Superconducting Spoke Cavities 
for Acceleration of Low Velocity Particles

Tsuyoshi Tajima  (tajima@lanl.gov)
Los Alamos National Laboratory (LANL)

Elliptical superconducting radio-frequency (RF) cavities have been used in electron storage rings for particle and nuclear physics, e.g., CEBAF of Thomas Jefferson National Accelerator Facility, HERA of DESY in Germany, KEKB of KEK in Japan and LEP of CERN in Switzerland.  There has been interest in using elliptical cavities for lower velocity particles such as protons.  However, due to lower velocity, the length of the acceleration gap has to be shorter for the same acceleration frequency.  The reduced gap leads to a much shorter elliptical structure with a larger diameter than for electrons, for example. For the velocities below b = v/c ~ 0.45, the elliptical structure becomes impractical from structural point of view.  In addition, there is an increased potential for multipacting.

A different type of accelerating structure called a spoke cavity has been studied at Argonne National Laboratory (ANL).  The spoke cavity has a cylindrical spoke with a small beam hole (~ 4 cm in diameter) in the middle. The spoke is oriented transverse to the beam direction and inside a cylindrical cavity.  See Figs. 1 and 2.  This structure is mechanically rigid and compact, i.e., about half the diameter of an elliptical cavity.  At LANL, a b = 0.29, 340-MHz spoke cavity, on loan from ANL, was tested and a very encouraging results were obtained.  Figure 3 shows the quality factor as a function of the accelerating field.  The maximum accelerating field reached 12.5 MV/m, which corresponds to maximum surface electric and magnetic fields of 40 MV/m and 1063 Oe, respectively. European laboratories involved in projects to demonstrate nuclear waste transmutation with a high power proton accelerator are also considering using this type of cavity.

Fig. 1: A schematic of a spoke cavity.

Fig. 2: A spoke cavity with an input power coupler and a vacuum valve.

Fig. 3: Quality factor versus accelerating gradient of a spoke cavity tested at LANL.