Effect of an axial magnetic field on multipactor in a dielectric-loaded accelerating structure

A recent paper suggested that an axial magnetic field near the cyclotron resonant value at the operating frequency can suppress multipactor loading in dielectric-loaded accelerating (DLA) structures. This occurs because the modification of electron trajectories caused by the magnetic field can reduce the electron impact energy at a given value of rf electric field, causing the electrons to strike the surface at energies below the first crossover of the secondary emission curve, and thus reducing the secondary electron yield below one. This effect should be enhanced at higher values of the rf electric field. We recently tested this prediction in a cylindrical alumina DLA structure using high-power 11.4 GHz radiation in the Magnicon Laboratory at the Naval Research Laboratory. The DLA structure had an inner diameter of 8.5 mm and an overall length of 14 cm, including tapered matching regions at either end, and employed a thick film silver exterior coating to create the external boundary condition for the propagating TM01 mode. The cyclotron resonant magnetic field was ~4 kG. The solenoidal magnet used in the experiment produced a magnetic field that peaked at the center of the DLA structure, and fell to half the central value at ±5 cm. The structure was conditioned at progressively higher powers up to ~9 MV/m, and the multipactor loading determined by measuring the incident, reflected, and transmitted microwave power at values of the applied magnetic field ranging from zero to the cyclotron resonant value. Without a magnetic field, multi-pactor increased at higher accelerating gradients, as seen in earlier experiments. Introducing a magnetic field first increased, and, at higher magnetic fields, decreased the multi-pactor loading, and this effect was most pronounced at higher gradients. However, multipactor loading was never fully suppressed, as predicted by Ref. 1. The persistence of multi-pactor in the structure may be caused by the lower magnetic field and- lower rf electric fields at the tapered ends of the structure. We plan to carry out further experiments to clarify this issue.