Volume Plasma Source Research Articles

An ion species model for positive ion sources: II. Analysis of hydrogen isotope effects

A one-dimensional model of the magnetic multipole volume plasma source has been developed for application to intense ion/neutral atom beam injectors. The model uses plasma transport coefficients for particle and energy flow to create a detailed description of the plasma parameters along an axis parallel to that of the extracted beam. In this paper the isotopic modelling of positive hydrogenic ions is considered and compared with experimental data from the neutral beam injectors of the Joint European Torus. The use of the code to gain insights into the processes contributing to the ratios of the ionic species is demonstrated and the conclusion is drawn that 75% of the atomic ion species arises from ionization of dissociated molecules and 25% from dissociation of the molecular ions. However, whilst the former process is independent of the filter field, the latter is sensitive to the change in distribution of fast and thermal electrons produced by the magnetic filter field and an optimum combination of field strength and depth exists. Finally, the code is used to predict the species ratios for the JET source operating in tritium and hence the neutral beam power injected into the plasma in the JET tritium campaign planned for 2016.

An ion species model for positive ion sources: I. Description of the model

A one-dimensional model of the magnetic multipole volume plasma source has been developed for use in intense ion/neutral atom beam injectors. The model uses plasma transport coefficients for particle and energy flow to create a detailed description of the plasma parameters along an axis parallel to that of the extracted beam. Primarily constructed for applications to neutral beam injection systems on fusion devices, the model concentrates on the hydrogenic isotopes but can be extended to any gas by substitution of the relevant masses, cross-sections and rate coefficients. The model considers the flow of fast ionizing electrons that create the ratios of the three hydrogenic isotope ion species, H+, , (and similarly for deuterium and tritium) as they flow towards the beam extraction electrode, together with the production of negative hydrogenic ions through volume processes. The use of detailed energy balance in the discharge allows the determination of the fraction of the gas density that is in an atomic state and also the gas temperature as well as the electron temperatures and plasma potential. Comparisons are made between the results of the model and experimental measurements in deuterium from a number of different filament driven sources used on beam heating facilities.

Industrial plasmas in academia

The present review, written at the occasion of the 2014 EPS Innovation award, will give a short overview of the research and development of industrial plasmas within the last 30 years and will also provide a first glimpse into future developments of this important topic of plasma physics and plasma chemistry. In the present contribution, some of the industrial plasmas studied at the CRPP/EPFL at Lausanne are highlighted and their influence on modern plasma physics and also discharge physics is discussed. One of the most important problems is the treatment of large surfaces, such as that used in solar cells, but also in more daily applications, such as the packaging industry. In this contribution, the advantages and disadvantages of some of the most prominent plasmas such as capacitively- and inductively-coupled plasmas are discussed. Electromagnetic problems due to the related radio frequency and its consequences on the plasma reactor performance, and also dust formation due to chemical reactions in plasma, are highlighted. Arcing and parasitic discharges occurring in plasma reactors can lead to plasma reactor damages. Some specific problems, such as the gas supply of a large area reactor, are discussed in more detail. Other topics of interest have been dc discharges such as those used in plasma spraying where thermal plasmas are applied for advanced material processing. Modern plasma diagnostics make it possible to investigate sparks in electrical discharge machining, which surprisingly show properties of weakly-coupled plasmas. Nanosecond dielectric barrier discharge plasmas have been applied to more speculative topics such as applications in aerodynamics and will surely be important in the future for ignition and combustion.Most of the commonly-used plasma sources have been shown to be limited in their performance. Therefore new, more effective plasma sources are urgently required. With the recent development of novel resonant network antennas for new advanced large area or large volume plasma sources, an important step towards high performance plasmas and new fast processes is made.

Cylindrical rf network antennas for coupled plasma sources Copper legs delayed in time system stability analysis

— In this article, Very Crucial subject discussed cylindrical (closed) RF network antennas for coupled plasma sources copper legs delayed in time system stability analysis. Resonant RF network antennas are important to plasma sources with many applications. The cylindrical resonant RF network antennas run as large volume plasma sources and have stability switching due to system's copper legs parasitic effects. The cylindrical RF network antenna structure is 16-leg cylindrical (Birdcage) RF antenna which has electrical circuit and opposite points of RF feeding and grounding. The vacuum vessel is a glass cylinder closed at the top and bottom by grounding metal plates. Generally there are two popular different resonant RF network assemblies: a cylindrical and a planar RF antenna. The cylindrical RF antenna is built as a high-pass Birdcage coil. The antenna is mounted outside a glass tube. The RF antenna consists of 16 copper legs equally spaced interconnected with capacitors. Due to RF antenna copper leg parasitic effect we get copper leg's current and current derivative with delay τ1-k and τ2-k (k is leg number index, k=1,…,16). The uncooled antenna is fed at the midpoint and operated with opposite grounded. Alternatively, it can be fed by another transmitter unit. Due to cylindrical antenna parasitic delayed in time, there is a stability issue by analyzing its operation. We consider for simplicity that all copper leg's current parasitic time delayed are equal (τ1-1= τ1-2=… =τ1-16) and current derivative parasitic time delayed are equal (τ2-1= τ2-2=… =τ2-16). The cylindrical RF network antennas delayed in time equivalent circuit can represent as a delayed differential equations which depend on variable parameters and delays. The investigation of our cylindrical network antenna with copper leg system, a differential equation is based on bifurcation theory [1], a study of possible changes in the structure of the orbits of a delayed differential equation depending on variable parameters. Cylindrical RF network antenna analysis is done under two series of different time delays respect to antenna's copper legs current and current derivative. All of that for optimization of a cylindrical RF network antenna circuit parameter analysis to get the best performance. The cylindrical network antenna with copper leg system can be represented as delayed differential equations which, depending on variable parameters and delays. There is a practical guideline that combines graphical information with analytical work to effectively study the local stability of models involving delay dependent parameters. The stability of a given steady state is determined by the graphs of some function of τi-1, ..,τi-16 (i=1, 2) [2] [3] [4].

Feasibility of a low-voltage discharge in pure molecular hydrogen

The feasibility of initiating a low-voltage discharge in pure (free of readily ionizable impurity) molecular hydrogen is studied theoretically. A discharge with cathode fall φ1 = 10 V, interelectrode gap L = 2 cm, and total hydrogen concentration \(N_{H_2 }^{(0)} = 2 \times 10^{15}\) cm−3 is considered by way of example. The plasma parameters, including concentration \(N_{H^ - }\) of negative hydrogen ions H−, are calculated. The concentration of H− ions is maximal in the near-anode plasma and may reach \(\left( {N_{H^ - } } \right)_{\max } = 0.5 \times 10^{12}\) cm−3. Concentration \(N_{H^ - }\) can be increased severalfold by introducing a small amount of cesium into the discharge, \(N_{Cs}^{(0)} \leqslant 10^{13}\) cm−3. Cesium ionizes completely and concentrates in narrow near-electrode layers, which are depleted with the plasma in the purely hydrogen discharge. The discharge modifications studied in this work may be of interest as low-voltage volume plasma sources of H− ions under conditions when a high concentration of cesium in the source plasma is undesirable.

Conditions for the formation of an H−/D− ion beam in a volume plasma source with a magnetic filter

A one-dimensional model for the transport of a plasma with two ion species across the magnetic field in a steady-state volume negative-ion source is proposed. The conditions in the magnetic filter adjacent to the plasma electrode optimum for the generation, formation, and extraction of an H−/D− ion beam are found. The theoretical results are in good agreement with the experimental data.

The increase of the effective rate constant of the dissociative attachment of electrons to hydrogen molecules due to the H2 vibrational excitation in a hydrogen stream flowing in the channel

There is a significant interest in the production of vibrationally excited H2 molecules because of their application to H- volume-plasma sources, plasma chemistry etc. We consider the vibrational kinetics in a hydrogen flow, which passes through a channel, the H2 molecules being previously excited in a hydrogen discharge. It is shown that an essential increase of the population of high H2 vibrational levels may be achieved in the hydrogen flow by means of a proper choice of channel wall material and by the appropriate organization of the flow. As a result, the significant enhancement of the effective rate constant of volume-plasma H- generation due to the dissociative attachment may be achieved. The effect depends only upon the mean H2 vibrational energy in the inlet of the channel.

Two-dimensional particle simulation of negative ion extraction from a volume source

Abstract Two-dimensional electrostatic particle simulation was done to study the extraction of negative ions from a volume plasma source. The simulation model is a rectangular system which consists of an extraction grid, a plasma grid, and a grounded wall. Full dynamics of electrons, ions, and negative ions are followed. Negative ions are extracted from the plasma region to the extraction grid through a slit in the plasma grid. for the lower value of extraction grid potential, the simulation results agree with the Child-Langmuir law, where the extracted negative ion current is proportional to the three-halves power of the potential of the extraction grid. For the higher value of extraction grid potential, the space charge effect of negative ions, which enter into the beamline at the top of the concavity of the positive ion boundary, reduces the negative ion current from the prediction of the Child-Langmuir law.

Extraction of intense H−beams from volume plasma sources

Abstract The rapid development of H− sources over the past few years has reauired the design and construction of accelerators which can collimate H∼ beams as well as suppress co-extracted electrons. This in tum has led to modifications in the design of the plasma source namely the incorporation of intense local magnetic fields to inhibit electron drift in the plasma and also the addition of cesium as a catalyst. Results from these experiments are presented and their significance for future development is assessed.

Particle simulation on extraction of positive and negative ions from a volume source

A two dimensional electrostatic particle simulation was done to study the extraction of positive (negative) ions from a volume plasma source. The simulation model is a rectangular system which consists of an extraction grid (left wall), a plasma grid, and a grounded wall (right wall). Upper and lower boundaries are connected by the periodic boundary condition. Full dynamics of charged particles are followed. Positive (negative) ions are extracted from the plasma region through a slit in the plasma grid to the extraction grid. Electrons are reflected by the magnetic filter and confined in the region to the right of the magnetic filter. Simulation results are compared with the Child–Langumuir law where the extracted ion current is proportional to the three-halves power of the potential of the extraction grid. In the case of the positive ion extraction, simulation results agree quite well with the Child–Langumuir law. Whereas, in the case of the negative ion extraction, simulation results agree with the law only for the lower value of extraction grid potential.

Development of H− and D− ion sources for plasma heating

The large size of future, and possibly present day, tokamaks requires high neutral atom beam energies for current drive and heating. These injectors must also have a high electrical efficiency and this can be achieved using negative ion sources to create the beam. These large tokamaks, such as ITER or NET require a neutral beam power of around 50–100 MW at beam energy of 1 MeV of D0. This power is achieved by operating several sources in parallel, each one of which would extract a 28-A beam of D− ions of which 57% are converted to D0 in a gas cell neutralizer and can be transmitted to the torus. The injector for one port consists of four units, each of which is a fully independent unit. This article describes the details of the injector unit which is based on volume plasma sources of D− ions.