Development of high current density surface ionization sources for heavy ion fusion applications

Abstract

Results are presented on the maximum current density obtained in pulsed (2 μs) contact ionization sources that utilize potassium and cesium atoms on porous tungsten substrates. Maximum values of 89 and 48 mA/cm2 for K+ and Cs+, respectively, were obtained in space charge limited mode for the source geometry used. Similar results were observed for potassium aluminosilicate (K20⋅Al2O3⋅4SiO2) sources. Neutral emission rates were measured in the temperature range of interest to heavy ion fusion (1000–1150 °C). Evidence is presented regarding a diffusion-limited flow of alkali atoms towards the emitting surface, which would be a determining factor in the source lifetime.

Development and testing of a lithium ion source and injector

“…By comparing the lifetime of emitters operated continuously to the lifetime of emitters for which the extraction voltage was applied intermittently (3%, 33%, and 100% duty cycle), the ratio of the ion emission rate ( i ) to the neutral emission rate ( o ) was found to be of order unity. This is in sharp contrast to K þ and Cs þ Al-Si emitters which yield a high current ( > 10 mA=cm 2 ) at 1025-1100 C and for which i = o ) 1 [16]. The neutral emission rate is expected to increase with temperature, but the magnitude of the neutral rate here is not understood in detail.…”

Developing high brightness beams for heavy ion driven inertial fusion

“…Detail results and comparison with simulations can be found in Bieniosek et al [12]. Measurements are also in progress to characterize high current density ionizers and the emission of neutral alkali vapor [13].…”

Diffusion of alkali species in porous tungsten substrates used in contact-ionization sources

“…This report summarizes a simple methodology applied to understand the importance of diffusive processes in the performance of this type of sources. Using previously measured [3] rates of neutral alkali atom desorption to match theoretical curves it was possible to estimate the diffusion constant of the alkali species in the porous tungsten substrate, in good agreement with the observed ion source lifetimes.…”

“…The above stages may be interpreted from neutral desorption rate data during the heating cycles of doped tungsten substrates [3]. Given the diffusion dynamics described above, the estimated effective diffusion constants ('D') at different temperatures do not scale simply with an Ahrrenius-type formula.…”

“…Equation (6) was used to fit the experimental measurements of neutral atom desorption of cesium and potassium atoms. These data exist for a collection of operating temperatures and at different points of the heating cycle of the emitter [3]. After applying an appropriate normalization factor to adjust the magnitude, the curve is fitted by varying a single parameter, the effective diffusion coefficient 'D'.…”