Ion Production Region Research Articles

Investigation of the Radio-Frequency Discharge in a High Current Negative Hydrogen Ion Source With a Global Enhanced Vibrational Kinetic Model

A numerical investigation of the radio-frequency hydrogen discharge in the high current negative hydrogen ion source (HCNHIS) is presented using a global enhanced vibrational kinetic model (GEVKM). The HCNHIS consists of a high-pressure (2–65 torr) radio-frequency discharge chamber where the main production of high-lying vibrational states of the hydrogen molecules occurs. The hydrogen plasma flow in the discharge chamber is reduced by a series of bypass tubes and enters through a nozzle into a low-pressure (1–15 mtorr) negative hydrogen ion production chamber where H− are generated mainly by the dissociative attachment of low-energy electrons to rovibrationally excited hydrogen molecules. The GEVKM is applied to the HCNHIS discharge and involves volume-averaged equations for 21 hydrogen species (atoms, ions, and molecules in excited states) and electrons. The GEVKM is supplemented with outlet boundary conditions for the nozzle and bypass tubes of the HCNHIS and accounts for compressibility, viscous, and rarefaction effects. GEVKM simulations of the RF discharge are performed with inlet flow rates of 5–1000 sccm and absorbed powers of 200–1000 W using the HCNHIS-2 design which is configured with an extractor grid attached to a short negative ion production region. These simulations investigate the effects of the absorbed power and the inlet flow rate on the chemical composition, electron and heavy particles temperature, wall temperature, the maximum extractable H− current in the discharge chamber, as well as optimum operational parameters of HCNHIS-2. GEVKM simulations of the HCNHIS-2 discharge are used to obtain estimates of the H− current and compared with Faraday cup measurements taken at the extraction grid.

A Global Enhanced Vibrational Kinetic Model for High-Pressure Hydrogen RF Discharges

A global enhanced vibrational kinetic model (GEVKM) is developed for multitemperature, chemically reacting hydrogen plasmas in inductively coupled cylindrical discharges for lowto high-pressure regimes. The species in a GEVKM are ground-state hydrogen atoms H and molecules H <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> , 14 vibrationally excited hydrogen molecules H <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> (v), v = 1 - 14, electronically excited hydrogen atoms H(2) and H(3), groundstate positive ions H <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">+</sup> , H <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">+</sup> , and H <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sub> <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">+</sup> , ground-state negative ions H <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">-</sup> , and electrons e. The GEVKM involves volume-averaged steady-state continuity equations for the plasma species, an electron energy equation, a total energy equation, a heat transfer equation to the chamber walls, and a comprehensive set of surface and volumetric chemical processes governing vibrational and ionization kinetics of hydrogen plasmas. The GEVKM is verified and validated by comparisons with previous numerical simulations and experimental measurements of a negative hydrogen ion source in the low-pressure (20-100 mtorr), low-absorbed-power-density (0.053-0.32 W/cm <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sup> ) regime and of a microwave plasma reactor in the intermediate to high-pressure (1-100 torr), high-absorbed-power-density (8.26-22 W/cm <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sup> ) regime. The GEVKM is applied to the simulation of a high-current negative hydrogen ion source (HCNHIS). The HCNHIS consists of a high-pressure (20-65 torr) radio-frequency discharge chamber in which the main production of high-lying vibrational states of the hydrogen molecules occurs, a bypass system, and a low-pressure (0.1-0.4 torr) negative hydrogen ion production region where negative ions are generated by the dissociative attachment of low-energy electrons to rovibrationally excited hydrogen molecules. The discharge pressure and negative hydrogen ion current predicted by the GEVKM compare well with the measurements in the HCNHIS.

Flow Energy Control of Nitrogen Ions Generated by Electron Cyclotron Resonance

The flow energy of nitrogen ions produced by an electron cyclotron resonance (ECR) discharge is controlled by electrostatic acceleration in a plasma synthesis method. The source consists of an ion production region using ECR and a plasma synthesis region. Two grids for potential control and electron reflection of the ion production region, and an electron emitter with mesh shape are installed. In the synthesis region, electrons emitted from the emitter and ions accelerated by electrostatic potential difference between these regions are synthesized, which yields the generation of ion flow in the synthesis region. The electron temperature in the plasma synthesis region can be reduced to about 0.5 eV because of a reflection of high-energy electrons in the ion production region and a supply of thermionic electrons. In addition, it is observed that the density ratio of nitrogen atomic to molecular ions in the synthesis region markedly increases compared with that in the ion production region.

Development of a low temperature plasma source providing ion flow energy control

A low electron temperature plasma source suitable for control of ion flow energy along magnetic-field lines is developed, which is made up of an ion-production region and a plasma-synthesis region. A mesh-shaped electron emitter is installed in order to uniformly supply the electrons to the synthesis region. Our diagnoses using Langmuir probes and electrostatic ion energy analyzers prove that only the ion flow energy can be controlled under constant electron density and temperature. Moreover, our method is found to achieve a low electron temperature (Te ≃ 0.2–0.4 eV), yielding almost uniform radial and axial profiles of the averaged electron density, temperature, space potential and the ion flow energy.

Simultaneous control of ion flow energy and electron temperature in magnetized plasmas

Ion flow energy along magnetic-field lines is precisely controlled by electrostatic acceleration in magnetized collisional and synthesized plasmas. The source is made up of an ion-production region and a plasma synthesis region; an electron emitter of mesh shape is installed between the regions and supplies the electrons to the synthesis region. The ion flow is generated by an electrostatic potential difference between these regions. Our experimental results demonstrate that only the ion flow energy can be controlled under constant electron density and temperature. Moreover, the electron temperature is also controllable and could be reduced to less than 0.5eV.

Development of an r.f. driven negative hydrogen ion source

A large r.f. driven negative hydrogen ion source has been constructed. R.f. negative ion sources have the potential of continuous and efficient operation for next-step neutral beam injection systems. The developed r.f. negative ion source is large for this fusion application. The r.f. plasma generator is a metal-walled multi-cusp bucket source with tent magnetic filter and has dimensions of 30 × 30 cm 2 cross-section and 19 cm depth. An internal induction coil is used as an r.f. antenna. The negative ion accelerator, which consists of three grids, produces nine beamlets with an energy of 60 keV. The r.f. power amplifier system generates 2 MH – 30 kW for 1 s. The r.f. plasma was produced successfully under impedance matching condition. An ion current density of 150 mA cm −2 was obtained and the electron temperature was high, 5–10 eV, in the antenna vicinity. In the chamber center which is the negative ion production region, the electron temperature is still high, more than 3 eV. As a result, a negative ion current was not detected at an extraction energy of 1.2 keV.

Ion flow experiments in a multipole discharge chamber

It has been customary to assume that ions flow nearly equally in all directions from the ion production region within an electron-bombardment discharge chamber. Ion flow measurements in a multipole discharge chamber have shown that this assumption is not true. In general, the electron current through a magnetic field can alter the electron density, and hence the ion density, in such a way that ions tend to be directed away from the region bounded by the magnetic field. When this mechanism is understood, it becomes evident that many past discharge chamber designs have operated with a preferentially directed flow of ions.

A model for mercury orificed hollow cathodes - Theory and experiment

The model outlined here provides a useful qualitative description of the basic physical processes taking place within a mercury orificed hollow cathode and can predict, to first order, important cathode operating parameters such as emission length and insert temperature. The analytical formulation of the model is based on the concept of an idealized production region/' which is defined as the volume circumscribed by the emitting portion of the insert. The energy exchange mean free path for primary electrons is used as a criterion for determining the length Le of this region. An ion production region aspect ratio D/Le of 2 is suggested as a design criterion for minimizing the keeper voltage. The model accounts for electrons produced in the ion production region by surface emission and volume ionization. Surface and volume energy balances are used to predict plasma density and plasma potential in this region. An empirical relation is presented that can be used to estimate cathode internal pressure (a necessary input to the model) from the discharge current and cathode orifice diameter. Calculations based on the model are compared with experimental results.