Increase In Plasma Density Research Articles

Optimization of Electron Beams for Ion Bombardment Secondary Emission Electron Gun

Abstract Electron beam fluorescence technology is an advanced non-contact measurement in rarefied flow fields, the fluorescence signal intensity is positively correlated with the electron beam current. The ion bombardment secondary emission electron gun is suitable for the technology. To enhance the beam current, COMSOL simulations and analyses were con-ducted to examine plasma density distribution in the discharge chamber under the effects of various conditions and the electric field distribution between the cathode and the spacer gap. The anode shape and discharge pressure conditions were optimized to increase plasma density. Additionally, an improved spacer structure was designed with the dual purpose of enhancing the electric field distribution between the cathode-spacer gap and improving vacuum dif-ferential effects. This design modification aims to increase the pass rate of secondary elec-trons. Both simulation and experimental results demonstrated that the performance of the optimized electron gun was effectively enhanced. When the electrode voltage remains con-stant and the discharge gas pressure is adjusted to around 8 Pa, the maximum beam current was increased from 0.9mA to 1.6 mA.

Influence of the RF-power on the etching yields and surface morphology in reactive ion beam etching of Si and photoresist

The change in ion energy distribution and composition of a reactive ion beam produced by an RF-excited ion beam source and operation with a mixture of CHF3 and O2 was investigated and correlated with the etching behavior. To this end, measurements were performed with an energy-selective mass spectrometer to determine ion energy distributions, current density measurements for the measurement of current density distributions of the ion beam, and tactile measurements to determine the etching rates of Si and photoresist. The morphology of the photoresist was measured with a scanning force microscope. In particular, alterations in the etching yield and surface morphology of the photoresist can be observed in response to changes in the applied RF-power. An increase in plasma density leads to an increase in fragmentation processes of the injected reactive gases, resulting in the formation of smaller fragments. These smaller fragments have a chemical impact on the substrate surface, which affects the etching performance. These effects can have significant consequences in the context of long-time reactive ion beam processing for patterning applications.

Effect of detachment on Magnum-PSI ELM-like pulses: direct observations and qualitative results

Abstract Conditions similar to those at the end of the divertor leg in a tokamak were replicated in the linear plasma machine Magnum-PSI. The neutral pressure in the target chamber is then increased to cause the target to transition from an attached to a detached state. Superimposed to this steady state regime, edge localised mode (ELM)-like pulses are reproduced, resulting in a sudden increase in plasma temperature and density, such that the heat flux increases transiently by half an order of magnitude. Visible light emission, target thermography, and Thomson scattering are used to demonstrate that the higher the neutral pressure the more energy is removed from the ELM-like pulse in the volume. If the neutral pressure is sufficiently high, the ELM-like pulse can be prevented from affecting the target and the plasma energy is fully dissipated in the volume instead (ID 4 in table 1). The visible light images allow the division of the ELM-plasma interaction process of ELM energy dissipation into 3 ‘stages’ ranging from no dissipation to full dissipation (the target plasma is detached). In the second publication related to this study, spectroscopic data is analysed with a Bayesian approach, to acquire insights into the significance of molecular processes in dissipating the plasma energy and particles.

Absorption of electromagnetic waves by cylindrical surface plasmon resonance

Abstract The absorption characteristics of cylindrical surface plasmon resonance (CSPR) have been studied. We demonstrated that a single plasma column with CSPRs can achieve high absorptivity at the plasmon frequency. We also studied the effects of plasma density and collision frequency on the absorptivity. As both of them increase, the corresponding absorptivity tends to increase first and then decrease, but with different reasons. Such manifestation is explained by analyzing transmission and reflection spectra in both cases, as well as magnetic field distribution patterns at the frequency of plasmon. On the one hand, the increase in plasma density leads to the enhancement of plasmons, improving transmittance; On the other hand, the increase in collision frequency leads to a weakening of plasmons and an increase in reflectivity. Finally, we investigated the absorption characteristics of plasmons in the plasma photonic crystals (PPCs) structure and overcame the absorption attenuation caused by the increase in plasma density by increasing the number of plasma columns. Meanwhile, the absorption generated by surface plasmon resonance is not affected by the lattice constant of PPCs. The research has shown that efficient absorption can be achieved using CSPR, resulting in extremely high absorptivity when using fewer plasma columns. Finally, we verified the absorption ability of CSPRs through experiments.

Latitudinal Characteristics of the Post‐Sunset Enhancements in Ionospheric Electron Density During the Geomagnetic Quiet Period in May 2021 Over East‐Asian Region

AbstractThis study investigated the latitudinal variations of post‐sunset enhancements in the ionospheric electron density during the geomagnetic quiet period in May 2021 with a combination of high‐precision ionospheric parameters obtained from four ionosondes, Beidou geostationary satellite (BD‐GEO) receiver network and Sanya incoherent scatter radar (SYISR). We identified four categories of post‐sunset enhancement phenomena (Types 1–4), each with unique spatial and temporal evolutions, yet uniformly accompanied by a decrease in hmF2. Measurements of plasma drift vector velocities from SYISR and hmF2 gradients across various latitudes provided pivotal insights, confirming that the ionospheric post‐sunset enhancements can result from downward plasma motion due to westward electric field, downward field‐aligned drift, or a combination of both. For Type 1, dominated by field‐aligned drift, plasma density enhancements not only intensify at low latitudes but may also extend to mid‐latitudes, exhibiting a distinct temporal delay with increasing latitude. In contrast, Type 4, primarily driven by the westward electric field, is characterized by modest increases in plasma density confined to localized low‐latitude regions, with no observable latitudinal time delay in the peak of enhancements. Types 2 and 3, which are subject to the combined influence of the westward electric field and field‐aligned drift, exhibit plasma density increases at certain low‐latitude areas, with Type 2 presenting a delayed pattern and Type 3 showing none with rising latitude. Meanwhile, neutral winds can partially account for the observed post‐sunset enhancement from low to middle latitudes. These findings offer new insights into the factors influencing ionospheric behavior after sunset.

Ion velocity separation mechanism during vacuum spark stage

Abstract Supersonic ion jets produced in vacuum arc discharges have a wide range of applications, where precise control of ion kinetic energy is crucial. However, a comprehensive understanding of the ion acceleration mechanism remains elusive, particularly regarding whether there is ion velocity separation in the vacuum spark stage. In this paper, a 1D spherical implicit particle-in-cell (PIC) with Monte Carlo collision (MCC) model is employed to investigate the ion velocity separation in multi-charged vacuum arc plasma with varying electrode bias voltages and plasma ion densities. The results show that ion kinetic energy can reach hundreds of electron volts due to continuous acceleration by the formed potential valley, which leads to ion velocity separation at low electrode bias voltage or low plasma density. An increasing electrode bias voltage flattens the potential valley, reducing the electric field acceleration. While increasing the plasma density deepens the valley and intensifies Coulomb collisions, resulting in nearly-equal velocities across ions in different charge states. These findings can theoretically explain the discrepancies observed in previous experiments regarding the dependence of the ion velocity on its charge state during the vacuum spark stage.

Plasma opacity induced by laser-driven movement of background ions

Abstract The transition threshold from relativistically transparent regime to opaque regime in the interaction of an ultra-intense laser pulse and a bulk plasma with mobile background ions is investigated. The threshold corresponds to the onset of laser hole-boring. We show that for an ultra-intense laser, the threshold depends on the ion composition of the plasma, and the corresponding plasma density is significantly lower than that with ion immobile plasma. These are supported by 1D PIC simulations by using hydrogen plasma and fully ionized carbon plasma. The movement of background ions modifies the dynamics and distribution of electrons in the plasma, which results in the Doppler-red-shift of the incident laser and the increase of the effective plasma density. An intuitive model, which gives the dependence of the transparency-opaqueness threshold on laser intensity and frequency, as well as the ion composition, is established. It is tested by multi-dimensional PIC simulations with hydrogen plasma.

SOLPS-ITER modeling of divertor detachment in MAST-U’s super-X double null configuration and comparison to experiment

The MAST-U tokamak has recently undergone upgrades to investigate any detachment advantages of the “Super-X Double-Null” magnetic configuration, a concept featuring up-down symmetry with two magnetic nulls and elongated divertor legs with strong wall baffling. This study presents SOLPS-ITER simulations of MAST-U Super-X Double Null discharges, in H-mode conditions, compared to initial 2D imaging measurements of divertor detachment characteristics. In the simulations, the Super-X divertor targets achieve detachment at a substantially lower upstream separatrix density compared to a shorter leg, conventional configuration (CD). As the density increases further in the Super-X configuration, the cold detached front moves upstream towards the X-point. Its speed in the poloidal plane, calculated as the change in front location divided by the corresponding increase in upstream plasma density, decreases by a factor of ∼ 3–4 as the front transitions from the baffled divertor chamber to the main plasma chamber, reaching a speed comparable to that observed in the CD configuration. In a chosen experimental Super-X density ramp discharge, the ionization front, identified using D2 Fulcher emission as a proxy, is already displaced from the target at the start of the divertor fueling ramp, whereas modeling of the same initial phase shows the ionization front still close to the target. As divertor fueling increases, both experiment and modeling show the ionization front moving further upstream towards the divertor throat. However, the associated variation in upstream density is ∼ 2 times larger in the modeling compared to the experiment, resulting in a broader detachment window. Comprehensive modeling efforts are underway to address these discrepancies, with several inaccurate modeling assumptions identified as potential causes of the deviation.

Improved axial confinement in the open trap by the combination of helical and short mirrors

The paper presents experimental results from the SMOLA device, which was built in the Budker Institute of Nuclear Physics for the verification of the helical mirror confinement idea. This concept involves active control of axial losses from the confinement zone in an open magnetic trap through the use of multiple mirrors that move in the plasma frame of reference. The discussed experiments focused on determining the cumulative effect of a helical mirror system in combination with a short segment of a stronger magnetic field. Combination of these two methods of axial flow suppression results in higher efficiency compared with each method individually. Different combinations of the mirrors were tested. The most effective flow suppression was observed if the short mirror was placed between the confinement region and the helical mirror. In this configuration, an effective mirror ratio of $R_{{\rm eff}} = 32.6\pm 7.8$ was achieved, along with a more than three-fold increase in plasma density within the confinement region. The possibility of a cumulative effect of different types of magnetic mirrors offers a way to improve the confinement performance of the reactor-grade mirror confinement devices.

Experimental and simulation study of argon helicon discharge in multiple plasma simulation linear device (MPS-LD)

Abstract To obtain a high-parameter plasma in the target region of a multiple plasma simulation linear device and to realize the experimental simulation environment of tokamak divertor plasma, experimental and numerical simulations of argon helicon discharge are carried out. Langmuir probes are used to diagnose the electron density (ne ) in the source and target regions with different experimental parameters (magnetic field, radio frequency power, puffing flow rate). A three-dimensional discharge model is developed using drift-diffusion equations of electron density and electron energy with the aid of COMSOL. Helicon discharge with a long straight plasma beam and a bright blue core is experimentally achieved. The simulation and experimental results are compared, validating the model. The corresponding spatial ne distribution is obtained, and the dependence of ne on the main experimental parameters is confirmed. The energy conversion relationship between the helicon and plasma is found. Helicon waves prefer to transfer energy to the plasma in the source region, where ne is significantly increased. This results in a strong ne gradient, which acts as a barrier to prevent the propagation of helicon waves. Therefore, localized standing helicon waves are formed, which limits the increase in plasma density in the target region. By increasing the magnetic field strength (B < 1500 G) and RF power (P < 1500 W), ne in the source region can be increased, but they have little effect on ne in the target region.

Design and Application of High-Density Cold Plasma Devices Based on High Curvature Spiked Tungsten Structured Electrodes

Advances in radar technology have driven efforts to develop effective countermeasures. Plasma is recognized as a highly effective medium for absorbing electromagnetic waves. Recent research has focused on enhancing plasma element performance. This paper achieved ultra-high-density, low-pressure cold plasma with a density of 1.15 × 1012 cm−3, surpassing similar studies by more than an order of magnitude. Tungsten electrodes with high-curvature spiked structures were invented to replace traditional iron–nickel alloy electrodes, increasing plasma density by 88.2% under the same conditions. Lightweight and cost-effective tubular and annular ultra-high-density, low-pressure cold plasma devices were developed, demonstrating exceptional performance in electromagnetic wave absorption, plasma transient antennas, and radar stealth technology. The influence of plasma on electromagnetic waves and its numerical relationship were analyzed. By measuring the radar cross-section (RCS), the reduction in radar detection rates was quantified. The results show that the ultra-high-density cold plasma devices exhibit very low intrinsic RCS values, suitable for plasma antenna applications. The array of plasma elements generates a large-area high-density low-pressure cold plasma. This plasma effectively reduces the radar cross-section (RCS) of metallic equipment in the S and C bands and shows attenuation in the X band. These effects highlight the superior characteristics of plasma technology in electronic warfare. This exploratory research lays the groundwork for further defense applications.

Generation of the vortex terahertz radiation by the interaction of two-color Laguerre–Gaussian laser with plasmas in the presence of a static magnetic field

The generation of vortex terahertz (THz) radiation by the interaction of a two-color Laguerre–Gaussian (LG) laser with plasmas under an external magnetic field is investigated theoretically and numerically. It is found that the vortex THz radiation with good monoenergetic properties can be generated successfully, and the orbital angular momentum of the LG lasers can be transferred to the radiation. In this scheme, the external magnetic field can not only enhance the intensity but can also break the spatial distribution symmetry of the vortex THz radiation. With the increase in the initial plasma density, the intensity of the vortex THz radiation increases significantly before reaching saturation and the spatial period of the radiation decreases, which indicates the monoenergetic peak of the vortex THz radiation can be well controlled through the initial plasma density. The relevant conclusions are verified by two-dimensional particle-in-cell simulations.

Plasma enhanced atomic layer deposition of silicon nitride using magnetized very high frequency plasma

To obtain high-quality SiN x films applicable to an extensive range of processes, such as gate spacers in fin field-effect transistors (FinFETs), the self-aligned quadruple patterning process, etc, a study of plasma with higher plasma density and lower plasma damage is crucial in addition to study on novel precursors for SiN x plasma-enhanced atomic layer deposition (PEALD) processes. In this study, a novel magnetized PEALD process was developed for depositing high-quality SiN x films using di(isopropylamino)silane (DIPAS) and magnetized N2 plasma at a low substrate temperature of 200 °C. The properties of the deposited SiN x films were analyzed and compared with those obtained by the PEALD process using a non-magnetized N2 plasma source under the same conditions. The PEALD SiN x film, produced using an external magnetic field (ranging from 0 to 100 G) during the plasma exposure step, exhibited a higher growth rate (∼1 Å/cycle) due to the increased plasma density. Additionally, it showed lower surface roughness, higher film density, and enhanced wet etch resistance compared to films deposited using the PEALD process with non-magnetized plasmas. This improvement can be attributed to the higher ion flux and lower ion energy of the magnetized plasma. The electrical characteristics, such as interface trap density and breakdown voltage, were also enhanced when the magnetized plasma was used for the PEALD process. Furthermore, when SiN x films were deposited on high-aspect-ratio (30:1) trench patterns using the magnetized PEALD process, an improved step coverage of over 98% was achieved, in contrast to the conformality of SiN x deposited using non-magnetized plasma. This enhancement is possibly a result of deeper radical penetration enabled by the magnetized plasma.

Power measurement analysis of moderate pressure capacitively coupled discharges

This study examines the transition of 13.56 MHz, capacitively coupled plasmas (CCP) from low to intermediate pressure regimes. Here, we investigate power deposition/plasma production in argon, nitrogen, and oxygen discharges as a function of pressure. These three feed gases were chosen as they provide a set of electropositive and electronegative gases and they are widely discussed in the existing literature. Experiments were conducted for all combinations of pressures: 0.5, 1.5, and 2.5 Torr, and nominal power density between 0.1 and 0.7 W/cm2 for each feed gas at a fixed electrode gap of 24 mm, a commonly employed gap in many industrial processes. Our study shows that increasing pressure results in an increase in current at a given electrode bias in argon and oxygen discharges, while there is no discernible pressure-induced change in nitrogen discharges. We attribute this increase to an increase in plasma density, which might result from a change in power deposition or ionization processes. It is likely that heating via secondary electrons becomes more important at intermediate pressures, resulting in increased plasma density and current. Specifically, based on our measurements, it appears that the mechanisms through which power is deposited into the plasma change with increasing pressure for both argon and oxygen discharges but not for nitrogen discharges. Our experimental results align with the outcomes of our simulations and the simulation results of CCP discharges conducted by other researchers under similar conditions.

Frequency Spectrum of Radiation Flux Generated by Beam-Plasma System with Ten Joules Energy Content in Microsecond Pulse

The work reports the achievement of an energy content of 10 J per microsecond pulse in a directed flux of electromagnetic radiation in the frequency range of ~ 0.2–0.3 THz. The flux is generated by a fundamentally new method, which is realized through the pumping of upper-hybrid plasma oscillations in a magnetized plasma column with a relativistic electron beam (REB) and their subsequent transformation into a flux of electromagnetic radiation. In the described experiments at the GOL-PET facility, this method to generate THz radiation is implemented in the following way a beam of electrons with energy E ~ 0.5 MeV with a current density of (1–2) kA/cm2 is passing through a magnetized (4 T) plasma column with a density of 1014–1015 cm–3. By comparing the experimentally measured spectral composition of the radiation flux with the calculated spectrum, it is proved that this process is realized through resonant pumping of the branch of upper-hybrid plasma waves by such beam. A coordinated increase in plasma density and beam current density opens up the prospect of advancement in the generation of multi-megawatt radiation fluxes in the region of one terahertz.

Correlation of source parameters and beam properties in the early operation of the full size ITER negative ion beam source

One of the requirements of Heating and current drive Neutral Beam injectors for ITER is a beam homogeneity greater than 90%, to achieve an optimal beam transmission while keeping the heat load consistently low on the acceleration electrodes. The large size and complexity of ITER negative ion source play a key role in determining the homogeneity of the negative ion current of each of the 1280 beamlets and their divergence, and it is studied in the full-scale prototype source SPIDER. In this work the plasma properties are studied by spectroscopic and electrostatic measurements in the drivers, where the plasma is generated, and in the expansion region, where the plasma drifts and negative ions are produced, and they are correlated with the properties of the beam. The non-homogeneous plasma density profile is related to the non-homogeneous availability of negative ions along the beam vertical profile, with and without cesium evaporation. Visible tomography, a technique capable of characterizing isolated beamlet properties, is used to study the beam’s dependence on plasma uniformity along the entire beam profile. Using these tools, it has been demonstrated how an increase in plasma density is linked to an improvement in beam homogeneity. The latter has been directly correlated with plasma homogeneity. The magnetic filter field and biases of the plasma grid and bias plate are responsible for the variation in plasma density and its homogeneity. Non-uniformities in the plasma’s top/bottom and left/right distributions have been studied and partially addressed experimentally. The first issue was resolved by adjusting the radio-frequency power supplied to the plasma in different vertical regions, while the second issue was addressed by reversing the direction of the magnetic filter field and increasing the plasma density.

Radiation reaction kinetics and collective QED signatures

Observing collective effects originating from the interplay between quantum electrodynamics and plasma physics might be achieved in upcoming experiments. In particular, the generation of electron–positron pairs and the observation of their collective dynamics could be simultaneously achieved in a collision between an intense laser and a highly relativistic electron beam through a laser frequency shift driven by an increase in the plasma density increase. In this collision, the radiation of high-energy photons will serve a dual purpose: first, in seeding the cascade of pair generation; and, second, in decelerating the created pairs for detection. The deceleration results in a detectable shift in the plasma frequency. This deceleration was previously studied considering only a small sample of individual pair particles. However, the highly stochastic nature of the quantum radiation reaction in the strong-field regime limits the descriptive power of the average behavior to the dynamics of pair particles. Here, we examine the full kinetic evolution of generated pairs in order to more accurately model the relativistically adjusted plasma density. As we show, the most effective pair energy for creating observable signatures occurs at a local minimum, obtained at finite laser field strength due to the trade-off between pair deceleration and the relativistic particle oscillation at increasing laser intensity. For a small number of laser cycles, the quantum radiation reaction may re-arrange the generated pairs into anisotropic distributions in momentum space, although, in the one-dimensional simulations considered here, this anisotropy quickly decreases.

Effects of charge exchange on plasma flow in the heliosheath and astrosheathes

ABSTRACT Shock boundary layers are regions bounded by a shock wave on the one side and tangential discontinuity on the other side. These boundary layers are commonly observed in astrophysics. For example, they exist in the regions of the interaction of the stellar winds with the surrounding interstellar medium. Additionally, the shock layers (SLs) are often penetrated by the flows of interstellar atoms, as, for instance, in the astrospheres of the stars embedded by the partially ionized interstellar clouds. This paper presents a simple toy model of an SL that aims to qualitatively describe the influence of charge exchange with interstellar hydrogen atoms on the plasma flow in astrospheric SLs. To clearly explore this effect, magnetic fields are neglected, and the geometry is kept as simple as possible. The model explains why the cooling of plasma due to charge exchange in the inner heliosheath leads to an increase in plasma density in front of the heliopause. It also demonstrates that the source of momentum causes changes in the pressure profile within the SL. The paper also discusses the decrease in plasma density near the astropause in the outer SL in the case of layer heating, which is particularly relevant in light of the Voyager measurements in the heliospheric SL.

Variation of Electron Lifetime Due To Scattering by Electrostatic Electron Cyclotron Harmonic Waves in the Inner Magnetosphere With Electron Distribution Parameters

AbstractThrough resonant wave‐particle interactions with electrostatic electron cyclotron harmonic (ECH) waves, low‐energy plasma sheet electrons can be scattered into the atmospheric loss cone and precipitate. This process can be investigated by calculating bounce‐averaged quasi‐linear diffusion coefficients. However, the numerical calculation of ECH wave‐induced scattering rates requires the specification of several parameters, including the properties of the hot plasma sheet electrons responsible for the wave excitation. The dependence of the diffusion coefficients on these parameters and the exact contribution of scattering by ECH waves to diffuse auroral precipitation is still poorly understood and has not been carefully quantified yet. In this study, we calculate bounce‐averaged quasi‐linear scattering rates and compare the resulting lifetimes to the strong diffusion limit. We vary the temperature, plasma density, and loss cone parameters of the hot component in the electron loss cone distribution over a large range based on previous studies and observations. By adopting a new model that derives the wave normal angle from the midpoint and the angular width of the growth rate profile, our findings suggest that the electron lifetime near the loss cone is not significantly affected by the hot electron temperature and loss cone parameters. However, there is an increase in electron lifetimes with increasing plasma density. For an electric field amplitude of 1 mV/m, the electron lifetime is below or equal to the lifetime calculated using the strong diffusion limit up to E = 0.25 keV when n = 1.5 cm−3, and up to E = 0.22 keV when n = 9 cm−3.

Quadruple Langmuir probe characterization of different fuel gases in a plasma deflagration accelerator

Astrophysical flows may be studied by reproducing similar conditions using a coaxial plasma accelerator operating in the deflagration regime (or plasma deflagration accelerator). This allows for the recreation and investigation of dynamics present in complex highly coupled plasma systems at the laboratory scale. We report on measurements of the plasma density, temperature, plasma potential and velocity found using a quadruple Langmuir probe (QLP) on such a deflagration accelerator in the form of the Stanford Coaxial High ENerGy (CHENG) device operating with multiple gases – specifically argon, nitrogen and hydrogen. Experiments show a general decrease in bulk plasma velocity with gas atomic mass from upwards of $120\ {\rm km}\ {\rm s}^{-1}$ with hydrogen to less than $30\ {\rm km}\ {\rm s}^{-1}$ with argon. There was an accompanying increase in peak plasma density with increasing atomic mass from ${\sim }3\times 10^{20}\ {\rm m}^{-3}$ with hydrogen to ${\sim }1.5 \times 10^{21}\ {\rm m}^{-3}$ with argon. It was found that the momentum flux and internal energy density also generally increase with atomic mass while the particle flux is constant between shots. Further investigation is needed to understand these correlations and the underlying physics. Lastly, comparisons with scaling laws show that while the CHENG device may be operated in such a way as to simulate the effects of bulk solar wind movement, it may not properly capture the thermal effects.