Low-temperature Hydrogen Plasma Research Articles

The development of a zero-dimensional collisional-radiative model for interpreting plasma emission in low temperature divertor plasmas in tokamaks

Collisional-radiative models are commonly used to analyse atomic and molecular processes in low temperature plasmas by determining the distribution functions of excited states as functions of various plasma parameters. This paper outlines the improvements to a zero-dimensional collisional-radiative model, developed at Keio University, for purposes relevant to the analysis of emission measurements in low temperature hydrogen plasmas. The extension of this 0D model centres on the inclusion of additional molecular species and reactions, a calculation of emission intensity in order to directly compare with experimental work, and the addition of a simple wall model to allow for a deeper understanding of recycling of atoms and molecules in low temperature plasmas close to plasma facing components in fusion machines. Results from the improved model are then compared with both an existing CR model for benchmarking and experimental emission spectroscopy data from an inductively-coupled plasma device at the University of Liverpool. These results show how the developments to the model have increased the relevance to experimental plasmas, such as those in the power exhaust regions of fusion machines, with the ratio of the Hβ/Hα line intensities outputted from the improved CR model mirroring more closely the emission measured experimentally in the ICP device.

Low temperature hydrogen plasma permeation in palladium and its alloys for fuel recycling in fusion systems

Superpermeation of hydrogen isotopes through metal foils is a critical component for efficient fuel recycling in fusion power systems. In that context hydrogen permeation through foils of palladium and its alloys with silver and copper was studied at low temperature (60 - 200°C) under plasma exposure. These alloys differ significantly in both bulk and surface properties, and comparisons can provide mechanistic insights. Permeation was observed only during plasma operation, confirming the negligible contribution of molecular hydrogen to the observed flux. As-received foils required surface treatment to achieve top performance. For Pd and Pd75Ag25 an oxidation treatment increased permeation an order of magnitude, but proved unstable as this desirable surface was reduced under hydrogen plasma exposure. In contrast, an Ar plasma cleaning step provided both high and stable flux. As-received FCC phase Pd60Cu40 foils required annealing to transform it into the high permeability BCC phase that delivered top performance. All foils displayed similar temperature dependence with flux declining with temperature, suggesting that the primary rate-limiting step is absorption of superthermal hydrogen. Among these foils the hydrogen flux through PdCu was 3 - 5X greater than that of Pd or PdAg, which were similar. The superiority of BCC PdCu is attributed to its superior hydrogen desorption kinetics. Using PdCu 100% permeation of supplied hydrogen was achieved, and the flux saturated with increasing plasma power at values >10−2 mol H·m−2·s−1, and under these conditions permeation rates are equivalent with or without the membrane present. The fluxes achieved are the highest reported to date at these conditions, and the results highlight the important roles of both surface and bulk properties.

Self-Controlled Flashing Nuclear Fusion in Stationary Magnetized Low-Temperature Plasma

The prerequisites and mechanism for the implementation of efficient pulsed (flashing) nuclear fusion in a low-temperature hydrogen plasma with a temperature of 10 to 20 eV in a constant magnetic field are considered. It is shown for the first time that the natural very frequent alternation of the processes of ionization of atoms and recombination of ions leads to the synchronous formation of coherent correlated states of hydrogen nuclei and its isotopes. The formation of such states leads to the generation of very large fluctuations of kinetic energy (up to 10 to 100 keV) at the initial stage of each ionization event, which exists for most of the lifetime of the ionized state before ion recombination. It is shown that the relatively long duration of the existence of these fluctuations and their very large amplitude are sufficient for efficient nuclear fusion in such a magnetized low-temperature plasma.

Surface and sub-surface investigations of the plasma influence on Cs doped Mo as surface converter for negative hydrogen ion sources for fusion(a)

The influence of low-pressure and low-temperature hydrogen plasma on a pure molybdenum sample implanted with caesium is investigated by means of sputtering-XPS. A portion of the sample was exposed to the plasma, while the rest of the sample was covered by stainless steel and thus shielded from direct plasma flux. The results show that the part of the sample exposed to the plasma exhibits both on and beneath the surface an increased amount of H2O and MoO2 with respect to the covered sample part which can be explained by a chemical reduction of MoO3 oxides by the hydrogen plasma species. The XPS Cs lines show oxidation peaks. A sputtering depth of roughly 13 nm is necessary to recover an identical spectrum, indicating that the plasma changes the chemical composition of the implanted samples not only on the surface but also inside the material.

Erosion research of CX-2002U carbon composites under low-temperature high-flux hydrogen plasma

The net erosion yield of CX-2002U carbon fiber composites under high-flux low-temperature hydrogen plasma is investigated using a linear plasma device. It is found that the net erosion yield decreases rapidly first, and then tends to saturate with the increase of hydrogen–plasma flux. When the temperature of the sample eroded by hydrogen plasma is above 300 °C, the hybridization of electrons outside the carbon atom would change. Then the carbon atoms combine with hydrogen atoms to form massive spherical nanoparticles of hydrocarbon compounds and deposit on the surface at the flux condition of 1.77 × 1022 m−2·s−1. Under the irradiation of hydrogen plasma loaded with negative bias, the surface morphology of the matrix carbon is changed dramatically. Moreover, the energy dependence of mass loss does not increase in proportion to the increase of hydrogen–plasma energy, but reaches a peak around 20 V negative bias voltage. Based on the analysis of different samples, it can be concluded that the enhancement of energy could make a contribution to chemical erosion and enlarge the size of pores existing on the surface.

Work function performance of a C12A7 electride surface exposed to low pressure low temperature hydrogen plasmas

The calcium aluminate electride [Ca24Al28O64]4+(e−)4 (C12A7:e−) is chemically and thermally stable in ambient atmosphere, offers strong electron donating capabilities and exhibits an intrinsic work function of 2.4 eV. Thus, it is an attractive material for electron emitter and low work function applications. For an implementation in negative hydrogen ion sources, where the electride could serve as a converter surface for the H− production, compatibility with moderate vacuum conditions and exposure to low pressure low temperature hydrogen plasmas must be ensured. Hence, the stability and work function performance of a polycrystalline C12A7:e− sample are investigated under ion source relevant vacuum (∼10−6 mbar) and plasma conditions (electron temperature ∼2 eV, electron density ∼1016m−3). The work function is measured via the photoelectric effect, showing that long-term hydrogen plasma exposure results in a steady-state surface work function of 2.75±0.10 eV. Since the electride is very sensitive to surface contaminations, an increase of the surface work function occurs as soon as the plasma is switched off. Vacuum heat treatment of the degraded C12A7:e− surface after plasma activation leads to a work function minimum of 2.9 eV for temperatures in the range of 670–770 K. Exceeding a temperature of ∼770 K leads to an increase of the C12A7:e− surface work function. The sample is stable during long-term hydrogen plasma exposure (up to 10 h total plasma-on time tested) and no plasma-induced erosion is observed.

Effect of a low pressure low temperature hydrogen plasma on the work function of europium

Europium is a ductile rare-earth element and has the lowest function among lanthanides (2.5 eV). Due to its low work function reachable without an activation procedure, Eu is a potential electron emitter. For its application to negative hydrogen ion sources for fusion, the work function of an Eu sample exposed to a low pressure low temperature hydrogen plasma is investigated here. First, a work function of 2.6±0.1 eV is achieved in the present vacuum conditions (∼10−6 mbar) by heating the sample up to 350°C. A short hydrogen plasma pulse of 1 min allows us to achieve the same low work function; however, a longer plasma exposure leads to an increase of the work function, which must be counteracted by heating the sample up to at least 400°C to maintain the low work function during plasma. Furthermore, the exposure to hydrogen plasma affects the reactivity of Eu to oxidation when it is exposed to air, showing evidence of a faster oxidation in the areas previously in contact with the plasma. In conclusion, due to the strong reaction to hydrogen plasma and the rapid oxidation in air, Eu is not suitable for application in negative ion sources for fusion.

Theoretical study of the T+D2 and D+T2 collisions in low-temperature hydrogen plasmas

The study of low-temperature hydrogen plasmas is relevant for many technological plasma applications. The theoretical investigation of elementary processes, especially those involving H, D, and T species, is crucial for the modeling of nonequilibrium plasma systems such as those found in fusion plasmas. We report fully quantum time-independent calculations of rate coefficients for the vibrationally inelastic and reactive collisions of T with ${\mathrm{D}}_{2}(v=0)$ and of D with ${\mathrm{T}}_{2}(v=0)$. Our calculations are based on the ${\mathrm{H}}_{3}$ global potential-energy surface of Mielke et al. [S. L. Mielke et al., J. Chem. Phys. 116, 4142 (2002)]. We obtain state-to-state rate coefficients for the formation of DT through $\mathrm{T}+{\mathrm{D}}_{2}$ and $\mathrm{D}+{\mathrm{T}}_{2}$ reactions and for the vibrational excitation of ${\mathrm{D}}_{2}$ by T and of ${\mathrm{T}}_{2}$ by D for temperatures up to 2500 K. The results are compared to existing ones for other hydrogen isotopes and significant differences are found between the different sets of results, confirming that specific data have to be obtained for all the collisional systems of interest of hydrogen plasmas. We expect that this data will allow a more accurate modeling of hydrogen plasmas of interest for technological applications.

Study of gasdynamic electron cyclotron resonance plasma vacuum ultraviolet emission to optimize negative hydrogen ion production efficiency.

Negative hydrogen ion sources are used as injectors into accelerators and drive the neutral beam heating in ITER. Certain processes in low-temperature hydrogen plasmas are accompanied by the emission of vacuum ultraviolet (VUV) emission. Studying the VUV radiation, therefore, provides volumetric rates of plasma-chemical processes and plasma parameters. In the past, we have used gasdynamic ECR discharge for volumetric negative ion production and investigated the dependencies between the extracted H- current density and various ion source parameters. It was shown that it is possible to reach up to 80 mA/cm2 of negative ion current density with a two electrode extraction. We report experimental studies on negative hydrogen ion production in a high-density gasdynamic ECR discharge plasma consisting of two simple mirror traps together with the results of VUV emission measurements. The VUV-power was measured in three ranges-Lyα, Lyman band, and molecular continuum-varying the source control parameters near their optima for H- production. It was shown that the molecular continuum emission VUV power is the highest in the first chamber while Lyα emission prevails in the second one. Modifications for the experimental scheme for further optimization of negative hydrogen ion production are suggested.

Formation of CuCo Alloy From Their Oxide Mixtures Through Reduction by Low-Temperature Hydrogen Plasma

Formation of CuCo Alloy From Their Oxide Mixtures Through Reduction by Low-Temperature Hydrogen Plasma

Etch Control and SiGe Surface Composition Modulation by Low Temperature Plasma Process for Si/SiGe Dual Channel Fin Application

In this study, we discuss Si-SiGe etch characteristics as well as SiGe surface composition modification. It is required to etch Si and SiGe simultaneously for Si/SiGe dual channel Fin-FETs. Therefore, etch control of these two materials is desired. However, not only halogen chemistries but also physical sputtering etch SiGe selective to Si. We found that Si can be etched faster than SiGe by hydrogen plasma. Our analysis presents that hydrogen bonds selectively with Si rather than Ge, which leads to Si selective removal. As for SiGe surface modification, realizing Si-rich surface in SiGe is known to improve SiGe/high-k interface quality in advanced CMOS. It is also presented that the low-temperature hydrogen plasma induces Si-surface segregation (i.e., Si-rich surface) in SiGe, which is confined near the top-surface region. We proposed this may be caused by ion-energy-driven surface reaction. Our study also shows that Ge/Si ratio increases with plasma exposure time, which has correlation with surface roughness. Using the hydrogen plasma and conventional halogen plasma, we successfully demonstrate to etch Si/SiGe dual channel fins with depth and CD value control.

Modification mechanisms of silicon thin films in low-temperature hydrogen plasmas

To achieve quasi-atomic precision etching of thin film materials in advanced transistors, a method based on light ion implantation and consisting of two sequential steps—(1) surface modification in hydrogen ICP or CCP plasmas, and (2) selective removal of modified layers in wet solutions or remote plasmas—was recently proposed. In this paper, to better understand this process (called Smart Etch thereafter), molecular dynamics simulations are performed to study the modification of crystalline Si (1 0 0) substrates under low-energy (x = 1–3) ion implantation and mixed ion/H radical bombardment. In agreement with the experiments, simulations of (x = 1–3) ion bombardment of Si show a self-limited implantation with a surface evolution composed of two stages: a rapid volume modification (with no etching) followed by a slow saturation and the formation of a stable [a:Si–H] layer at a steady state. The mechanisms of ion-induced damage (Si–Si bond breaking, formation of H2 molecules, creation of SiHx groups) are investigated and allow us to bring new insights to both the well-known Smart Cut™ and more recent Smart Etch technologies. Si exposure to both ions and H radicals (H2 plasma) also shows a self-limited transformation but the modified layers are simultaneously etched during the implantation. Both the modified layer thickness and the ion dose required to reach the steady state increase with the ion energy. The ion composition and the radical-to-ion flux ratio (Γ) must be considered as well. In particular, the etch rate strongly increases with Γ, compromising even the possibility to achieve a Smart Etch of silicon.

In2O3 nanofibers surface modified by low-temperature RF plasma and their gas sensing properties

Hierarchical structure In2O3 nanofibers were synthesized by traditional electrospinning technology. Then, In2O3 nanofibers were treated 30 min by using low-temperature RF oxygen plasma and hydrogen plasma, respectively. The morphology, structure, composition and element content of the In2O3 nanofibers modified by oxygen plasma and hydrogen plasma were characterized and analyzed by XRD, SEM, BET and XPS. After modification by plasma, the morphology, structure and element contents of the In2O3 nanofibers were significantly changed. Gas sensors were fabricated based on In2O3 nanofibers modified by oxygen plasma and hydrogen plasma, respectively. Gas sensing properties of modified In2O3 sensors were investigated by a static test system to a variety of VOC gases in the concentration range of 0.3–500 ppm. The test result shows that In2O3 gas sensor modified by oxygen plasma exhibits excellent acetone sensing properties, It indicates that surface adsorption properties of In2O3 nanofibers can be improved by surface modification technology using low-temperature RF plasma, so as to improve the gas sensing properties of sensors.

Plasma-assisted oxide removal from ruthenium-coated EUV optics

An experimental study of oxide reduction at the surface of ruthenium layers on top of multilayer mirrors and thin Ru/Si films is presented. Oxidation and reduction processes were observed under conditions close to those relevant for extreme ultraviolet lithography. The oxidized ruthenium surface was exposed to a low-temperature hydrogen plasma, similar to the plasma induced by extreme ultraviolet radiation. The experiments show that hydrogen ions are the main reducing agent. Furthermore, the addition of hydrogen radicals increases the reduction rate beyond that expected from simple flux calculations. We show that low-temperature hydrogen plasmas can be effective for reducing oxidized top surfaces. Our proof-of-concept experiments show that an in situ, EUV-generated plasma cleaning technology is feasible.

Low-Temperature RF Plasma Treatment Effect on Junctionless Pd-Al2O3-Ingaas Misfet Operation

The junctionless (JL) device concept for silicon-on-insulator MOSFETs was introduced by Colinge et al. in 2010 [1], demonstrating considerable gains in terms of process simplicity when compared to conventional inversion-mode MOSFETs. The objective of this work is to implement the JL device concept in an In0.53Ga0.47As channel, where the SiO2 insulator in [1] is replaced by a wider bandgap p-type In0.52Al0.48As barrier layer and to investigate the effect of low-temperature RF hydrogen plasma treatment on the main electrical parameters of such devices. The JL device architecture is particularly well suited to III-V channel materials. Firstly, the high doping concentration (Nd) present in the channel of a JL MOSFET is less problematic for In0.53Ga0.47As than it is for Si. Indeed, the bulk electron mobility in Si is ~100 cm2/V·s at Nd = 1×1019 cm-3, while in In0.53Ga0.47As the bulk mobility is ~4,000 cm2/V·s at a similar Nd level. Moreover, the JL architecture circumvents the difficulties associated with the implantation or regrowth techniques generally used to form the source/drain (S/D) regions of III-V inversion-mode MOSFETs. However in order to form a good Ohmic S/D contacts it is necessary to anneal the device at 350-400°C that could lead to diffusion of In atoms into the gate dielectric. In this work low-temperature RF plasma treatment (RFPT) [2] is proposed to improve S/D contacts and enhance the properties of JL MOSFETs. A structure consisting of a 32-nm-thick n- In0.53Ga0.47As (Nd = 2×1018 cm-3) on a 500-nm-thick p-In0.52Al0.48As (Na = 8×1015 cm-3) barrier was grown by metal organic vapour-phase epitaxy (MOVPE) on a p+-InP wafer (Fig. 1 (a)). In order to form a channel for the JL devices, the In0.53Ga0.47As layer was thinned using a 10% H2O2/10% HCl digital wet etching process to achieve channel thicknesses of 24, 20 and 16 nm. A gate enclosed device layout was employed to simplify the fabrication process flow (Fig. 1(b-e)). A surface passivation in 10% (NH4)2S for 30 min was performed before atomic layer deposition (ALD) of an 8.5-nm-thick Al2O3 gate oxide. A Pd gate was formed by e-beam evaporation and lift-off. The Al2O3 on the S/D contact areas was etched in dilute HF. The S/D contact formation was performed by e-beam evaporation of a Au/Ge/Au/Ni/Au stack and lift-off. The RFPT (13.6 MHz) was performed in forming gas (10%H2+90%N2) with additional heating of the sample holder (up to 200°C). Temperature of the samples at the RFPT did not exceed 250°C [2]. Well behaved Id-Vg characteristics measured on the 24-nm-thick In0.53Ga0.47As channel device are shown in Fig. 2. A threshold voltage (VT) of -0.55 V was extracted using the second-derivative method. Scaling the In0.53Ga0.47As channel thickness down to 16 nm improved the subthreshold swing (SS) but degraded the maximum Id and ION/IOFF due to the combined effect of S/D series resistance (RSD) and mobility degradation. As a result, the lowest SS value (115 mV/dec) was extracted on the 16-nm-thick In0.53Ga0.47As channel device and 190 mV/dec - on the 24-nm-thick In0.53Ga0.47As channel one, but the highest ION/IOFF value (1.5×105) was obtained on the 20-nm-thick In0.53Ga0.47As channel device and 2.0×104 - on the 24-nm-thick In0.53Ga0.47As channel one. The gate-to-channel Cgc-Vg characteristic featured a low frequency dispersion near the accumulation region, suggesting a low density of interface traps (Dit) in the upper part of the In0.53Ga0.47As bandgap. Moreover, the conductance analysis yielded low Dit values ranging from 1.5×1012 to 3.5×1012 cm-2×eV-1 from the conductance band edge to midgap, correspondingly. Measurements of RSD on circular transfer length method (CTLM) structures yielded the value of 12 kOhm on the 20-nm-thick In0.53Ga0.47As channel devices and contacts were strongly rectifying. RF plasma treatment with power 0.5 W/cm2 at 150°C resulted in Ohmic SD contact, a strong decrease of RSD down to 45 Ohm (Fig. 3) and increases the ION by two orders of magnitude (Fig. 2). Additionally, SS for the plasma treated devices decreases from 190 mV/dec to 150 mV/dec, and the threshold voltage shifts to positive values by 0.4 V (Fig.2). This is associated, correspondingly, with a decrease of Dit and negative charge in the gate dielectric. The conductance analysis yielded reduction of Dit at midgap down to 1.0×1012 cm-2×eV-1. [1] Colinge J.-P., Lee C.-W., Afzalian A., Akhavan N.D., Yan R., Ferain I., Razavi P., O'Neill B., Blake A., White M., Kelleher A.-M., McCarthy B., Murphy R. Nanowire Transistors Without Junctions. Nature Nanotechnology. Vol. 5, P. 225-229 (2010). [2] Nazarov A.N., Lysenko V.S., Nazarova T.M. Hydrogen Plasma Treatment of Silicon Thin-film Structures and Nanostructured Layers. Semiconductor Physics, Quantum Electronics & Optoelectronics. Vol. 11, P. 101-123 (2008). Figure 1

Hydrogen loss and its improved retention in hydrogen plasma treated a-SiN :H films: ERDA study with 100 MeV Ag7+ ions

Hydrogen loss from a-SiNx:H films under irradiation with 100 MeV Ag7+ ions using elastic recoil detection analysis (ERDA) experiments is reported. The results are explained under the basic assumptions of the molecular recombination model. The ERDA hydrogen counts are composed of two distinct hydrogen desorption processes, limited by rapid molecular diffusion in the initial stages of irradiation, and as the fluence progresses a slow process limited by diffusion of atomic hydrogen takes over. Which of the aforesaid processes dominates, is determined by the continuously evolving Hydrogen concentration within the films. The ERDA measurements were also carried out for films treated with low temperature (300 degrees centigrade) hydrogen plasma annealing (HPA). The HPA treated films show an improved diffusion of atomic hydrogen, resulting from healing of weak bonds and passivation of dangling bonds. Further, upon HPA, films also show evolution of hydrogen with significantly higher counts, at advanced fluences, relative to the as-deposited films. These results indicate the potential of HPA towards improved H retention in a-SiNx:H films. The study distinguishes clearly the presence of two diffusion processes in a-SiNx:H whose diffusion rates differ by an order of magnitude, with hydrogen radicals not being able to diffuse beyond ~1 nm from the point of their creation. The results are very relevant for the passivation applications of a-SiNx:H

Formation of Copper–Nickel Alloy from Their Oxide Mixtures Through Reduction by Low-Temperature Hydrogen Plasma

The paper presents experimental results of a new method of production of nanoparticles of copper–nickel alloys from the reduction of the mixture of cupric oxide (CuO) and nickel oxide (NiO) by low-temperature hydrogen plasma in a microwave assisted hydrogen plasma set-up. The microwave power and hydrogen flow-rate used for the current investigation are 750 W and 2.5 × 10−6 m3 s−1 respectively. The addition of NiO–CuO, in proportion to result in alloys of 90Cu:10Ni and 70Cu:30Ni, not only removed the induction period from the kinetic plot of CuO reduction but also, improved the reduction rate of CuO. The XRD analysis of the product exhibited a single-phase peak with a d-spacing lying between Cu and Ni, which satisfies the Vergard’s law, indicating the Cu–Ni alloy formation. The lattice parameter decreases from 3.6221 (90Cu:10Ni) to 3.595 Ǻ (70Cu:30Ni), due to the smaller atomic radius of Ni (0.1246 nm) than that of Cu (0.1278 nm). The crystallite size, calculated by applying Scherrer’s formula, in both cases is found to be 31.7 nm.

Reduction of Oxide Mixtures of (Fe2O3 + CuO) and (Fe2O3 + Co3O4) by Low-Temperature Hydrogen Plasma

The paper presents experimental results pertaining to the reduction of oxide mixtures namely (Fe2O3 + CuO) and (Fe2O3 + Co3O4), by low-temperature hydrogen plasma in a microwave hydrogen plasma set-up, at microwave power 750 W and hydrogen flow rate 2.5 × 10−6 m3 s−1. The objective was to examine the effect of addition of CuO or Co3O4, on the reduction of Fe2O3. In the case of the Fe2O3 and CuO mixture, oxides were reduced to form Fe and Cu metals. Enhancement of reduction of iron oxide was marginal. However, in the case of the Fe2O3 and Co3O4 mixture, FeCo alloy was formed within compositions of Fe70Co30, to Fe30Co70. Since the temperature was below 841 K, no FeO formed during reduction and the sequence of Fe2O3 reduction was found to be Fe2O3 → Fe3O4 → Fe. Reduction of Co3O4 preceded that of Fe2O3. In the beginning, the reduction of oxides led to the formation of Fe–Co alloy that was rich in Co. Later Fe continued to enter into the alloy phase through diffusion and homogenization. The lattice strain of the alloy as a function of its composition was measured. In the oxide mixture in which excessive amount of Co3O4 was present, all the Co formed after reduction could not form the alloy and part of it appeared as FCC Co metal. The crystallite size of the alloy was in the range of 22–30 nm. The crystal size of the Fe–Co alloy reduced with an increase in Co concentration.

Increase atom/molecular ratio of the hydrogen discharge

The high atomic to molecular ion ratio is important for hydrogen ion sources, but in the usual low-temperature hydrogen plasma the molecular ion component is dominating. Proton component can be increased by means of increasing plasma temperature, but in this case energy spectrum will be widened. Single electron collision with a hydrogen molecule may not only lead to the formation of the molecular ion, but also results in the formation of the proton. However, the cross section of the second process is much smaller than of the first (approximate ratio between them being 1:200). Thus, a single electron-molecule collision cannot be an efficient means of proton formation. Atomic hydrogen ions are produced by ionization of neutral hydrogen atoms. As a rule, a lifetime of neutral atoms is too short for ionization. The increase of the hydrogen atom lifetime by reducing the wall recombination coefficient allows us to get more protons. In this work we study the influence of water vapor injection on atom/molecular ratio of hydrogen plasma.

Progress of the ELISE test facility: towards one hour pulses in hydrogen

In order to fulfil the ITER requirements, the negative hydrogen ion source used for NBI has to deliver a high source performance, i.e. a high extracted negative ion current and simultaneously a low co-extracted electron current over a pulse length up to 1 h. Negative ions will be generated by the surface process in a low-temperature low-pressure hydrogen or deuterium plasma. Therefore, a certain amount of caesium has to be deposited on the plasma grid in order to obtain a low surface work function and consequently a high negative ion production yield. This caesium is re-distributed by the influence of the plasma, resulting in temporal instabilities of the extracted negative ion current and the co-extracted electrons over long pulses. This paper describes experiments performed in hydrogen operation at the half-ITER-size NNBI test facility ELISE in order to develop a caesium conditioning technique for more stable long pulses at an ITER relevant filling pressure of 0.3 Pa. A significant improvement of the long pulse stability is achieved. Together with different plasma diagnostics it is demonstrated that this improvement is correlated to the interplay of very small variations of parameters like the electrostatic potential and the particle densities close to the extraction system.