Optimization of ion transport in a combined RFQ Cooler with axial magnetic field

Abstract

The accurate mass spectrometry (with resolution goal 1:20000) of exotic ions requests beams with low energy spread (goal is about 0.5 eVrms or lower) and low transverse emittance, so it is necessary to cool ions produced by a fission source. In a radiofrequency (rf) quadrupole cooler (RFQC), collisions decrease ion kinetic energy, while rf and DC voltages confine and reaccelerate ions towards the extraction, where the cold ion beam is formed. Operation is based on carefully chosen tunings of voltages and of gas pressure p g, which requires an adequate pumping system; efficient simulations of extraction systems, which critically depend on residual speed of ions, are also important. Progresses in the experimental setup are described. Indications from simple ray tracing and ‘ray+collision’ tracing are compared. Results are applied to a simple triode extraction system, and more flexible configurations are considered.

Ion transport and gas collision effects in a radio frequency quadrupole cooler: installation in the Eltrap solenoid and beam calculations

“…A standard tool in beam transport simulations is the so-called ray tracing (precise calculation of single particle trajectories), which can resolve the rapid variations of ion speeds and DC fields (for example in the passage through the V8 pupil) with an adequately refined mesh (0.2 mm here) and time integrators. But ray tracing can treat only the friction force (Cavenago et al 2019), and not the diffusion (or straggling) arising from collisions. The theoretical analysis presented in the following allows us to approximately describe the diffusion with the time evolution of its correlations, which can be added to the ray-tracing solver, obtaining a so-called 'augmented solver'.…”

“…where ν m is the collision frequency, E p = −∇φ p , E s = −∇φ s with φ s the DC potential, indicates the statistical average, η is a random kick, with its second-order moment given by the diffusion tensor D, and k, n = 1, 2, 3 or x, y, z are coordinate indices. A preliminary study of the overall ion trajectories (from the source to the emittance meter EMI1) was given in Cavenago et al (2019) considering the friction force, but not straggling effects.…”

“…In paraxial approximation v x , v y |v z |, and the D non-zero components are D xx = D yy and D zz , calculated from interatomic potentials, in the same way as the mobility and collision frequencies (Schwarz 2006;Cavenago et al 2019). Calculating averages on the collision impact parameter as usual, we obtained ν m = n g f 1 |v|σ m and for example the component D xx = n g f 2 1 |v| 3 σ xx , with σ m the momentum cross-section (McDaniel 1973;Lieberman & Lichtenberg 1994;Cavenago et al 2022) and σ xx the transverse diffusion cross-section given in (5) of Cavenago et al (2022).…”

“…The RFQC insertion inside the Eltrap machine was described elsewhere; for example, see figure 1 of Cavenago et al (2019). The Eltrap machine features a high uniformity magnetic solenoid (1.5 m long) optimized for experiments with a Malmberg-Penning trap (Amoretti et al 2003); this trap has been used over the last 20 years to perform experiments on the dynamics of an electron plasma (Maero et al 2016).…”

“…A box encloses the RFQC to maintain the gas pressure p g necessary for cooling. The extraction line (Cavenago et al 2019) is actually cantilevered on this RFQC box, which lies on a 2 m girder (or support bar), where the injection line also sits; the girder is rigidly connected to a CF250 flange, forming a so-called 'plugin' module.…”