Collisional Power Research Articles

First Thomson scattering results from AWAKE’s helicon plasma source

Abstract We present the first results of electron density and temperature measurements obtained from Thomson scattering at the helicon plasma source (HPS) for the AWAKE project. These measurements are compared to simulation results from a 1D power and particle balance model (PPM), confirming that the plasma can be fully sustained by collisional power dissipation. The variations in plasma parameters under different experimental conditions are evaluated in the PPM framework. We discuss current limitations of the model and propose possible improvements. Additionally, we suggest modifications to the existing HPS setup to enhance axial plasma homogeneity.

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 7×1015 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 1.2×1015 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 target ITER ion densities and confinement times.

Coupling plasma physics and chemistry in the PIC model of electric propulsion: Application to an air-breathing, low-power Hall thruster

This work represents a first attempt to include the complex variety of electron-molecule processes in a full kinetic particle-in-cell/test particle Monte Carlo model for the plasma and neutral gas phase in a Hall thruster. Particular emphasis has been placed on Earth’s atmosphere species for the air-breathing concept. The coupling between the plasma and the gas phase is self-consistently captured by assuming the cold gas approximation and considering gas-wall and gas recycling from the walls due to ion neutralization. The results showed that, with air molecular propellants, all the most relevant thruster performance figures degraded relative to the nominal case using Xe propellant. The main reasons can be ascribed to a reduced ionization cross-section, a larger gas ionization mean free path due to lighter mass air species, and additional electron collisional power losses. While vibrational excitations power losses are negligible, dissociation and electronic excitations compete with the ionization channel. In addition, for molecular oxygen, the large dissociation leads to even faster atoms, further reducing their transit time inside the discharge channel. Future studies are needed to investigate the role of non-equilibrium vibrational kinetics and metastable states for stepwise ionization.

Characterization of a radio-frequency inductively coupled electrothermal plasma thruster

A radio-frequency (RF) inductively coupled electrothermal plasma thruster operating with argon is experimentally characterized for different mass flow rates, RF powers, and propellant injection configurations. Depending on the propellant mass flow rate, significant neutral gas heating is observed with effective stagnation temperatures around 2000 K (giving a maximum estimated thrust and specific impulse of about 100 mN and 125 s, respectively) for absorbed powers between 300 and 500 W. A self-consistent theoretical discharge model is developed and used to study the basic physics and operation of RF electrothermal thrusters, and predictions of the gas temperature are in good agreement with experimental measurements. The model identifies primary power inefficiencies as electron-neutral excitation losses and neutral gas heat losses to the thruster walls. Both experimental and theoretical results indicate that a relatively high stagnation pressure (of the order of 100 Torr or higher) is critical for high performance. For pressures significantly below this the electron-neutral collisional power transfer is too low to effectively heat the neutral gas.

Impact of lithium wall conditioning and wave-frequency on high density lower hybrid current drive experiment on EAST

Abstract A series of dedicated lower hybrid current drive (LHCD) experiments on EAST shows that lithium wall conditioning extends LH current drive and heating up to the line-averaged density of n - e ≈ 4x1019 m−3 for both 2.45 and 4.6 GHz. Current drive at such a high density is crucial for the development of long-pulse non-inductive scenarios on EAST. With lithiation, the LH power injection of 1.5 MW at 2.45 GHz resulted in a drop of loop voltage of ~ 0.3 V, which is a comparable loop voltage drop observed with 1.1 MW at 4.6 GHz. The observed decrease in loop voltage is attributed mostly to the RF heating effect. Another LHCD experiment suggests that lithium wall coating has a more significant impact on the scrape-off-layer (SOL) properties than changes in the Greenwald fraction. LHCD at 2.45 GHz still suffers from a loss of efficiency. Enhanced power ionization in front of the launcher may cause the onset of density-dependent wave instabilities. The rise in the midplane SOL density may also accelerate a transition in the divertor regime, leading to additional ionization and collisional losses in the X-point divertor plasma. Ray-tracing modeling supports that a lower wave frequency is more prone to collisional power loss. The experiments confirm that lithiation is a useful tool to control the SOL plasma, and suggest that density control in front of the launcher may be critical to mitigating power loss mechanisms in the plasma boundary.

Multiphysics simulation of plasma channel formation during micro-electrical discharge machining

A 2D axisymmetric plasma model for micro-electrical discharge machining (μEDM) is developed, and the discharge phenomenon is discussed in this paper. Variations in different plasma properties, such as density, temperature, and collisions of the electrons bombarding the anode and cathode electrodes, were simulated to comprehensively explain the discharge process. The said properties of the plasma channel will be extremely helpful in determining the heat flux available at the tool and workpiece of μEDM. The governing equations of electrostatics, drift-diffusion, and heavy species transport were coupled together and solved simultaneously for computing the properties of the plasma channel in water vapor. The simulation describes the movement of electrons and ions in the inter-electrode gap during the discharge initiation under the applied electric field. The anode spot responsible for the material removal was formed much earlier compared to the cathode spot formed at the tool. Both the temperature and the density of the electrons were observed to be higher near the workpiece, compared to the tool electrode. The temperature of the electrons and the current density of the plasma obtained during the simulation will be useful to determine the heat flux responsible for the material removal. The non-equilibrium nature of the plasma sheath is responsible for the steep changes in the collisional power loss and higher capacitive power deposition near the workpiece electrode.

Characterization of cold background plasma during the runaway electron beam mitigation experiments in the JET tokamak

Disruptions are a major threat to future tokamaks including ITER. They generate excessive electromagnetic forces, heat loads and multi-MeV runaway electrons. The runaway electron beam carries the risk of in-vessel component damage and even the structures beyond them. Thus, prevention of the runaway beam generation or the mitigation of the developed beam is of prime importance. In JET ITER-like wall, the runaway electron beams triggered by massive gas injection (MGI) coexists with a cold background plasma. Lines corresponding to the higher ionization states of argon are observed in VUV spectra outside of the runaway region suggesting a hot background plasma.Using the quantitative analysis of the VUV spectroscopy, the temperature profiles of the background plasmas are estimated using a synthetic line ratios method. The background plasmas at JET-ILW are found to be hotter than other tokamaks where mitigation of the runaway electron beam was unconditionally successful. The volume-averaged Te is found to increase linearly with the gas amount used to trigger the disruption and the electron density in the far scrape-off layer. It is independent of other background plasma properties. A 0D/1D power balance of the post-disruption physical systems is made using the characteristics of the background plasma. The collisional power loss of the runaway electron beam is the primary power source heating the background plasma.

Identification of an Optimized Heating and Fast Ion Generation Scheme for the Wendelstein 7-X Stellarator.

A Doppler shifted resonance minority species ion cyclotron range of frequency (ICRF) scheme for heating neutral beam ions has been identified and optimized for the Wendelstein 7-X stellarator. Compared with more conventional methods, the synergetic scheme increases the normalized core collisional power transfer to the background plasma, and induces larger concentrations of energetic ions. Simulations in the intricate 3D magnetic stellarator geometry reveal an energetic distribution function that is only weakly anisotropic, and is thus relevant to fast ion and alpha particle driven Alfvén eigenmode experimental preparation. Quasilinear theory and simulations of the Joint European Torus indicate that the excellent confinement properties are due to increased velocity diffusion from ICRF interaction along the magnetic field lines. Agreement is found between SCENIC simulations and Joint European Torus experimental measurements for the total neutron rate and the energy distribution of the fast ions.

The effect of external magnetic field on the collisional power absorption in helicon plasma sources

The paper reports on the effects of uniform and non-uniform external magnetic field on the power absorption in a helicon plasma source, which are computationally investigated by the CST Microwave Studio code. RF (13.56 MHz) power deposition was studied using one designs of antennas, namely, the Nagoya type-III. Argon was used as the plasma working gas at the operating pressure of 15 mTorr. We have focused on the collisional power absorption utilizing WKB approximation to describe the plasma inhomogeneity. We put discrete plasma density layers in CST. The obtained results show that the radial inhomogeneity has different effects on the power absorption at the uniform and the non-uniform external magnetic fields. It is found that at uniform external magnetic fields (i.e., B0 = 200 G), there is a specific density (nc) ranging from 1 × 1018 m−3 to 5 × 1018 m−3, before and after which the radial inhomogeneity decreases and increases the absorbed power, respectively. On the other hand, at uniform external magnetic fields (i.e., B0 = 900 G), the inhomogeneity has no regular effect on the power absorption in various plasma densities. In addition, for a given plasma density (e.g., n = 1018 m−3), as the magnetic field increases, the radial inhomogeneity effect on the power absorption would decrease for the Nagoya type-III.

Spectral collocation method for collisional power absorption in radially inhomogeneous helicon plasmas.

A spectral collocation method using Chebyshev polynomials is applied to compute the electromagnetic fields and the collisional power absorption in radially inhomogeneous helicon plasmas. The governing equation has a singularity at the cylindrical axis, which has been resolved by imposing the appropriate boundary conditions. The results are compared with those of the finite difference method. The present work not only shows the superior accuracy of the spectral collocation method with a proper treatment of the singularity, but also discusses the advantages of using Chebyshev nodes for helicon plasmas in particular. It is possible to extend the range of parameters efficiently in cases for which the finite difference method fails to obtain reliable solutions without drastically decreasing the grid size. The spectral collocation method is shown to be especially useful near the lower hybrid resonance condition.

Numerical Simulation and Parametric Study of Carbon Deposition During Graphene Growth in PECVD System

The aim of the present work is to understand the carbon deposition during graphene growth in plasma enhanced chemical vapor deposition system containing Ar/C2H2/H2 gas mixture and to optimize the process parameters for the enhanced growth of graphene. In this regard, many numerical simulations have been carried out using two-dimensional axis-symmetrical inductively coupled plasma module in COMSOL multi physics 5.2a simulation software. From the simulation outcomes, it is found that the electron density, electron energy density, electric potential, and collisional power losses decrease with an increase in temperature. The number densities of various dominant ions were found to decrease and number densities of dominant neutrals were found to increase with an increase in temperature. During the carbon deposition, the formation of non-uniform horizontal graphene layer has been observed along the substrate and vertically oriented graphene sheet has been observed at the edges of the substrate. It is found that the thickness of horizontal graphene along the substrate and height of vertical graphene at the edges of the substrate increases with an increase in gas temperature. Moreover, the reduction in the carbon deposition on the substrate (growth of parallel graphene) and enhanced growth of vertical graphene sheet have been observed on applying substrate biasing. Our numerical simulation results are in good agreement with existing experimental observations and confirms the adequacy of the computational and simulation approach.

The effect of magnetic equilibrium on auxiliary heating schemes and fast particle confinement in Wendelstein 7-X

The performance of the auxiliary heating systems ion cyclotron resonance heating and neutral beam injection is calculated in three different magnetic mirror configurations foreseen to be used in future experiments in the Wendelstein 7-X stellarator: low, standard and high mirror. This numerical work is implemented with the SCENIC code package, which is designed to model three-dimensional magnetic equilibria whilst retaining effects such as anisotropy and the influence of including a finite orbit width of the particles. The ability to simulate NBI deposition in three-dimensional equilibria, the implementation of the realistic beam injector geometry, and the modification of the SCENIC package to permit the investigation of the 3-ion species heating scheme, are recent developments. Using these modifications, an assessment of the advantages and disadvantages of these two fast-ion producing auxiliary heating systems is made in the three different magnetic mirror equilibria. For NBI heating, the high mirror configuration displays the best global confinement properties, resulting in a larger collisional power transfer to the background plasma. The standard mirror has the best particle confinement in the core region, but the worst towards the edge of the plasma. The low mirror has the largest lost power and thus the lowest total collisional power. For ICRH, the displacement of the RF-resonant surface significantly impacts the heating performance. Due to the large toroidal magnetic mirror in the high mirror equilibrium, resonant particles easily become trapped and cannot remain in resonance, generating only small energetic particle populations. Despite this, global confinement is still the strongest in this equilibrium. The low mirror is the only equilibrium to produce peaked on-axis collisional power deposition, with associated peaked on-axis fast ion pressure profiles. A highly energetic particle population is then produced but this results in larger lost power as this equilibrium is not sufficiently optimised for fast ion confinement. A comparison between the two heating methods concludes that NBI produces a smaller fraction of lost to input power, and a reduced sensitivity of the performance to variations of the toroidal magnetic mirror. The main limit of NBI which does not apply to ICRH is the production of highly energetic particle populations, with predictions of energetic particles of E ∼ 0.45 MeV.

A Simulation Study of the Factors Affecting the Collisional Power Dissipation in a Helicon Plasma

Plasma density and magnetic field strength are modified to maximize the collisional power absorption in the helicon plasma. Simulations were done for two common types of RF antennas, namely, half-helix and Nagoya type-III. The simulations were performed with the CST Microwave Studio code. The finite-integration technique approach in the frequency domain has been used for the calculations. The power deposition profile calculated in this paper is validated against HELIC code. The method proved to be reliable, with accuracy depending on the appropriate number of constant radial density shells. CST code has the capabilities that calculate the power absorption for different antenna types at a given plasma density profile at various electron temperatures, magnetic fields, and plasma densities. At high magnetic fields power absorption is rather symmetric, independent to the plasma density. Our finding indicates that power profile is approximately symmetric at low plasma densities in the range of values explored for magnetic fields. It seems that the ratio of the plasma density to the magnetic field has an extreme effect on the axial power deposition profile.

The effect of radial inhomogeneity on the collisional power absorption in helicon plasma sources

The paper reports on the effects of plasma radial inhomogeneity on the power absorption in a helicon plasma source, which are computationally investigated by the CST Microwave Studio code. RF (13.56 MHz) power deposition was studied using three designs of antennas, namely, the Nagoya type-III, the fractional helix, and the single loop. Argon was used as the plasma working gas at the operating pressure of 15 mTorr. We have focused on the collisional power absorption utilizing WKB approximation to describe the plasma inhomogeneity. The obtained results show that the radial inhomogeneity has different effects on the power absorption at the low and the high magnetic fields. It is found that at low magnetic fields (i.e., B0=0.01 T), there is a specific density (nc) ranging from 5×1018 m−3 to 1×1019 m−3, before and after which the radial inhomogeneity decreases and increases the absorbed power, respectively. On the other hand, at high magnetic fields (i.e., B0=0.1 T), the inhomogeneity has no regular effect on the power absorption in various plasma densities. In addition, for a given plasma density (e.g., n=1018 m−3), as the magnetic field increases, the radial inhomogeneity effect on the power absorption would decrease for the Nagoya type-III and the fractional helix designs. However, for the single loop antenna design, this effect is negligible.

Numerical study of the effect of gas flow in low pressure inductively coupled Ar/N2 plasmas

Abstract The effect of gas flow in low pressure inductively coupled Ar/N2 plasmas operating at the rf frequency of 13.56 MHz and the total gas pressure of 20 mTorr is studied at the gas flows of 5–700 sccm by coupling the plasma simulation with the calculation of flow dynamics. The gas temperature is 300 K and input power is 300 W. The Ar fractions are varied from 0% to 95%. The species taken into account include electrons, Ar atoms and their excited levels, N2 molecules and their seven different excited levels, N atoms, and Ar+, N+, N2 +, N4 + ions. 51 chemical reactions are considered. It is found that the electron densities increase and electron temperatures decrease with a rise in gas flow rate for the different Ar fractions. The densities of all the plasma species for the different Ar fractions and gas flow rates are obtained. The collisional power losses in plasma discharges are presented and the effect of gas flow is investigated.

Is media shape important for grinding performance in stirred mills?

Models for understanding the basic concepts of fine grinding and how they apply to the design of stirred media mills have not yet matured. While spherical media in tower mills has previously been studied, real grinding media shape in stirred mills can range from spherical (steel/ceramic balls) to highly non-spherical (sand or slag) resulting in very different media and grinding dynamics. Handling the contact mechanics of non-spherical particles is a challenge for numerical models, and very few studies dealing with non-spherical particle shape exist in the literature. Discrete Element Method (DEM) simulations of dry media flow in a pilot-scale tower mill are performed for four cases with different shaped grinding media, in order to understand how flow and energy utilisation within a stirred mill depend on media shape. Differences in media transport, stress distribution, energy dissipation, and liner wear were observed in the tower mill for the spherical and non-spherical cases. A significant departure from sphericity of the media leads to strong dilation of the bed, reduced bulk density, and a reduction in active volume and collisional power levels leading to a reduction in power draw for the mill. In addition, highly non-spherical media tend to pack tightly near the mill walls forming a near solid layer around the inside of the mill shell which results in poorer transport and mixing, as well as increased wear rates on the screw impeller. Grinding performance in stirred mills appears to deteriorate strongly when using highly non-spherical media.

Collisional power absorption near mode conversion surface in helicon plasmas

Central core heating in helicon plasmas was simulated in some full wave codes considering classical collision without knowing the clear central heating mechanism since the damping rate of helicon wave was known to be low. However, collisional damping of a fast helicon wave may not be negligible when the fast wave approaches mode conversion surface (MCS) in an inhomogeneous plasma. In this study collisional damping rate of a helicon wave is estimated near MCS from the cold plasma dispersion relation. In the presence of sufficient collision, the WKBJ approximation in inhomogeneous helicon plasmas can be applied even near MCS. The collisional damping rate near MCS is found to be high enough to explain the central heating of helicon plasmas by the helicon wave itself. In this heating mechanism, high density near MCS suffices for the helicon mode and it is closely related to operating parameters such as magnetic field, driving frequency and antenna geometry.

Distortion of ion velocity distributions in the presence of ICRH: A semi-analytical analysis

On the assumption that the distribution function of ions heated by ion cyclotron resonance absorption is essentially isotropic, analytical and semi-analytical approximations are derived for the distribution function. The result is used to evaluate the weighted velocity space averages of the distribution, which determine the fusion reactivity and the collisional power transfer to plasma background particles, and to study their scaling with RF parameters such as absorbed power and perpendicular wave number. The importance of higher order finite Larmor radius effects on the formation of RF induced high energy tails is particularly emphasized. Comparisons based on full 2-D numerical calculations show good agreement with the semi-analytical results.

The probability distribution of S+ gyrospeeds in the Io torus

We report a measured probability distribution for S+ gyrospeeds in the Io torus derived from [S II] 6716 Å, 6731 Å emission line profiles recorded at axial distance 5.9 RJ from Jupiter. This ion species is not in kinetic thermodynamic equilibrium. An artificial spectral decomposition in terms of three Maxwellian distributions finds perpendicular temperatures T⊥ = 2.7, 28, and 192 eV with weights 0.28, 0.50, and 0.22, respectively. The average temeprature is about 60 eV, which would be seriously underestimated by a simple measurement of the line width. We find upper limits of ∼5% or ∼20% to the mixing ratios of S+ ions having the pickup speed of ionized ballistic atoms, assuming a flat or isotropic pitch angle distribution, respectively. Due to the low S+ density and charge, Coulomb collisional power transfer from the observed S+ population to the ambient electrons delivers only 0.01 eV cm−3 s−1, or 1/40 the UV emission power requirement. Existing [S III] linewidth temperatures indicate S2+ is hotter than S+, indicating that kinetic thermodynamic equilibrium exists neither between nor within species in the hot, outer plasma torus.

CURRENTS DRIVEN BY HIGH FREQUENCY ELECTROMAGNETIC WAVES

The mechanism of rf-driven currents by high frequency electromagnetic waves is discussed. It is shown that the current will be driven both by the electric force and Lorentz force, and either the resonant effects or the nonresonant effects may be domi-nant,and there exist some differences between the l0 resonances. It is also shown that under some conditions the collisional power dissipation may be stronger than the resonant power dissipation.