Higher-order mode rf guns

Traditional photocathode rf gun design is based around the use of TM0,1,0-m ode cavities. This is typically done in the interest of obtaining the highe st possible gradient per unit supplied rf power and for historical reasons. In a multicell, aperture-coupled photoinjector, however, the gun as a whol e is produced from strongly coupled cavities oscillating in a pi mode. This design requires very careful preparation and tuning, as the field balance and resonant frequencies are easily disturbed. Side-coupled designs are oft en avoided because of the dipole modes introduced into the cavity fields. T his paper proposes the use of a single higher-order mode rf cavity in order to generate the desired on-axis fields. It is shown that the field experie nced by a beam in a higher-order mode rf gun is initially very similar to t raditional 1.5- or 2.5-cell pi -mode gun fields, and projected performance in terms of beam quality is also comparable. The new design has the advanta ges of much greater ease of fabrication, immunity from coupled-cell effects , and simpler tuning procedures. Because of the gun geometry, the possibili ty also exists for improved temperature stabilization and cooling for high duty-cycle applications.