Measured Electron Energy Distribution Research Articles

Cross-calibration and first vertical ECE measurement of electron energy distribution in the TCV tokamak

Abstract This paper describes the first vertical electron cyclotron emission measurement of non-thermal electron distributions in the Tokamak à Configuration Variable. These measurements were conducted in runaway electron scenarios and in the presence of electron cyclotron current drive. Measured intensities of linearly polarized X- and O-mode radiation from fast electrons allow the analysis of the energy distribution. The measurements were made possible through the creation of an operational regime for the diagnostic that is free of thermal background radiation, in relaxed electron density operations. This operational regime notably enables the cross-calibration of the diagnostic system, relying on thermal plasma measurements and modeling with the ray-tracing code SPECE.

On the coupling effect in the RF-biased inductively coupled plasma with the synchronous control

The coupling effects between the bias power and the inductive power in the RF-biased inductively coupled plasma with synchronous control are investigated by measuring electron energy distribution function using a compensated Langmuir probe. With synchronous control, the inductive power and the bias power are driven at an identical phase and frequency. The experimental results show that the inductive power lowers the self-bias voltage, while the bias power changes the plasma density by introducing extra power absorption and dissipation. The bias power also enhances the electron beam confinement, leading to an increase in electron density at a low pressure. Furthermore, in the E and H mode transition, with the bias power increasing, the hysteresis power reduces, and the electron density jump decreases.

Double-sided electron energy analyzer for measurement of non-Maxwellian electron energy distributions.

A double-sided electron energy analyzer is developed for studies of magnetic reconnection. It can measure electron energy distribution functions along two directions opposite to each other at the same time. Each side is composed of a floating reference grid, an energy selector grid, and a collector plate. The voltage of the selector grid is swept from -40 to 0V with respect to the reference grid with a frequency of 1MHz. This fast sweeping is required to resolve sub-Alfvénic changes in plasma quantities of the Magnetic Reconnection Experiment, where the typical Alfvénic time is a few microseconds. The reliability of the energy analyzer is checked in Maxwellian plasmas away from the reconnection region. In this case, the electron temperature computed from the electron energy distribution function agrees with measurements of a reference triple Langmuir probe. When it is located near the reconnection region, the temperatures of the tail electron population from both sides, facing into the electron flow and facing away from it, exceed the bulk electron temperature measured by the Langmuir probe by a factor of about 2.

Measurements of electron energy distribution on electron cyclotron resonance plasma operated with hydrogen gas

This article deals with the production of electron cyclotron resonance hydrogen plasma in a closed gas chamber called Menja device. For this purpose, 2.45 GHz frequency and various power i.e. 0 - 500 W are employed. The flow of hydrogen gas is controlled manually with the range 1.5 – 10 sccm maintaining pressure range 10-5 – 10-4 mbar. The Langmuir probe technique is used to drag the electrons and ions by biasing the probe positively and negatively to study the key parameters such as ion saturation current, floating potential, plasma potential, electron temperature and plasma density. Besides these, to obtain information on electron energies and their interaction with plasma, the electron energy distribution function (EEDF) has been studied with the help of 2nd derivative of current obtained from the differentiator. For this purpose, an analogue differentiation circuit was built and tested.

Bright betatron radiation from direct-laser-accelerated electrons at moderate relativistic laser intensity

Direct laser acceleration (DLA) of electrons in a plasma of near-critical electron density (NCD) and the associated synchrotron-like radiation are discussed for moderate relativistic laser intensity (normalized laser amplitude a0 ≤ 4.3) and ps length pulse. This regime is typical of kJ PW-class laser facilities designed for high-energy-density (HED) research. In experiments at the PHELIX facility, it has been demonstrated that interaction of a 1019 W/cm2 sub-ps laser pulse with a sub-mm length NCD plasma results in the generation of high-current well-directed super-ponderomotive electrons with an effective temperature ten times higher than the ponderomotive potential [Rosmej et al., Plasma Phys. Controlled Fusion 62, 115024 (2020)]. Three-dimensional particle-in-cell simulations provide good agreement with the measured electron energy distribution and are used in the current work to study synchrotron radiation from the DLA-accelerated electrons. The resulting x-ray spectrum with a critical energy of 5 keV reveals an ultrahigh photon number of 7 × 1011 in the 1–30 keV photon energy range at the focused laser energy of 20 J. Numerical simulations of betatron x-ray phase contrast imaging based on the DLA process for the parameters of a PHELIX laser are presented. The results are of interest for applications in HED experiments, which require a ps x-ray pulse and a high photon flux.

Increasing the Measurement Range of Plasma Electron Parameters in the Single Langmuir Probe Method

It is proposed to use a new signal type fed to the single Langmuir probe to measure current—voltage characteristics. It is shown that such an approach makes it possible to increase the range of measured electron energy distribution functions.

Electron emission characteristics of wet spun carbon nanotube fibers

Wet spun carbon nanotube fibers were characterized using both field emission and electron energy distribution measurements. Fowler-Nordheim analysis of the field emission results showed that the carbon fibers demonstrated a large effective emission area, 2 x 10-12 m2, which resulted in a reduced brightness of 1.84 x 1010 A/m2/sr/V. By considering the emission and number of carbon nanotube emitters it can be shown that the brightness is consistent with previous reports for single nanotube emitters. Additionally, using the effective emission area determined from the Fowler-Nordheim analysis an emittance value around 0.70 μm was found. These characteristics are useful metrics in determining the applicability of using wet spun carbon nanotube fibers for field emission devices.

Helium ion beam induced electron emission from insulating silicon nitride films under charging conditions

Abstract Secondary electron emission from thin silicon nitride films of different thicknesses on silicon excited by helium ions with energies from 15 to 35 keV was investigated in the helium ion microscope. Secondary electron yield measured with Everhart-Thornley detector decreased with the irradiation time because of the charging of insulating films tending to zero or reaching a non-zero value for relatively thick or thin films, respectively. The finiteness of secondary electron yield value, which was found to be proportional to electronic energy losses of the helium ion in silicon substrate, can be explained by the electron emission excited from the substrate by the helium ions. The method of measurement of secondary electron energy distribution from insulators was suggested, and secondary electron energy distribution from silicon nitride was obtained.

Using Poisson-regularized inversion of Bremsstrahlung emission to extract full electron energy distribution functions from x-ray pulse-height detector data

We measured Electron Energy Distribution Functions (EEDFs) from below 200 eV to over 8 keV and spanning five orders-of-magnitude in intensity, produced in a low-power, RF-heated, tandem mirror discharge in the PFRC-II apparatus. The EEDF was obtained from the x-ray energy distribution function (XEDF) using a novel Poisson-regularized spectrum inversion algorithm applied to pulse-height spectra that included both Bremsstrahlung and line emissions. The XEDF was measured using a specially calibrated Amptek Silicon Drift Detector (SDD) pulse-height system with 125 eV FWHM at 5.9 keV. The algorithm is found to out-perform current leading x-ray inversion algorithms when the error due to counting statistics is high.

Evolution of electron temperature in inductively coupled plasma

It is generally recognized that the electron temperature Te either remains constant or decreases slightly with plasma power (plasma density). This trend can be simply verified using a single-step or multi-step fluid global model. In this work, however, we experimentally observed that Te evolved with plasma power in radio frequency (RF) inductively coupled plasmas. In this experiment, the measured electron energy distributions were nearly Maxwellian distribution. In the low RF power regime, Te decreased with increasing plasma power, while it increased with plasma power in the high RF power regime. This evolution of Te could be understood by considering the coupling effect between neutral gas heating and stepwise ionization. Measurement of gas temperature via laser Rayleigh scattering and calculation of Te using the kinetic model, considering both multi-step ionization and gas heating, were in good agreement with the measured value of Te. This result shows that Te is in a stronger dependence on the plasma power.

Synthesis of hydrogenated diamondlike carbon thin films using neon–acetylene based high power impulse magnetron sputtering discharges

Hydrogenated diamondlike carbon (DLC:H) thin films exhibit many interesting properties that can be tailored by controlling the composition and energy of the vapor fluxes used for their synthesis. This control can be facilitated by high electron density and/or high electron temperature plasmas that allow one to effectively tune the gas and surface chemistry during film growth, as well as the degree of ionization of the film forming species. The authors have recently demonstrated by adding Ne in an Ar-C high power impulse magnetron sputtering (HiPIMS) discharge that electron temperatures can be effectively increased to substantially ionize C species [Aijaz et al., Diamond Relat. Mater. 23, 1 (2012)]. The authors also developed an Ar-C2H2 HiPIMS process in which the high electron densities provided by the HiPIMS operation mode enhance gas phase dissociation reactions enabling control of the plasma and growth chemistry [Aijaz et al., Diamond Relat. Mater. 44, 117 (2014)]. Seeking to further enhance electron temperature and thereby promote electron impact induced interactions, control plasma chemical reaction pathways, and tune the resulting film properties, in this work, the authors synthesize DLC:H thin films by admixing Ne in a HiPIMS based Ar/C2H2 discharge. The authors investigate the plasma properties and discharge characteristics by measuring electron energy distributions as well as by studying discharge current characteristics showing an electron temperature enhancement in C2H2 based discharges and the role of ionic contribution to the film growth. These discharge conditions allow for the growth of thick (>1 μm) DLC:H thin films exhibiting low compressive stresses (∼0.5 GPa), high hardness (∼25 GPa), low H content (∼11%), and density in the order of 2.2 g/cm3. The authors also show that film densification and change of mechanical properties are related to H removal by ion bombardment rather than subplantation.

Laser-initiated explosive electron emission from flat germanium crystals

Flat Sb-doped germanium (100) crystals were investigated in the triode configuration under pulsed tunable laser illumination (pulse duration tlaser = 3.5 ns and photon energy hν = 0.54–5.90 eV) and under DC voltages <104 V. Large bunch charges up to ∼1 μC were extracted from the cathodes for laser pulses >1 MW/cm2 corresponding to a high quantum efficiency up to 3.3% and cathode currents up to 417 A. This laser-induced explosive electron emission (EEE) from Ge was characterized by its voltage-, laser power- and hν-sensitivity. The analysis of the macroscopic surface damage caused by the EEE is included as well. Moreover, we have carried out first direct measurements of electron energy distributions produced during the EEE from the Ge samples. The measured electron spectra hint for electron excitations to the vacuum level of the bulk and emission from the plasma plume with an average kinetic energy of ∼0.8 eV.

Comparative analyses of plasma probe diagnostics techniques

The subject of this paper is a comparative analysis of the plasma parameters inferred from the classical Langmuir probe procedure, from different theories of the ion current to the probe, and from measured electron energy distribution function (EEDF) obtained by double differentiation of the probe characteristic. We concluded that the plasma parameters inferred from the classical Langmuir procedure can be subjected to significant inaccuracy due to the non-Maxwellian EEDF, uncertainty of locating the plasma potential, and the arbitrariness of the ion current approximation. The plasma densities derived from the ion part of the probe characteristics diverge by as much as an order of magnitude from the density calculated according to Langmuir procedure or calculated as corresponding integral of the measured EEDF. The electron temperature extracted from the ion part is always subjected to uncertainty. Such inaccuracy is attributed to modification of the EEDF for fast electrons due to inelastic electron collisions, and to deficiencies in the existing ion current theories; i.e., unrealistic assumptions about Maxwellian EEDFs, underestimation of the ion collisions and the ion ambipolar drift, and discounting deformation of the one-dimensional structure of the region perturbed by the probe. We concluded that EEDF measurement is the single reliable probe diagnostics for the basic research and industrial applications of highly non-equilibrium gas discharge plasmas. Examples of EEDF measurements point up importance of examining the probe current derivatives in real time and reiterate significance of the equipment technical characteristics, such as high energy resolution and wide dynamic range.

Observation of ion heating during stimulated Buneman instability in a temporally growing plasma

Observations and conclusive measurements of ion heating during stimulated Buneman instability (BI) are reported in a temporally growing plasma, created by short-pulse electromagnetic waves. The instability lasts for during the pulse on time and arises when the applied electric-field amplitude supersedes the threshold critical electric field for free acceleration. During the initial period of plasma development after the launch of the pulsed waves, the maximum growth of BI is registered at . The analysis of time-resolved measurements of electron and ion energy distributions indicates an anomalous collision frequency and a hot ion energy tail that persists for more than 700 ns. The energy from the instability is responsible for bringing about the intense and rapid ionization observed upon its termination.

Electron energy distribution function by using probe method in electron cyclotron resonance multicharged ion source

We are constructing a tandem type electron cyclotron resonance (ECR) ion source (ECRIS). High-energy electrons in ECRIS plasma affect electron energy distribution and generate multicharged ion. In this study, we measure electron energy distribution function (EEDF) of low energy region (≦100 eV) in ECRIS plasma at extremely low pressures (10(-3)-10(-5) Pa) by using cylindrical Langmuir probe. From the result, it is found that the EEDF correlates with the electron density and the temperature from the conventional probe analysis. In addition, we confirm that the tail of EEDF spreads to high energy region as the pressure rises and that there are electrons with high energy in ECR multicharged ion source plasma. The effective temperature estimated from the experimentally obtained EEDF is larger than the electron temperature obtained from the conventional method.

Plasma diagnostic method using intermodulation frequencies in a Langmuir probe

A plasma diagnostic method using intermodulation frequencies is developed. When dual-frequency (ω1,ω2) voltage signals are applied to a probe, the intermodulation frequencies (ω2±ω1, ω2±2ω1) between the signals are generated due to the nonlinearity of the sheath. From the analysis of the intermodulation frequencies, the plasma parameters, such as the electron temperature and the plasma density, can be obtained. The measured plasma parameters from this method are compared to the results from the measured electron energy distribution function, and they are in good agreement. Because the intermodulation currents originated from the plasma not from the stray component of the measurement system, an accurate measurement of the plasma parameters is achievable.

Spatial measurements of electron energy distribution and plasma parameters in a weakly magnetized inductive discharge

Spatial characteristics of plasma parameters such as electron temperature, plasma density, plasma potential, and electron energy distribution (EED) were studied in inductively coupled plasma with an axial dc magnetic field. With dc magnetic field, the measured EEDs in the total electron energy scale are spatially coincided except cutting of the low electron energy part indicating the conserved non-local electron kinetics in an axial direction, even though the dc magnetic field is applied. Spatial distributions of the plasma densities at axial positions have almost same trends with various magnetic field strengths. We also discuss the reduction of the ambipolar potential along the axial direction as the applied magnetic field increased.

Investigation of capacitively coupled argon plasma driven at various frequencies and validation of surface waves excitation

The influence of excitation frequency (13.56–96 MHz) on the characteristics of capacitively coupled argon plasma is investigated by means of Langmuir probe and a high-voltage probe. Measurements are performed in argon pressure of 40 and 60 mTorr at a fixed discharge voltage Vpp=200 V. The measured electron energy distribution function EEPFs are a bi-Maxwellian type irrespective of the driving frequency and gas pressure. The electron density and temperatures show peak over frequency range of 54–72 MHz, beyond which it decreases. The non-monotonic dependences of plasma parameters with driving frequency were interpreted in terms of excited surface wave excited at the powered electrode.

Correlation between X-ray yield and electron spectra in laser-cluster interaction

Measurements of electron energy distribution obtained for several experimental conditions show evidence for a strong correlation between X-ray yields and the high energy part of the electron spectra in laser-cluster interaction.

Energy distributions of electrons emitted from reflection-mode Cs-covered GaAs photocathodes

By calculating the energy distributions of electrons reaching the photocathode surface and solving the Schrödinger equation for an electron tunneling through the surface potential barrier, we have obtained an equation to calculate the energy distributions of electrons emitted from reflection-mode Cs-covered GaAs photocathodes based on a two-minima diffusion model. According to the equation, we studied the effects of incident photon energies, diffusion lengths, and surface potential barrier on the electron energy distributions. The equation was also used to fit the measured electron energy distribution curves and the cathode performance parameters were obtained from the fitting. The Γ and L peaks in the theoretical curves are in agreement with the peaks in the experimental curves. The fitted barrier thickness 1.7 Å exactly reflects the GaAs-Cs dipole layer thickness.