Characterization of TRIUMF dc H− ion sources for enhanced brightness

Abstract

At TRIUMF, H− ion sources have been characterized on a teststand to improve the understanding of their performance. Measured beam characteristics such as current, emittance, and e∕H− ratio were correlated with source conditions as a function of relevant plasma parameters. Plasma densities, temperatures, and plasma potentials were measured with a Langmuir probe and correlated with beam properties for different confining magnetic fields and different values of arc power and gas pressure. The mechanism of beam extraction was studied by correlating plasma potential profiles with the corresponding plasma electrode bias voltages obtained from optimizing ion source outputs. Experiments with collar biasing and noble gas mixing were unsuccessful because of undesirable plasma potential profiles.

VUV-diagnostics of a filament-driven arc discharge H− ion source

“…Typical plasma density profiles reported in Refs. [1,5] suggest that the total plasma emission is at least 50% of the given maximum (Fig 6 a)). The results also suggest that at least 15 -30 % of the discharge power is dissipated via VUV-emission within the observed range (120-250 nm).…”

“…Filament-driven multi-cusp arc discharge ion sources operated in volume production mode are often used for H − /D − production for cyclotrons and electrostatic accelerators requiring DC beams [1,2,3].Their relatively simple design, ease of use and maintenance, and properties of the extracted beams i.e. brightness and stability make them attractive for this purpose.…”

“…Plasma electrons can be divided into two populations; thermal electrons with temperature of 2-5 eV and hot electrons distributed over wide range of energies up to filament potential [1,4,5,6]. The plasma density is typically 10 11 -10 13 cm −3 [1], ionization degree below 1% and estimated fraction of atomic hydrogen ∼ 1% of molecules [7]. The range of pressures is (0.1-1.5)·10 −2 mbar corresponding to typical gas feed rate of 1-15 sccm.…”

Photoelectron emission from metal surfaces induced by VUV-emission of filament driven hydrogen arc discharge plasma

“…The estimated total PE flux depends on the radial plasma profile because it defines the ratio of observed emission volume to the total emission volume. In the radial direction the plasma density profile is typically almost uniform decaying only near the plasma chamber wall due to the multi-cusp field [17,18]. The measurement was limited to line-of-sight in the axial direction at the plasma center as illustrated in Fig.…”

“…The emitted electrons are further accelerated by the plasma sheath potential. For example, the plasma potential in the TRIUMF source has been measured to be about 5 V [18]. However, extracted negative ions are predominantly created near the (biased) plasma electrode due to short survival length of the negative ions [24].…”