wakefield acceleration revolutionized the field of particle accelerators by generating gigavolt-per-centimeter fields. To compete with conventional radio-frequency (RF) accelerators, plasma technology must demonstrate operation at high repetition rates, with a recent research showing feasibility at megahertz levels using an Argon source that recovered after about 60 ns. Here we report about a proof-of-principle experiment that demonstrates the recovery of a Hydrogen plasma at the sub-nanosecond timescale. The result is obtained with a pump-and-probe setup and has been characterized for a wide range of plasma densities. We observed that large plasma densities reestablish their initial state soon after the injection of the pump beam ( < 0.7 ns). Conversely, at lower densities we observe the formation of a local dense plasma channel affecting the probe beam dynamics even at long delay times ( > 13 ns). The results are supported with numerical simulations and represent a step forward for the next-generation of compact high-repetition rate accelerators.

Direct Laser Acceleration in Underdense Plasmas with Multi-PW Lasers: A Path to High-Charge, GeV-Class Electron Bunches.

The direct laser acceleration (DLA) of electrons in underdense plasmas can provide hundreds of nC of electrons accelerated to near-GeV energies using currently available lasers. Here we demonstrate the key role of electron transverse displacement in the acceleration and use it to analytically predict the expected maximum electron energies. The energy scaling is shown to be in agreement with full-scale quasi-3D particle-in-cell simulations of a laser pulse propagating through a preformed guiding channel and can be directly used for optimizing DLA in near-future laser facilities. The strategy towards optimizing DLA through matched laser focusing is presented for a wide range of plasma densities paired with current and near-future laser technology. Electron energies in excess of 10GeV are accessible for lasers at I∼10^{21} W/cm^{2}.

On the Issue of Possibility of Reducing Anomalous Absorption of Extraordinary Pump Wave in a Wide Range of Plasma Densities

On the Issue of Possibility of Reducing Anomalous Absorption of Extraordinary Pump Wave in a Wide Range of Plasma Densities

Distinctive Features of Anomalous Plasma Radiation in Experiments on Electron Cyclotron Heating Using Extraordinary Wave at the Second Resonance Harmonic in a Wide Density Range

A detailed interpretation of the effect of anomalous radiation is given, which was first observed at the TEXTOR tokamak in experiments on electron cyclotron resonance heating by the extraordinary wave at the second resonance harmonic in a wide range of plasma densities.

Absorption of Microwaves During Plasma Heating at the Second Harmonic of Electron Cyclotron Resonance in Tokamaks and Stellarators: Linear Theory and Experiment

We study the microwave absorption during electron cyclotron resonance heating (ECRH) by the extraordinary wave at second harmonic (X2 mode) in the T-10 tokamak and TJ-II stellarator in a wide range of plasma densities, and compare experiments with the classical formulas for the absorption of the injected ECR power. Empirical relations for the absorption efficiency and for the critical plasma density {{n}_{{{text{cr}}}}}, which separates the regions with full and partial absorption of the injected ECR power, are obtained using the numerical simulation of the heat transport with the transport model of canonical profiles. It is shown that in both devices, the range of densities exists, where the absorption predicted by the classical formulas is almost full, while according with the empirical formula, only a small fraction of the power is absorbed. The obtained relations allow ones to optimize the conditions of ECRH in toroidal systems for magnetic plasma confinement.

Radial spatiotemporal evolution of rf magnetic field and plasma current in a very-high-frequency plasma source over a wide range of plasma density and pressure.

We present detailed spatiotemporal measurements of rf magnetic fields and plasma conduction current as well as the corresponding plasma density profiles when very-high-frequency 100-MHz waves are introduced to a parallel plate plasma. The magnetic probe, optical emission, and plasma absorption probe measurements support a model in which the interaction of high-frequency waves with the moving plasma sheath at specific phases of the very high frequencies (VHF) period leads to local reversals in the vertical rf electric field which can lead to conduction current reversals and circulations. The presence of current reversals and circulation correlate with measured plasma uniformity. In particular, measured electric field and current reversals coincide with the strongly center-peaked plasma density profiles frequently observed in VHF plasma. The magnitude of the center peak increases with the number and magnitude of the current reversal cells. The scale length of the current reversal cells is on the order of what is expected for plasma surface waves. Measurements indicate that plasma conductivity is the common scaling factor for both plasma uniformity and the circulation. The propagation of high-frequency waves and the generation of rf current reversal in one phase of the rf cycle rather than another results in the appearance of harmonics in Fourier decomposed magnetic probe wave forms. These harmonics turn out not to be actual waves or evidence of standing waves at these frequencies but rather consequences of packets of wave propagation that occur periodically.

First evidence of dominant influence of E × B drifts on plasma cooling in an advanced slot divertor for tokamak power exhaust

Addressing power exhaust in tokamaks is presently recognized as one of the major remaining open issues for the development of fusion reactors. At the forefront of this endeavor is the effort to develop an advanced divertor by maximizing dissipation of plasma power and momentum inside the divertor. Here, we demonstrate, for the first time, that the electromagnetic ( E × B ) drifts exert a key influence on plasma and gas dynamics in a new advanced slot divertor in the DIII-D tokamak, named the small angle slot (SAS). SAS leverages the effect of drifts to achieve a highly dissipative divertor with electron temperature T e ≲ 10 eV over a wide range of plasma densities, for ion B × ∇ B away from the divertor, as used for advanced tokamak operation on DIII-D. Modeling with the SOLPS-ITER code shows that for this drift direction, the E × B flow carries particles toward the outer common flux region (CFR) via the private flux region (PFR), reinforcing neutral recycling and enhancing divertor dissipation. In contrast, for the opposite field direction, the E × B flow carries particles away from the outer CFR into the PFR, offsetting the anticipated SAS geometric effects. This finding is an important step in the understanding of the behavior of advanced divertors for achieving a power exhaust solution for fusion reactors.

Transport model of plasma heating at the second harmonic of the electron cyclotron frequency

The optical thickness of the plasma is often insufficient to fully absorb the microwaves during heating at the second harmonic of the electron cyclotron frequency. An analysis of the experiments to the T-10 tokamak allows us to find the criteria for the full absorption, and to construct a canonical profile transport model for the full and partial absorptions of microwaves. The conditions to the equivalence of discharges in different tokamaks, and in a pair of tokamaks with the optimized W7-X stellarator are formulated. For equivalent discharges, calculations to the T-15MD tokamak under construction with the obtained model coincide with measurements of electron and ion temperatures in the W7-X over a wide range of plasma densities. The validated model is used to analyze future shots of T-15MD.

New low temperature multidipole plasma device with a magnetic X-point and its properties

A new low temperature multidipole plasma device with a magnetic X-point is developed. With a usual multidipole configuration generated by permanent neodymium magnets, a pair of axially flowing electrical currents up to 1.0 kA in the chamber creates figure-eight shaped poloidal magnetic fields with the X-point which separates plasmas into three distinct regions of core, edge and private regions. This new device, magnetic X-point simulator system (MAXIMUS), is equipped with end-plate wall filaments, core filaments and a LaB6 cathode as DC plasma sources. A wide range of plasma densities from 108 to 1012 cm−3 with electron temperatures of 0.4 to 3 eV is achieved. Plasmas in MAXIMUS are highly correlated with the shape of the magnetic fields as electrons are magnetized. Furthermore, electron velocity distribution functions can be significantly modified from usual Maxwellian distributions due to the strong grad-B and curvature drifts of electrons, resulting in high skewness and excess kurtosis. Such a capability of controlling the distribution function as well as having closed circular magnetic fields will allow us to systematically investigate effects of non-Maxwellian distribution functions and curved magnetic fields on various physical phenomena such as cross-field diffusion process, plasma waves and many nonlinear physics including solitons, shock waves and three-wave interactions. Tokamak edge physics correlated with neutral particles is also to be investigated with MAXIMUS.

Conceptual design of the heavy ion beam probe diagnostic for the T-15MD tokamak

The conceptual design of the heavy ion beam probe (HIBP) for T-15MD tokamak (R = 1.5 m, a = 0.67 m, Bt = 2 T, Ipl = 2 MA) is presented. The location of the HIBP injection and detection points is chosen based on the probing beam trajectory calculations. Observation area covers the whole radial interval 0 < r < a for Bt = 1 T, Ipl = 1 MA with the beam energy Eb <260 keV for Tl+ probing ions. It allows to measure two-dimensional maps of plasma parameters in an area which covers a large fraction in the upper right quarter of the plasma cross-section. In order to provide the measurements in a wide range of plasma density, high intensity long-focusing probing beam is needed. To develop such a beam a high voltage test bench is designed. It will allow to study the ion-optics system of the HIBP injector and the properties of the thermionic emitters and their lifetime.

A high field side multijunction launcher with aperture impedance matching for lower hybrid current drive in DIII-D advanced tokamak plasmas

A high field side (HFS) lower hybrid current drive launch scenario improves wave accessibility, has single pass damping at r/a ~ 0.6–0.8 and good current drive efficiency in DIII-D advanced tokamak discharges. The DIII-D experiment is an opportunity to validate HFS wave propagation, absorption and scrape-off layer benefits. A HFS multi-junction launcher is designed and simulated in COMSOL over a range of plasma edge conditions to evaluate n|| spectrum, directivity, and return loss. The COMSOL model utilizes a lossy anisotropic dielectric modeled with the cold plasma dispersion relation cross validated against ALOHA and Petra-M codes. The COMSOL model seamlessly includes a realistic plasma model and coupler that allows for rapid optimization of a single launcher module, while Petra-M allows more complex simulation of an eight-module array including curvature, and warm plasma effects. The resulting design utilizes a traveling wave poloidal power divider to minimize peak electric fields in the vacuum region of the coupler, and an internal aperture matching structure provides an impedance match to the plasma over a wide range of plasma density and density gradient edge conditions.

Millimeter wave control using a plasma filled photonic crystal resonator

The manipulation of millimeter-scale microwaves is studied using plasma discharges formed within a vacancy defect of a two-dimensional photonic crystal consisting of a hexagonal lattice of sapphire rods. When an externally driven discharge pulse is generated within the defect, the photonic crystal response is to transmit microwave pulses with variable properties, such as pulse widths and delays as small as 300 ns. Without external drive of the discharge, the device is also shown to behave as a nonlinear power limiter and attenuates the incoming power by 10–15 dB in less than one microsecond when the input power surpasses the threshold of microwave breakdown within the cavity, even at modest incident power of order 1 W. Lastly, by combining external pulse drive with high incident field power but below self-ignition limits, we show using a simple model analysis of the resonator response that the plasma formed spans a wide range of plasma densities, dependent upon the sustaining microwave frequency.

Response of narrow cylindrical plasmas to dense charged particle beams

By combining the linear theory and numerical simulations, we study the response of a radially bounded axisymmetric plasma to relativistic charged particle beams in a wide range of plasma densities. We present analytical expressions for the magnetic field generated in the dense plasma, demonstrate vanishing of the wakefield potential beyond the trajectory of the outermost plasma electron, and follow the wakefield potential change as the plasma density decreases. At high plasma densities, wavefronts of electron density and radial electric field are distorted due to beam charge and current neutralization, while wavefronts of wakefield potential and longitudinal electric field are not. At plasma densities lower than or of the order of the beam density, multiple electron flows develop in and outside the plasma, resulting in a nonzero wakefield potential around the plasma column.

Electron capture and excitation processes in collisions of H+ with H in finite-temperature quantum plasmas

Electron capture and excitation processes in H+-H collisions imbedded in warm dense quantum plasmas are studied by the two-center atomic orbital close-coupling (TC-AOCC) method in the energy range 1–200 keV. The screened potential which is used to describe the interaction between charged particles incorporates the quantum electron degeneracy, exchange-correlation and finite-temperature gradient correlation effects. The total and state-selective electron capture and excitation cross-sections for this collision system with this potential are calculated in a wide range of plasma densities and temperatures. It is demonstrated that the screening of Coulomb interactions affects the collision dynamics and the magnitude and energy behavior of the excitation and electron capture cross-sections. The results at very low and high temperatures are also compared with those obtained for zero-temperature quantum plasmas and Debye plasmas, respectively.

Efficient full wave code for the coupling of large multirow multijunction LH grills

The full wave code OLGA, for determining the coupling of a single row lower hybrid launcher (waveguide grills) to the plasma, is extended to handle multirow multijunction active passive structures (like the C3 and C4 launchers on TORE SUPRA) by implementing the scattering matrix formalism. The extended code is still computationally fast because of the use of (i) 2D splines of the plasma surface admittance in the accessibility region of the k-space, (ii) high order Gaussian quadrature rules for the integration of the coupling elements and (iii) utilizing the symmetries of the coupling elements in the multiperiodic structures. The extended OLGA code is benchmarked against the ALOHA-1D, ALOHA-2D and TOPLHA codes for the coupling of the C3 and C4 TORE SUPRA launchers for several plasma configurations derived from reflectometry and interferometery. Unlike nearly all codes (except the ALOHA-1D code), OLGA does not require large computational resources and can be used for everyday usage in planning experimental runs. In particular, it is shown that the OLGA code correctly handles the coupling of the C3 and C4 launchers over a very wide range of plasma densities in front of the grill.

Directional power absorption in helicon plasma sources excited by a half-helix antenna

This paper deals with the investigation of the power absorption in helicon plasma excited through a half-helix antenna driven at The simulations were carried out by means of a code, HELIC. They were carried out by taking into account different inhomogeneous radial density profiles and for a wide range of plasma densities, from to The magnetic field was and A three-parameter function was used for generating various density profiles with different volume gradients, edge gradients and density widths. The density profile had a large effect on the efficient Trivelpiece–Gould (TG) and helicon mode excitation and antenna coupling to the plasma. The fraction of power deposition via the TG mode was extremely dependent on the plasma density near the plasma boundary. Interestingly, the obtained efficient parallel helicon wavelength was close to the anticipated value for Gaussian radial density profile. Power deposition was considerably asymmetric when the ratio was more than a specific value for a determined density width. The longitudinal power absorption was symmetric at approximately irrespective of the magnetic field supposed. The asymmetry became more pronounced when the plasma density was The ratio of density width to the magnetic field was an important parameter in the power coupling. At high magnetic fields, the maximum of the power absorption was reached at higher plasma density widths. There was at least one combination of the plasma density, magnetic field and density width for which the RF power deposition at both side of the tube reached its maximum value.

Spectral properties of hydrogen-like ions in finite-temperature quantum plasmas

The oscillator strengths and radiative transition probabilities for the Lyman and Balmer series for transitions up to n = 5 of a hydrogen-like ion embedded in a finite-temperature quantum plasma are calculated in a wide range of plasma densities and temperatures for the first time. The plasma screened Coulomb interaction includes the quantum electron degeneracy, exchange-correlation, and finite-temperature gradient correction effects. The results for the oscillator strengths at temperatures 0.35 eV and 3.5 keV are compared with those for zero-temperature quantum plasmas and for Debye plasmas, respectively.

First-principles equation-of-state table of silicon and its effects on high-energy-density plasma simulations

Using density-functional theory-based molecular-dynamics simulations, we have investigated the equation of state for silicon in a wide range of plasma density and temperature conditions of ρ=0.001-500g/cm^{3} and T=2000-10^{8}K. With these calculations, we have established a first-principles equation-of-state (FPEOS) table of silicon for high-energy-density (HED) plasma simulations. When compared with the widely used SESAME-EOS model (Table3810), we find that the FPEOS-predicted Hugoniot is ∼20% softer; for off-Hugoniot plasma conditions, the pressure and internal energy in FPEOS are lower than those of SESAME EOS for temperatures above T ≈ 1-10 eV (depending on density), while the former becomes higher in the low-T regime. The pressure difference between FPEOS and SESAME 3810 can reach to ∼50%, especially in the warm-dense-matter regime. Implementing the FPEOS table of silicon into our hydrocodes, we have studied its effects on Si-target implosions. When compared with the one-dimensional radiation-hydrodynamics simulation using the SESAME 3810 EOS model, the FPEOS simulation showed that (1) the shock speed in silicon is ∼10% slower; (2) the peak density of an in-flight Si shell during implosion is ∼20% higher than the SESAME 3810 simulation; (3) the maximum density reached in the FPEOS simulation is ∼40% higher at the peak compression; and (4) the final areal density and neutron yield are, respectively, ∼30% and ∼70% higher predicted by FPEOS versus the traditional simulation using SESAME 3810. All of these features can be attributed to the larger compressibility of silicon predicted by FPEOS. These results indicate that an accurate EOS table, like the FPEOS presented here, could be essential for the precise design of targets for HED experiments.

Ionization competition effects on population distribution and radiative opacity of mixture plasmas

Ionization competition arising from the electronic shell structures of various atomic species in the mixture plasmas was investigated, taking SiO2 as an example. Using a detailed-level-accounting approximation, we studied the competition effects on the charge state population distribution and spectrally resolved and Planck and Rosseland mean radiative opacities of mixture plasmas. A set of coupled equations for ionization equilibria that include all components of the mixture plasmas are solved to determine the population distributions. For a given plasma density, competition effects are found at three distinct temperature ranges, corresponding to the ionization of M-, L-, and K-shell electrons of Si. Taking the effects into account, the spectrally resolved and Planck and Rosseland mean opacities are systematically investigated over a wide range of plasma densities and temperatures. For a given mass density, the Rosseland mean decreases monotonically with plasma temperature, whereas Planck mean does not. Although the overall trend is a decrease, the Planck mean increases over a finite intermediate temperature regime. A comparison with the available experimental and theoretical results is made.

BABE – a brush cathode discharge for thermal fluctuation measurements

For experimental tests of fluctuation theory in ideal plasmas and plasmas seeded with dust, the ideal environment would be that of stable quiescent plasma. In most laboratory plasmas the homogeneous state of the positive column is often unstable, rare exceptions are the so-called brush cathode discharges, proposed in the 60s, where a specially manufactured cathode allows stable operation in the abnormal glow regime and the only fluctuations present are those due the thermal motion of the particles. Such a device, the BAri Brush Electrode (BABE), has recently been built in a novel configuration that combines the advantages of the inverse design with those of the reflex geometry. The region between the two anodes is essentially field-free and extremely stable in wide range of plasma densities and collisionalities. Unprecedented low fluctuation levels of δn/n ⩽ 10−5 in He and δn/n ⩽ 5 × 10−6 in Ar discharges have been achieved.