Recent performance of the SNS H− ion source and low-energy beam transport system

Abstract

Recent measurements of the H(-) beam current show that SNS is injecting about 55 mA into the RFQ compared to ∼45 mA in 2010. Since 2010, the H(-) beam exiting the RFQ dropped from ∼40 mA to ∼34 mA, which is sufficient for 1 MW of beam power. To minimize the impact of the RFQ degradation, the service cycle of the best performing source was extended to 6 weeks. The only degradation is fluctuations in the electron dump voltage towards the end of some service cycles, a problem that is being investigated. Very recently, the RFQ was retuned, which partly restored its transmission. In addition, the electrostatic low-energy beam transport system was reengineered to double its heat sinking and equipped with a thermocouple that monitors the temperature of the ground electrode between the two Einzel lenses. The recorded data show that emissions from the source at high voltage dominate the heat load. Emissions from the partly Cs-covered first lens cause the temperature to peak several hours after starting up. On rare occasions, the temperature can also peak due to corona discharges between the center ground electrode and one of the lenses.

Cs-doped Mo as surface converter for H−/D− generation in negative ion sources: First steps and proof of principle

“…For example, in the frame of neutron therapy [1], for spallation neutron sources [2] as well as for neutral beam injection (NBI) systems on LHD [3]. For ITER, H -/D -NBI systems based on the small [4,5] and "half-size" [6,7] IPP prototype sources will be used.…”

Recent performance and ignition tests of the pulsed SNS H− source for 1-MW neutron production

“…Here the beam current can be measured reliably because secondary electrons are unable to escape in the field of the nearby second lens [3].…”

“…Accordingly, the required ~50 mA H -beams and the ~6% duty factor in SNS are significant challenges. These are met with the LBNL designed and built H -source [2], which continues to be improved [3][4][5][6][7][8]. Briefly, the cesium-enhanced, multicusp ion source has a 2.5 turn antenna inside the plasma chamber as seen in Fig.…”

Low energy beam transport system developments

“…With this design the commissioning of the SNS accelerator chain was accomplished at low duty factor. A history of SNS commissioning and RF ion source and ELEBT modification is presented in [1, 2,15,16,42]. Now the SNS uses an electrostatic, two-lens (G3 and G5)~12 cm long ELEBT, which focuses the 65-keV, ~ 6% duty factor, high-current H -ion beam (up to 60 mA) into the radio frequency quadrupole (RFQ) accelerator with the required Twiss parameters 1.7 and 0.06 mm/mrad.…”

“…A strong electrostatic focusing LEBT has been successfully adopted for reliable transportation of high current Hbeams in the SNS Front-End [1, 2,15]. Some modifications of such electrostatic LEBTs can be expected to improve the reliable transport of more intense positive and negative ion beams without greatly degrading their brightness and intensity.…”