Development of a low-energy, high-brightness $\mu^+$ beam line
  We are developing a beam line which compresses the phase space of a standard surface $\mu^+$ beam by 10 orders of magnitude with an efficiency of $10^{-3}$. Phase space compression occurs in a He gas target and consists of three consecutive stages: Transverse (perpendicular to the beam axis) compression, longitudinal compression and re-extraction into vacuum. Transverse compression was observed for the first time and longitudinal compression has been measured to occur within 2.5 $\mu$s with high efficiency. 
A. Eggenberger, I. Belosevic, G. Wichmann
hep-ph physics.acc-ph