Hydrogen Glow Discharge Research Articles

Confinement analysis in low-confinement mode of hydrogen isotope experiments on the Tokamak Fusion Test Reactor

The effect of isotope on confinement in high-recycling, L-mode plasmas is studied on the Tokamak Fusion Test Reactor (TFTR) [see D. M. Meade, J. Fusion Energy 7, 107 (1988)] by comparing hydrogen and deuterium plasmas with the same magnetic field and similar electron densities and heating power, with both Ohmic and deuterium-neutral-beam heating. Following a long operational period in deuterium, nominally hydrogen plasmas were created through hydrogen glow discharge and hydrogen gas puffing in Ohmic plasmas, which saturated the exposed limiter surface with hydrogen and raised the H/(H+D) ratio from 10±3% to 65±5%. Ohmic deuterium discharges obtained higher stored energy and lower loop voltage than hydrogen discharges with similar limiter conditions. Neutral-beam power scans were conducted in L-mode plasmas at minor radii of 50 and 80 cm, with plasma currents of 0.7 and 1.4 MA. To minimize transport differences from the beam deposition profile and beam heating, deuterium neutral beams were used to heat the plasmas of both isotopes. Total stored energy increased approximately 20% from nominally hydrogen plasmas to deuterium plasmas during auxiliary heating. Of this increase about half can be attributed to purely classical differences in the energy content of unthermalized beam ions. Kinetic measurements indicate a consistent but small increase in central electron temperature and total stored electron energy in deuterium relative to hydrogen plasmas, but no change in total ion stored energy. No significant differences in particle transport, momentum transport, and sawtooth behavior are observed. Overall, only a small improvement (∼10%) in global energy confinement time of the thermal plasma is seen between operation in hydrogen and deuterium.

Spectroscopic investigations of a cathode fall region of the Grimm-type glow discharge

This paper presents the results of the spectroscopic study of the cathode fall region of a plane cathode Grimm-type glow discharge in pure hydrogen and in argon with small admixtures of hydrogen. In contrast with the discharge in an argon-hydrogen mixture, the volt-ampere characteristics of the pure hydrogen discharge show a maximum typical for an abnormal glow-to-arc transition. This maximum in the V-A curve is explained here as being due to the increasing role of self-sputtering of the cathode material in sustaining the discharge at higher currents. For the measurements of the electric fields in the cathode fall region, Stark spectroscopy of the hydrogen Balmer lines is employed. Consistent results were obtained from H β and H γ recordings in a pure hydrogen discharge. Some of the difficulties in applying Stark spectroscopy for the diagnostics of a spatially inhomogeneous electric field inherent to Grimm glow discharges are discussed in detail. The experimental results are used to test the theoretical predictions of the electric field distribution in the cathode fall region. Reasonable agreement between theories and experiment is reported. Doppler spectroscopy of the same Balmer lines is used to determine the energies of the excited hydrogen atoms in the discharge. In the cathode fall region of a pure hydrogen discharge, two groups of excited atoms are detected: “slow”, in the range 3.4–8.2 eV, and “fast”, in the range 80–190 eV. The relative concentrations of “slow” and “fast” excited hydrogen atoms in the cathode fall region are determined. In addition, the relative concentration of hydrogen atoms with temperatures around 0.1 eV, excited in the plasma of the negative glow region, is also determined. The origin of these “slow” and “fast” hydrogen atoms is related to the presence of H + and H 3 + ions, respectively. In the cathode fall region of an argon-hydrogen mixture discharge, only excited hydrogen neutrals with energies of 32–43 eV are detected. Their origin is related to the dominant role of H 3 + ions in this discharge. For both gases, in the negative glow region, an increase in the temperature of excited hydrogen atoms is detected, and is explained by the additional excitation of energetic neutrals in collisions with electrons. The axial intensity distributions of the hydrogen Balmer lines, in comparison with other atomic and ionic lines, show different shapes with maxima in the vicinity of the cathode surface. These shapes are explained by the excitation of reflected high-energy neutral atoms in collisions with the matrix gas.

Behavior of hydrogen atoms in boron films during H 2 and He glow discharge and thermal desorption

Hydrogen absorption and desorption characteristics in boron films deposited on a graphite liner have been studied. The number of hydrogen atoms absorbed in the films is estimated from a decrease in hydrogen pressure during a hydrogen glow discharge. It was 1.9 × 10 17 atoms cm 2 in the 1 h discharge after an evacuation of H atoms contained in the original boron films by thermal desorption. Hydrogen atoms were absorbed continuously without saturation for 3 h during the discharge. The number of H atoms absorbed reached 2.6 × 10 17 atoms cm 2 at 3 h. A discharge in helium was carried out to investigate H desorption characteristics from hydrogen implanted boron films. It was verified that capacity for hydrogen absorption was recovered after the He discharge. Hydrogen atoms were accumulated in the films by repetition of alternate He and H 2 discharge. Thermal desorption experiments have been carried out by raising the liner temperature up to 500 °C for films after 1 h, 3 h hydrogen discharge and 6 times repetition of H 2 He discharges. Most of the H atoms in the films were desorbed for all these cases. The slow absorption process was confirmed through the thermal desorption experiments.

Surface modification of a-SiC thin films with ex-situ hydrogenation

We present a study of surface modifications induced in a-SiC thin films grown by sputtering and subjected to ex-situ hydrogenation in a radiofrequency hydrogen glow discharge. Measurements of X-ray photoelectron spectroscopy, optical reflectivity and X-Ray diffraction lead to the conclusion that atomic hydrogen causes preferential etching of silicon from the surface and promotes the formation of a surface polycrystalline hydrogenated carbon layer with a small oxygen content. Furthermore, surface characterisation using scanning electron microscopy and atomic force microscopy shows that the surface carbon layer is grooved. The groves form angles with values very close to those of a hexagonal structure and they have an average peak-to-peak height of 20±5nm.

Niobium as a construction material for a hydrogen energy system

An investigation of the electronegative surface impurities effect on the processes of interaction between niobium and glow discharge hydrogen plasma at room temperature is realized using the method of plasmochemical thermogravimetry and spectroscopy of Auger electrons.

Wall conditioning with lithium evaporation

Thin lithium layers are deposited on metal or graphite walls by evaporation in vacuum. Clean lithium surfaces have wide chemical activities on such gases as O 2, CO and CH 4. In particular, a strong gettering effect on oxygen is observed; the maximum number of O atoms gettered by the lithium layer is about one half the total number of Li atoms deposited on the wall, thus suggesting the formation of Li 2O. On the other hand, H 2 gas hardly reacts with the lithium surface which, however, displays a large pumping effect in a hydrogen glow discharge. The maximum number of H atoms pumped by the lithium layer below 180°C is approximately equal to the number of Li atoms in the vessel, probably due to the formation of LiH. Helium glow conditioning of the used lithium layer allows the partial recovery of lithium from losses to hydrogen pumping or oxygen gettering. This lithium evaporation method has been applied to JIPP T-IIU. 100–250 mg of lithium was deposited onto a limited vessel area of ∼ 1 m 2. The lithium coating leads to 20–50% reduction in oxygen and carbon impurities with less hydrogen recycling in ohmic and NBI discharges.

Design of carbon sheet pump for LHD and demonstration of hydrogen pumping

The first design of Carbon Sheet Pump (CSP) proposed in the Large Helical Device (LHD) project to achieve high plasma performances with low overall recycling ratio is presented. The CSP sheets of C/ C composite are designed to be mounted on the vessel wall near the divertor plate, and operated in the cycle of (1) pumping of charge-exchanged (CX) fast H neutrals by trapping them in the sub-surface layers of the carbon at temperatures below 200°C in the total duration time of about 100 s, (2) degassing of trapped hydrogen by direct Joule heating of the carbon sheet at a temperature above 800°C in a period between main discharges, and then (3) cooling down to the operation temperature below 200°C. The first demonstration of CSP carried out successfully in a hydrogen glow discharge chamber is presented.

Two-photon laser induced fluorescence and amplified spontaneous emission atom concentration measurements in O2 and H2 discharges

Multiphoton laser-induced fluorescence (LIF) and amplified spontaneous emission (ASE) are used to detect ground-state atoms in oxygen and hydrogen glow discharge plasmas. Experimental results and modeling are analyzed to establish the validity criteria for the use of LIF and ASE as diagnostic probes in the measurements of ground-state atom concentrations. An absolute calibration of the LIF signals is obtained by vacuum ultraviolet absorption experiments, either on the 130 nm resonant line of the atomic oxygen, or the 125 nm resonant line of the hydrogen atom. Under typical operating conditions of dc glow discharges, that is, for gas pressure between 0.5 and 5.0 Torr and discharge current from 1 to 50 mA, the densities of the atomic species are measured in the range 1013 cm−3–1015 cm−3. Under the same conditions the hydrogen atom temperature varies from (336±43) K to (1580±90) K.

Light-induced current in hydrogen glow-discharge plasma.

We report the experimental evidence of the new optogalvanic phenomenon--light-induced current (LIC) in plasma. The current was induced by laser radiation in a positive column of hydrogen glow discharge. The ratio of LIC signal to optogalvanic signal is 1.5% in experiment. The LIC signal profile is found to follow qualitatively the derivative profile of the absorption line. The experimental results are in agreement with a prediction of the theoretical estimation. Light-induced current may have been an important mechanism in the magnetic-field generation in stellar atmospheres, in particular, in the solar atmosphere.

Hydrogen plasma degradation of SnO2:F films prepared by the APCVD method

Degradation of textured SnO2:F films in glow-discharge hydrogen plasmas at different temperatures and for different times of exposure has been investigated. After exposure, measurement of transmittance and reflectance spectra, scanning electron micrograph observation and Auger analysis have been carried out to determine the extent of degradation. It has been found that the NIR reflectance spectra are sensitive to minute superficial changes, which are not reflected in the visible transmittance spectra and can be used to obtain plasma durability thresholds in textured SnO2:F films. The haze value, measured by a haze meter may not necessarily be an indication of the true surface morphology of a film and its quality.

Dissociative excitation of hydrogen in rf and dc glow discharges through H 2

The spectral and spatial profile of H β from the low pressure rf and dc glow discharges in hydrogen is studied in order to reveal the excitation mechanism of the fast excited H fragments. Measurements were performed both for the normal and abnormal dc glow discharges. Spatial distributions of the Balmer β radiation reflect the local plasma conditions in the discharge, especially the excitation efficiency which is used to determine the excitation kinetics in hydrogen discharges. Spectral H β profiles were measured and used to determine the kinetic energy of excited H atoms and to check which of the mechanisms describes best the results observed in our experiment. We have also calculated the number densities of vibrationally excited levels by solving a set of vibrational master equations for the conditions similar to those of our experiments, as excitation from the vibrationally excited ground-state hydrogen molecules may be used to explain the changes in the intermediate wing component of the line profile with the changing current.

Electric field measurement in the cathode sheath of a hydrogen glow discharge

Spatially resolved electric field measurements in the cathode region of a dc glow discharge in hydrogen are performed using polarization-dependent Stark broadening of plasma-induced emission of the Balmer lines. The large concentrations of excited atoms in the sheath provide an accurate, sensitive measure of discharge electric fields.

Effect of plasma surface modification on electrical conduction in polyethylene

AbstractHigh‐density polyethylene films 50 μm thick were exposed to the downstream plasma flow of RF glow discharges in argon, hydrogen, or oxygen. It was found that many transvinylene and unreacted radicals remain after the argon or hydrogen plasma treatment, and that carbonyl groups are formed in the oxygen plasma treatment.The conduction current was smaller in the hydrogen‐plasma treated samples and the samples exposed to oxygen or ozone after hydrogen‐plasma treatment than in the untreated samples. Thermal pulse current measurements indicate that the treated surface layer traps electrons or holes and reduces the field strength at the electrode surface, thus suppressing further injection of electrons or holes.

7369. Spectroscopic investigation of energetic atoms in a dc hydrogen glow discharge: C Barbeau and J Jolly, J Phys D: Appl Phys, 23, 1990, 1168–1174

7369. Spectroscopic investigation of energetic atoms in a dc hydrogen glow discharge: C Barbeau and J Jolly, J Phys D: Appl Phys, 23, 1990, 1168–1174

Low Temperature Dry Etching of Graphite Anodes in Hydrogen Glow Discharges

To discribe the temperature dependence of the etching rate of graphite in a hydrogen glow discharge a model is discussed taking into accout the transition from CH4 to CH as the main etching product with increasing temperature.

Particle-induced desorption of hydrogen from a-C: H at low impact energies

The release of hydrogen from the carbonized walls of a tokamak by plasma bombardment is an important process intervening in hydrogen recycling. The desorption of hydrogen from amorphous, hydrogen containing carbon films (a-C: H) by low energy hydrogen (deuterium) and noble gas ions has been studied in an UHV test device coated with a-C: H films. The dynamics and quantities of the released fluxes as well as the implanted fluxes by exposure to low energetic (200–350 eV) hydrogen and noble gas ions have been measured. Evidence is presented for the preferential incorporation of low energy hydrogenic species in a growing a-C: H layer from CH 4/H 2 mixtures. In subsequent RG discharges in pure hydrogen the desorption of hydrogen from the film leads to a change of the recycling coefficient R from initial values R > 1 to stationary values R≈1. The more effective removal of hydrogen by glow discharges in noble gases yields finally a strong receptivity for hydrogen of the a-C: H layer. This behaviour can be observed while operating a glow discharge in pure hydrogen following such a conditioning procedure. Starting reimplantation of hydrogen from values R < 1 the a-C: H coating approaches a stationary state in which the hydrogen concentration is almost constant and the recycling coefficient reaches again values of about R = 1. Results from TEXTOR show the relevance of the experiments for tokamak operation.

Spectroscopic investigation of energetic atoms in a DC hydrogen glow discharge

The analysis of the Doppler profile of the Balmer Halpha line in the cathode sheath and negative glow of a hydrogen glow discharge shows an important concentration of anisotropic energetic atoms. This translational energy is regarded as mainly resulting from reactions including ions accelerated by the cathode sheath electric field. The mechanisms involved are gas-phase charge-exchange collisions producing fast atoms moving toward the cathode and simultaneous neutralisation and backscattering of the ions at the cathode surface leading to energetic atoms moving in the other direction. Quantitative analysis shows that most of the emission of the fast atoms results from excitation directly produced by these ion impact reactions.

Effect of changes in viewing window transmission on high-temperature Thomson scattering data

Unmonitored changes in the transmission of viewing windows owing to deposited films can produce errors in Thomson scattering temperature measurements. This effect is illustrated by a recent run on the Tokamak Fusion Test Reactor (TFTR) where apparent errors of over 25% in peak temperatures of 9 keV owing to carbon films were noted. Since coatings can also be removed by hydrogen discharges, the transmission of a window may change with time, resulting in variable errors in the temperature. It is proposed that these changes be monitored by calibration in situ with the aid of a low-pressure hydrogen glow discharge.

Hydrogen recycling in carbon films

Amorphous carbon films (∼40 nm thick) were prepared on the wall of a discharge vessel using methane plasma CVD. The films were exposed to a hydrogen and helium glow discharge. The hydrogen recycling phenomena in these films have been investigated. Measurements of the pressure change induced by the discharge allow the evaluation of hydrogen retention, and of hydrogen release. It has been found that the recycling depends on the wall temperature and on the energy of the hydrogen ions. The results are consistently interpreted in terms of a Desorbable-phase model) (D-phase moddl) with consideration of the dependence of the hydrogen depth profile in the films on the incident energy of the ions.

Heat treatment in hydrogen gas and plasma for transparent conducting oxide films such as ZnO, SnO 2 and indium tin oxide

The effect of heat treatments in hydrogen gas and glow discharge and electron cyclotron resonance (ECR) hydrogen plasmas on indium tin oxide (ITO), tin oxide (TO) and aluminum-doped zinc oxide (AZO) has been studied. The AZO films in comparison with the ITO and TO films have been found to be more resistant to these hydrogen treatments. These hydrogen treatments for the ITO and TO films lead to a chemical reduction with a colouring and then the vaporization of the corresponding metals. In contrast, the AZO films exhibit no colouring because of simultaneous development of the reduction and the vaporization. It is found that treatments utilizing the ECR hydrogen plasma are useful as a low temperature dry etching process for various transparent conducting oxide films.