Magnetic multipole line-cusp plasma generator for neutral beam injectors

Effects of a dielectric material in an ion source on the ion beam current density and ion beam energy

To understand a strong focusing phenomenon that occurs in a low-energy hydrogen ion beam, the electron temperature, the electron density, and the space potential in an ion source with cusped magnetic fields are measured before and after the transition to the focusing state using an electrostatic probe. The experimental results show that no significant changes are observed before or after the transition. However, we found unique phenomena that are characterized by the position of the electrostatic probe in the ion source chamber. Specifically, the extracted ion beam current density and energy are obviously enhanced in the case where the electrostatic probe, which is covered by a dielectric material, is placed close to an acceleration electrode.

High current density ion beam obtained by a transition to a highly focused state in extremely low-energy region

A high current density (≈3 mA/cm(2)) hydrogen ion beam source operating in an extremely low-energy region (E(ib) ≈ 150-200 eV) has been realized by using a transition to a highly focused state, where the beam is extracted from the ion source chamber through three concave electrodes with nominal focal lengths of ≈350 mm. The transition occurs when the beam energy exceeds a threshold value between 145 and 170 eV. Low-level hysteresis is observed in the transition when E(ib) is being reduced. The radial profiles of the ion beam current density and the low temperature ion current density can be obtained separately using a Faraday cup with a grid in front. The measured profiles confirm that more than a half of the extracted beam ions reaches the target plate with a good focusing profile with a full width at half maximum of ≈3 cm. Estimation of the particle balances in beam ions, the slow ions, and the electrons indicates the possibility that the secondary electron emission from the target plate and electron impact ionization of hydrogen may play roles as particle sources in this extremely low-energy beam after the compensation of beam ion space charge.

Electron density profile measurements at a self-focusing ion beam with high current density and low energy extracted through concave electrodes

The self-focusing phenomenon has been observed in a high current density and low energy ion beam. In order to study the mechanism of this phenomenon, a special designed double probe to measure the electron density and temperature is installed into the chamber where the high current density ion beam is injected. Electron density profile is successfully measured without the influence of the ion beam components. Estimated electron temperature and density are ∼0.9 eV and ∼8 × 10(8) cm(-3) at the center of ion beam cross section, respectively. It was found that a large amount of electrons are spontaneously accumulated in the ion beam line in the case of self-forcing state.

Self-focusing of a high current density ion beam extracted with concave electrodes in a low energy region around 150 eV

Spontaneous self-focusing of ion beam with high current density (Jc ∼ 2 mA/cm(2), Ib ∼ 65 mA) in low energy region (∼150 eV) is observed in a hydrogen ion beam extracted from an ordinary bucket type ion source with three electrodes having concave shape (acceleration, deceleration, and grounded electrodes). The focusing appears abruptly in the beam energy region over ∼135-150 eV, and the Jc jumps up from 0.7 to 2 mA/cm(2). Simultaneously a strong electron flow also appears in the beam region. The electron flow has almost the same current density. Probably these electrons compensate the ion space charge and suppress the beam divergence.

The dependence of extracted current on discharge gas pressure in neutral beam ion sources on HL-2A tokamak

The discharge gas pressure is a key factor to influence the extracted current of ion source. In this paper, the dependence of extracted current on discharge gas pressure was investigated in detail at different arc discharge currents. The discharge gas pressure with a very broad range (0.1 Pa-2.7 Pa) was scanned for the first time. It is turned out that, with the increasing of discharge gas pressure, the extracted current increases and the arc voltage decreases at different arc currents; however, when the discharge gas pressure exceeds a certain value, the extracted current decreases. For the same discharge gas pressure, the higher the arc current, the higher the arc voltage and the extracted current are. The arc efficiency was also calculated, and its dependence on gas pressure was almost the same with the dependence of extracted current on gas pressure, but at the same discharge gas pressure, the lower the arc current, the higher the arc efficiency is and the lower the extracted current is.

Characteristics of the Berkeley multicusp ion source

The performance of a cubical permanent magnet generated line-cusp ion source has been investigated for use with neutral beam injectors. This source has been operated with discharge currents greater than 500 A and ion current densities higher than 400 mA/cm2 at the extraction grid. The uniformity of the density profile across the extraction area is found to be dependent on the gas pressure. By using a fast Langmuir probe sweeping circuit, the electron temperature and the plasma density and potential have been analyzed for different discharge powers and gas pressures. The heat load on the plasma grid when it is electrically floating or connected to the negative cathode has been compared calorimetrically. The use of lanthanum hexaboride and impregnated oxide cathodes have been investigated for the purpose of long pulse operation. The phenomenon of mode flipping is found to occur quite frequently during a discharge with these magnetic-field-free cathodes. Species composition as a function of discharge power and chamber length is measured by a mass spectrometer.