C. Courteille

Investigation of a large volume negative hydrogen ion source

The electron and negative ion densities and temperatures are reported for a large volume hybrid multicusp negative ion source. Based on the scaling laws an analysis is made of the plasma formation and loss processes. It is shown that the positive ions are predominantly lost to the walls, although the observed scaling law is n+∝I0.57d. However, the total plasma loss scales linearly with the discharge current, in agreement with the theoretical model. The negative ion formation and loss is also discussed. It is shown that at low pressure (1 mTorr) the negative ion wall loss becomes a significant part of the total loss. The dependence of n−/ne versus the electron temperature is reported. When the negative ion wall loss is negligible, all the data on n−/ne versus the electron temperatures fit a single curve.

Primary electron drift in a volume hybrid multicusp H− ion source

A new, large, hybrid volume negative ion source is described. Observations of curvature and grad B primary electron drift in the multicusp magnetic field are reported. The direction of the electron drift changes from one filament to the neighboring one, because of the change in the direction of the magnetic field. For a given discharge current the extracted negative ion current and the plasma density at the center of the extraction region are affected by the direction of the primary electron drift of the active filaments. It was shown, however, that the negative ion current is controlled by the plasma density at the center of the extraction region. A trajectory calculation, effected in the guiding center approximation, allows to estimate the average drift velocity.

H- and D- temperature in volume sources

A new technique using laser induced photodetachment has been developed for measuring the negative ion temperature in H− and D− sources. Using this technique, we have investigated the dependence of the negative ion temperature on source parameters such as pressure, discharge current, and electron temperature. Simultaneous measurements of negative ion density, temperature, and extracted current lead to the conclusion that the extracted negative ion current is, at most, equal to the thermal flux.

Investigation of two negative hydrogen and deuterium ion sources: Effect of the volume

We have investigated by probes and by photodetachment the plasma properties of two negative ion sources of different volume. We compared the data relevant to these two sources and identify the effects induced by the change in volume and the isotope effects. The observed scaling law for the electron temperature is in each case Te∼I0.27d. Positive ions are predominantly lost to the walls, but due to the increasing influence of their loss by dissociative recombination when the source volume is increased, the scaling laws are ne∼I0.7d, for the small source, and ne∼I0.59d, for the large source. Atoms limit the production of negative ions and destroy them. Thus we see an isotope effect, with higher negative ion density in hydrogen than in deuterium, and the effect of the source volume, with the negative ion density larger in the small source than in the large one.

Beam intensity and brightness in H− and D− hybrid sources: Measurement of basic parameters

The photodetachment technique developed for measuring negative ion density and temperature has been used to study the difference between hydrogen and deuterium operation of the hybrid source. It was found that the negative ion temperature scales linearly with the electron temperature in both hydrogen and deuterium, but is higher in hydrogen than in deuterium for the same electron temperature. This dependence is affected by the volume of the source chamber. Recent progress in the photodetachment diagnostics of negative ions is reported. The presence of a capacitive component was observed in the photodetachment signal recorded using conventional, conductive probes. Capacitive probes have been constructed and tested. A bare probe at floating potential leads to workable signals. Experiments proving that the capacitive signal is proportional to the negative ion density are described.

Negative deuterium ion thermal energy measurements in a volume ion source

1407_64A diagnostic technique has been developed to determine the negative hydrogen ion (H-/D-) thermal energy in a variety of conditions in a volume production ion source. The method is based on Nd:YAG laser photodetachment of the negative ions. The photodetachment produces local changes in the ion and electron densities, whose space and time evolution are traced by the use of a probe and a time-delayed second Nd:YAG laser pulse. The results of measurements using this technique for deuterium ions are described. Information was collected from the center of the source to near the plasma electrode. The effects of the presence of electric and magnetic fields near the probe during measurements are addressed. The present study demonstrates that the extracted ion current is equal to the thermal flux through the extraction aperture.

Investigation of two negative hydrogen ion sources. Effect of the volume

Investigation of a large volume negative hydrogen ion source. Effect of the volume. We have investigated by probes and photodetachment the plasma properties of the large volume source Camembert III. We will compare these results with the data from a similar, but smaller source, Camembert II. We will tempt to identify the effects induced by the change in volume.

Isotope effect in negative ion production

We present the results of a numerical study of isotope effects in volume negative ion sources. We have studied both hydrogen and deuterium discharges over a range of discharge current. For basic plasma parameters, the model reproduces the behavior measured in the discharge region of a multicusp source. A predicted peak in negative ion production Te‐opt≊0.6–0.8 eV finds support from our analysis of D− and H− densities measured in a hybrid multicusp source. The most significant isotope effect suggested by this modeling effort is a stronger V–T cooling of the vibrational distribution in deuterium, which reduces negative ion production in high power deuterium discharges below that in equivalent hydrogen discharges.

Effect of primary electron drift on the negative ion efficiency of a volume hybrid multicusp H− ion source

A new, large, hybrid volume negative ion source is described. Observations of curvature and grad B primary electron drift in the multicusp magnetic field are reported. The direction of the electron drift changes from one filament to the neighboring one, because of the change in the direction of the magnetic field. For a given discharge current the extracted negative ion current and the plasma density at the center of the extraction region are affected by the direction of the primary electron drift of the active filaments. It was shown, however, that the negative ion current is controlled by the plasma density at the center of the extraction region. A trajectory calculation, effected in the guiding center approximation, allows to estimate the average drift velocity.

A comparison of the plasma characteristics of two multicusp negative ion sources

Electron temperature, electron density and negative ion fraction measurements in two different multicusp negative hydrogen ion sources, operating under similar conditions one at Queen’s University and one at Ecole Polytechnique, are compared. Some data from a third source at FOM are also included in the comparisons. The agreement between electron temperature and density measurements is generally good. There are however considerable discrepancies between the negative ion fraction measurements. This illustrates the great sensitivity of negative ion fractions to the exact source conditions.