Upgrading the LANSCE accelerator with a SNS RF-driven H- ion source.

The LANSCE accelerator is currently powered by a filament-driven, biased converter-type H- ion source that operates at 10%, the highest plasma duty factor for this type of source, using only ∼2.2 SCCM of H2. The ion source needs to be replaced every 4 weeks, which takes up to 4 days. The measured negative beam current of 12-16 mA falls below the desired 24 mA acceptance of the LANCSE accelerator. The SNS (Spallation Neutron Source) RF-driven, H- ion source injects ∼50 mA of H- beam into the SNS accelerator at 60 Hz with a 6% duty factor and an availability of >99.5% but requires ∼30 SCCM of H2. Up to 7 A h of H- have been produced during the 14-weeks-long source service cycles, which is unprecedented for small emittance, high-current, pulsed H- ion sources. The emittance of the SNS source is slightly smaller than the emittance of the LANSCE source. The SNS source also features unrivaled low Cs consumption and can be installed and started up in <12 h. LANSCE and SNS are working toward the use of SNS H- ion sources on the LANSCE accelerator because they could (a) fill the LANSCE accelerator to its capacity, (b) decrease the source replacement time by a factor of up to 7, and (c) increase source lifetime by a factor of about 4. This paper discusses some of the challenges that emerge when trying to match a different H- source into an existing injector with significantly different characteristics and operating regimes.

Forty years of surface plasma source development.

The cesiation effect, a significant enhancement of negative ion emission from a gas discharge with decrease of co-extracted electron current below negative ion current, was observed for the first time on July 1, 1971 by placing into the discharge a compound with 1 mg of cesium. Subsequent developments of surface plasma sources (SPS) for highly efficient negative ion production caused by the interaction of plasma particles with electrodes on which the adsorbed cesium reduced the surface work function are described. In the last 40 years, the intensity of negative ion beams has increased by cesiation up to 10(4) times from 3 mA to tens of amperes. Here, the main attention is concentrated on earlier SPS developments because recent results are well known and widely available.

Surface plasma source with anode layer plasma accelerator.

Proposed plasma generation system can be used for high current negative ion beam production and for directed deposition by flux of sputtered neutrals and negative ions. The main mechanism of negative ion formation in surface plasma sources is the secondary emission from low work function surface bombarded by a flux of positive ion or neutrals. The emission of negative ions is enhanced significantly by introducing a small amount of cesium or other substance with low ionization potential. In the proposed source are used positive ions generated by Hall drift plasma accelerator (anode layer plasma accelerator or plasma accelerator with insulated channel, with cylindrical or race track configuration of emission slit). The target-emitter is bombarded by the ion beam accelerated in crossed ExB fields. Negative ions are extracted from the target surface with geometrical focusing and are accelerated by negative voltage applied between emitter and plasma, contacting with the plasma accelerator. Hall drift ion source has a special design with a space for passing of the emitted negative ions and sputtered particles through the positive ion source.

Absolute beam brightness detector.

In generally accepted emittance measurement, main attention is concentrated on emittance areas ɛ(x), ɛ(y) occupied by desired part of ion beam in transverse phase space and shape of these areas. The absolute beam phase density (brightness) as usually is not measured directly and the average beam brightness B is calculated from a beam intensity I and the transverse emittances. In the ion source and low energy beam transport (LEBT) optimization, it is important to preserve the beam brightness because some aberration of ion optic and beam instabilities can decrease the brightness of the central part of ion beam significantly. For these brightness measurements, it is convenient to use an absolute beam brightness detector with the brightness determination from one short considered in this article.

Radio frequency discharge with control of plasma potential distribution.

A RF discharge plasma generator with additional electrodes for independent control of plasma potential distribution is proposed. With positive biasing of this ring electrode relative end flanges and longitudinal magnetic field a confinement of fast electrons in the discharge will be improved for reliable triggering of pulsed RF discharge at low gas density and rate of ion generation will be enhanced. In the proposed discharge combination, the electron energy is enhanced by RF field and the fast electron confinement is improved by enhanced positive plasma potential which improves the efficiency of plasma generation significantly. This combination creates a synergetic effect with a significantly improving the plasma generation performance at low gas density. The discharge parameters can be optimized for enhance plasma generation with acceptable electrode sputtering.

Features of semiplanotron surface plasma sources.

Features of the semiplanotron surface plasma sources (SPS) with cesiation used for high efficient negative ion beam production from first development to modern condition are considered. Design features of semiplanotrons SPS with cylindrical and spherical geometric focusing and the features of the negative ion production in the semiplanotrons are reviewed. Several versions of semiplanotrons with efficiency up to 0.1 A of H(-) per kW of discharge power are discussed. Modifications of the semiplanotrons for dc operation and for heavy negative ion production are reviewed.

Spallation neutron source saddle antenna H- ion source project.

In this project we are developing an H(-) source which will synthesize the most important developments in the field of negative ion sources to provide high current, high brightness, good lifetime, high reliability, and high power efficiency. We describe two planned modifications to the present spallation neutron source external antenna source in order to increase the plasma density near the output aperture: (1) replacing the present 2 MHz plasma-forming solenoid antenna with a 13 MHz saddle-type antenna and (2) replacing the permanent multicusp magnetic system with a weaker electromagnet.

Electrode activation in cesium-free negative ion sources.

Features of emission electrode activation leading to enhancement of negative ion emission in cesium-free discharges are discussed. In some ion sources with cesium-free discharges, the emission of negative ions has been increased significantly by emission electrode activation using strong heating of the negative biased electrode by discharge plasma. A simple explanation of this enhancement is that it is due to an accumulation on the emission surface of the plasma electrode of impurities with low ionization potential that decreases in surface work function and increases the secondary emission of negative ions similar to "Cesiation." The negative biasing of emission surface is important for accumulation and trapping the impurities on the emission surface. To effectively control the activation process it is important to directly detect the evolution of the work function and the impurity concentration during electrode activation with enhancement of negative ion emission.

Cesium control and diagnostics in surface plasma negative ion sources.

For efficient and reliable negative ion generation it is very important to improve a cesium control and diagnostics. Laser beam attenuation and resonance fluorescence can be used for measurement of cesium distribution and cesium control. Resonant laser excitation and two-photon excitation can be used for improved cesium ionization and cesium trapping in the discharge chamber. Simple and inexpensive diode lasers can be used for cesium diagnostics and control. Cesium migration along the surface is an important mechanism of cesium escaping. It is important to develop a suppression of cesium migration and cesium accumulation on the extraction system.