Extracted Ion Current Density Research Articles

Numerical study of the collisionless interaction between positive and negative ion beams

We study, through two dimensional Particle-In-Cell simulations, the expansion of an ion-ion beam in vacuum. This beam is generated by a continuous extraction of positive and negative ions from two adjacent gridded ion sources. The grid systems are biased to extract and accelerate, continuously, positive and negative ions from two distinct ion sources. The ion sources are positioned such that their grid systems form an angle θ. In this work, we study two configurations, θ = π and θ = π/2. The proposed device constitutes an alternative approach to the usual positive ion beam neutralized by electrons. This work aims, on the one hand, to demonstrate that the neutralization of a continuously extracted ion beam space charge might be achieved by the use of oppositely charged ions. On the other hand, this work investigates the physical properties of the generated ion-ion beam. Our results show that, for θ = π, potential barriers form in the close vicinity of the acceleration grids. These potential barriers oppose to extraction and induce an ion backflow. This backflow increases with the increase in extracted ion current density. It represents ∼60% to ∼80% of extracted ions for an extracted ion current density ranging from 1 to 10 A/m2. Moreover, for θ = π/2, the potential barriers are located downstream the grids, typically at one source diameter. For this configuration, we found that the backflow is drastically reduced to about 25% of the extracted ions.

Extracted ion current density in close-coupling multi-antenna type radio frequency driven ion source: CC-MATIS

Positive ions are extracted by using a small extractor from the Close-Coupling Multi-Antenna Type radio frequency driven Ion Source. Two types of RF antenna are used. The maximum extracted ion current density reaches 0.106 A/cm(2). The RF net power efficiency of the extracted ion current density under standard condition is 11.6 mA/cm(2)/kW. The efficiency corresponds to the level of previous beam experiments on elementary designs of multi-antenna sources, and also to the efficiency level of a plasma driven by a filament in the same chamber. The multi-antenna type RF plasma source is promising for all metal high density ion sources in a large volume chamber.

Design and analyses of a one-dimensional CFC calorimeter for SPIDER beam characterisation

In the framework of the activities for the design of Padova Research on Injector Megavolt Accelerated (PRIMA), the beam source experiment SPIDER (Source for the Production of Ions of Deuterium Extracted from RF plasma), (SPIDER)] has a key role for the next generation of sources for the ITER Neutral Beam Injectors (NBIs). The main purpose of SPIDER test facility is the investigation and optimisation of a negative ion beam produced by the full size ion source for ITER NBIs, aiming at fulfilling ITER requirements in terms of extracted negative ion current density, its spatial uniformity and co-extracted electron current. At the exit of the beam source the negative ion beam is composed of an array of 4 × 4 beamlet groups, each one made of 16 × 5 beamlets. In order to study and optimise the beam uniformity and extracted ion current density, a movable calorimeter has been developed. The thermal image of the beamlet footprints that appears on the rear side of the calorimeter panels is observed by thermal cameras, which permit to reconstruct the beam shape. The calorimeter is V-shaped and composed of two panels made of one-dimensional Carbon Fibre Composite (CFC) tiles that can intercept the beam up to 10 s at its full power. The panel support system can remotely move forward/backward and can open/close the panels, in vacuum. The CFC can sustain the high heat power loads (up to 20 MW/m 2) and the fibres are oriented along the thickness in order to transmit a clear image through the tiles enhanced by very low thermal conductivity in the in-plane directions. Each tile is electrically insulated to give a measure of the ion current reaching it and it is instrumented with a thermocouple array for absolute calibration of thermal camera measurements. According to the physical and mechanical requirements, a feasibility analysis has been carried out and a preliminary design has been concluded. This paper gives an overview of the design and thermal and structural analyses.

Emissive Membrane Ion Thruster Concept

Experiments conducted on solid-state ionic membranes that convert atoms into ions that can in turn be extracted into energetic beamlets and thereby produce thrust are described. The capability of ionizing and transporting ions is shown to be both rapid (over 10 mA/cm 2 ) and efficient (of order 1 eV/ion). Although similar in appearance to cesium contact sources, solid-state ionic membrane devices are shown to operate successfully at lower temperatures (400 to 700 °C) thus making them much more efficient than cesium- based devices. The process of extracting the ions by field emission is shown to limit the extracted ion current density. The need to select an emissive surface material/propellant ion combination with a low work function is suggested. Based on successful demonstrations of ion extraction, the Emissive Membrane Ion Thruster is proposed and shown to offer the potential for substantial reductions in ion thruster system cost and complexity as well as improvements in scalability and reliability compared to existing ion thruster designs

Cold-cathode source of ribbon gaseous ion beams

A ribbon ion beam source based on a magnetically confined low-pressure glow discharge has been developed. The electrode system of the ion source consists of four series-installed discharge chambers of the inverted magnetron type with a thin-wire anode. Slots are made in walls between the chambers to equalize the linear distribution of the extracted ion current density. A two-electrode multislit ion optical system 44×3 cm in size forms an ion beam having an energy of up to 20 keV. The operation voltage of the glow discharge with a current of up to 1.6 A in a magnetic field of 2–6 mT at a gas pressure (argon) of 0.015–0.03 Pa is 350–650 V. The ion beam current accounts for 8%–14% of the discharge current. The inhomogeneity of the current density distribution along the ribbon beam axis is 10%–20%.

Operation of a cw rf driven ion source with hydrogen and deuterium gasa)

We will describe the operation of a cw rf driven multicusp ion source designed for extraction of high current hydrogen and deuterium beams. The source is driven at 2 MHz by a 2.5 turn induction antenna immersed in the plasma. Bare stainless-steel and porcelain-coated Cu antennas have been used. The plasma load is matched to the rf generator by a variable tap N:1 transformer isolated to 46 kV, and an LC network on the secondary. With H2 gas the source can be operated at pressures between 5 and 60 mT with power reflection coefficients <0.01. The extracted ion current density with a porcelain-coated antenna is approximately given by 35 mA/cm2/kW with an 80 G dipole filter field for input powers from 3.5 to 6.6 kW. The current density remained constant for operation with a 6 and an 8 mm aperture. The source has been operated for 260 h at 3.6 kW with a single-porcelain-coated antenna. Mass spectrometer measurements of the extracted beam at this power show a species mix for H+:H+2:H+3:OH+ of 0.49: 0.04: 0.42: 0.04. The calculated beam divergence using the IGUN code is compared with the measured divergence from an electrostatic sweep emittance scanner designed for high-power cw beam diagnostics. Phase space measurements at 40 kV and 23 mA beam current result in a normalized rms emittance of 0.09 π mm mrad.

Operation of a cw rf driven ion source with hydrogen and deuterium gas (abstract)

We will describe the operation of a cw rf driven multicusp ion source designed for extraction of high current hydrogen and deuterium beams. The source is driven at 2 MHz by a 2.5 turn induction antenna immersed in the plasma. Bare stainless-steel and porcelain-coated Cu antennas have been used. The plasma load is matched to the rf generator by a variable tap N:1 transformer isolated to 46 kV, and an LC network on the secondary. With H2 gas the source can be operated at pressures between 5 and 60 mT with power reflection coefficients <0.01. The extracted ion current density with a porcelain-coated antenna is approximately given by 35 mA/cm2/kW with an 80 G dipole filter field for input powers from 3.5 to 6.6 kW. The current density remained constant for operation with a 6 and an 8 mm aperture. The source has been operated for 260 h at 3.6 kW with a single-porcelain-coated antenna. Mass spectrometer measurements of the extracted beam at this power show a species mix for H+:H+2:H+3:OH+ of 0.49: 0.04: 0.42: 0.04. The calculated beam divergence using the IGUN code is compared with the measured divergence from an electrostatic sweep emittance scanner designed for high-power cw beam diagnostics. Phase space measurements at 40 kV and 23 mA beam current result in a normalized rms emittance of 0.09 π mm mrad.

Introduction of cesium vapor into the arc chamber of a high-brightness H−/D− volume source and resulting beam characteristicsa)

Addition of cesium vapor to H−/D− multicusp volume sources has been shown to result in significantly increased extracted ion current density accompanied by reduced electron current density. However, using a cesium oven, the initial introduction of Cs vapor causes high-voltage breakdowns that persist for extended periods. In the extreme case, source operation may no longer be possible without shutting down and thoroughly cleaning source components. If operation can be recovered, the increase in extracted ion current density persists for many days, usually until filament burn-out or other failure unrelated directly to Cs introduction. We have refined the procedure for Cs introduction to produce less catastrophic initial source operation and shorter recovery times to consistent operation. The new procedure has produced consistent H− currents of 35–42 mA at 30 keV. Higher currents, up to 64 mA, have been achieved for short periods soon after recovery. These data were obtained for a 6.4-mm-diam emission aperture using a modified Pierce geometry. In this paper, the new procedure for introduction of Cs will be described with the associated recovery characteristics. This paper will compare the emittance changes as a function of H− current, equivalent current, and plasma electrode bias. The behavior of the output over many days and shutdown-operation-shutdown cycles will also be described.

Introduction of cesium vapor into the arc chamber of a high-brightness H−/D− volume source and resulting beam characteristics (abstract)

Addition of cesium vapor to H−/D− multicusp volume sources has been shown to result in significantly increased extracted ion current density. However, using a cesium oven, the initial introduction of Cs vapor causes high-voltage breakdowns that persist for extended periods. In the extreme case, source operation may no longer be possible without shutting down and thoroughly cleaning source components. If operation can be recovered, the increase in extracted ion current density persists for many days, usually until filament burn-out or other failure unrelated directly to Cs introduction. We have refined the procedure for Cs introduction to produce less catastrophic initial source operation and shorter recovery times to consistent operation. The new procedure has produced consistent H− currents of 35–42 mA at 30 keV. Higher currents, up to 64 mA, have been achieved for short periods soon after recovery. These data were obtained for a 6.4-mm-diam emission aperture using a modified Pierce geometry. In this paper, the new procedure for introduction of Cs will be described with the associated recovery characteristics. This paper will compare the emittance changes as a function of H− current, equivalent current, and plasma electrode bias. The behavior of the output over many days and shutdown-operation-shutdown cycles also will be described.