Review of Plasma Techniques Used to Trap Antihydrogen*

Recently, the ALPHA collaboration at CERN trapped antihydrogen atoms.\footnote{\textit{Trapped antihydrogen}, Nature,~\textbf{468}, 673, 2010.} To date,\footnote{\textit{Confinement of antihydrogen for 1,000 seconds}, Nature Physics, \textbf{7}, 558, 2011. } over three hundred antiatoms have been confined, some for as long as 1000s. This was the first time that antiatoms had ever been trapped. The ultimate goal of the ALPHA collaboration is to test CPT invariance by comparing the spectra of hydrogen and antihydrogen, and to measure the gravitational attraction between matter and antimatter. Such studies might resolve the baryogenesis problem: why is there very little antimatter in the Universe? The ALPHA experiment brought together techniques from many different fields of physics, but the crucial breakthroughs were in plasma physics. The essential problem is this: How does one combine two Malmberg-Penning trapped plasmas, one made from antiprotons, and the other positrons, which have opposite electrostatic potentials of nearly one volt, in such a manner that the antiprotons traverse the positrons with kinetic energies of less than 40$\mu $eV, this latter being the depth of the superimposed neutral antihydrogen trap? The plasma techniques ALPHA developed to accomplish this include: \begin{itemize} \item Minimizing the effects of the neutral trap multipole fields on the positron and antiproton plasma confinement. \item Compressing antiprotons down to less than 0.5mm. \item Using autoresonance to inject antiprotons into the positrons with very little excess energy. \item Evaporative cooling of the electrons and antiprotons to record low temperatures. \item Development of charge, radial profile, temperature, and antiproton loss location diagnostics. \item Careful and lengthy manipulations to finesse the plasmas into the best states for optimal antihydrogen production and trapping. \end{itemize} The plasma techniques necessary to trap antihydrogen will be reviewed in this talk.