Effects of discharge chamber length on the negative ion generation in volume-produced negative hydrogen ion source

Abstract

In a volume-produced negative hydrogen ion source, control of electron temperature is essential due to its close correlation with the generation of highly vibrationally excited hydrogen molecules in the heating region as well as the generation of negative hydrogen ions by dissociative attachment in the extraction region. In this study, geometric effects of the cylindrical discharge chamber on negative ion generation via electron temperature changes are investigated in two discharge chambers with different lengths of 7.5 cm and 11 cm. Measurements with a radio-frequency-compensated Langmuir probe show that the electron temperature in the heating region is significantly increased by reducing the length of the discharge chamber due to the reduced effective plasma size. A particle balance model which is modified to consider the effects of discharge chamber configuration on the plasma parameters explains the variation of the electron temperature with the chamber geometry and gas pressure quite well. Accordingly, H(-) ion density measurement with laser photo-detachment in the short chamber shows a few times increase compared to the longer one at the same heating power depending on gas pressure. However, the increase drops significantly as operating gas pressure decreases, indicating increased electron temperatures in the extraction region degrade dissociative attachment significantly especially in the low pressure regime. It is concluded that the increase of electron temperature by adjusting the discharge chamber geometry is efficient to increase H(-) ion production as long as low electron temperatures are maintained in the extraction region in volume-produced negative hydrogen ion sources.

Negative hydrogen ion production mechanisms

“…153,154 The RF wavefrequency dependence of H À ion production and extraction in a DESY type ion source was also studied at this University. 155,156 The plasma density and temperature determined using a Langmuir probe are correlated to the extracted H À beam current at different frequencies. 155 In 2002, a new concept of ECR plasma production that may be suitable for an accelerator size source has been proposed by Lacoste et al 156 The design of this source, designated as dipolar plasma source, is shown in the inset in Figure 21(b).…”

“…155,156 The plasma density and temperature determined using a Langmuir probe are correlated to the extracted H À beam current at different frequencies. 155 In 2002, a new concept of ECR plasma production that may be suitable for an accelerator size source has been proposed by Lacoste et al 156 The design of this source, designated as dipolar plasma source, is shown in the inset in Figure 21(b). It consists of two main parts: (i) a cylindrical drilled permanent magnet with an azimuthal symmetry around its magnetization axis and (ii) a microwave (2.45 GHz) applicator constituted by a coaxial line in the direction of the magnetization vector, which ends at the rear pole of the magnet.…”

Kinetics of electrons and neutral particles in radio-frequency transformer coupled plasma H−ion source at Seoul National University

“…This method is based on the interaction process between H − ion and photons. A Nd:YAG laser of 532 nm in wavelength is used to detach electrons from H − ions and the photo-detached electrons are collected by a single probe biased higher than the plasma potential [7]. A typical waveform measured by the probe during the laser photo-detachment is shown in figure 6.…”

“…The characteristic time for ion wall loss, t + , H is determined by the charged particle balance and the quasineutrality [7]. The characteristic wall loss times for molecular ions are weighted by a square root of their masses, because the ion wall loss is closely related to the Bohm velocity.…”

“…Influence of RF wave frequency and surface material on H − ion production were examined [5,6]. We also found that the electron temperature could be easily increased by shortening the length of the discharge chamber [7]. However, the overall increase of electron temperature failed to filter high energy electrons near the extraction hole efficiently [8].…”