Performance Limitations of Linear Gas Stoppers

Presently used linear gas cell concepts have performance limitations:

• Beam-rate dependent efficiencies limit experimental reach.
  A rate-dependent efficiency for linear gas cells has been observed in a variety of systems [1,2]. As shown in the figure, measured extraction efficiencies of existing linear gas cells drop precipitously with ionization rate density (rate of generation of ion pairs, IP, per volume). The drop in efficiency is attributed to space-charge effects, which hinder the extraction of ions and grow with ionization rate density. Next generation facilities will offer exotic beam rates of 10⁹/s, requiring the efficient handling of ionization rate densities of about 10¹¹ IP/cm³/s indicated by the orange band.

• Long extraction times do not match the advantage of fast-beam production. Linear gas cells require either a high gas pressure p or a large length L. (p × L = 0.5 bar × m). Limited by the maximum applicable electric field the average extractions time is about 100 ms. This leads to decay losses for short-lived isotopes and does not match the time scale (<1 ms) to create fast exotic beams and to transport them to the gas cell.
  
  
  Measured extraction efficiencies for linear gas cells decrease with ionization rate density. Future facilities require efficient performance at ionization rate densities of 10¹¹ (IP/cm³/s).

The cyclotron gas stopper is an improved concept that minimizes both performance limitations. It separates the region (at large radius) where stopping occurs and large space charge is produced from the extraction region (at small radius). It also features quicker stopping and extraction times.

References
[1] Wada et al., Nucl. Inst. Meth. B 204 (2005) 570.
[2] Weissman et al., Nucl. Inst. Meth. A 522 (2004) 212.