Compact cyclotron resonance high-power accelerator for electrons

Abstract

Exact numerical solutions for the single particle equations of motion have revealed conditions for highly nonlinear rapid acceleration near cyclotron resonance for electrons injected into a ${\mathrm{TE}}_{111}$-rotating-mode cylindrical microwave cavity immersed in a uniform axial magnetic field. We dub this the eCRA interaction. Magnitudes of acceleration energy in eCRA are shown to exceed to a large degree the limits for the related cyclotron autoresonance acceleration (CARA) interaction, wherein autoresonance acceleration is sustained for traveling rotating ${\mathrm{TE}}_{11}$-mode waves in a cylindrical waveguide. As with CARA, all injected electrons in an idealized eCRA enjoy equal energy gain without bunching. Injection of high currents that involve heavy beam loading allow acceleration in eCRA to multi-MeV levels for beams with average powers of hundreds of kW and rf-to-beam power efficiencies that exceed 80%. It is shown, to cite one example, that an effective acceleration gradient of over $90\text{ }\text{ }\mathrm{MV}/\mathrm{m}$ can be sustained with a maximum cavity surface field of only $40\text{ }\text{ }\mathrm{MV}/\mathrm{m}$, when producing a 4.5 MeV, 300 kW average power electron beam, with an rf-to-beam efficiency of about 86%. In that example, the cavity operates at 2.856 GHz, and the cavity's average surface heating rate is $100\text{ }\text{ }\mathrm{W}/{\mathrm{cm}}^{2}$. Other examples are given for beams with over one MW levels of average power and energies up to about 20 MeV. This paper's goal is only to elucidate and give examples of the basic mechanism for the strongly nonlinear acceleration that is predicted to occur in eCRA, rather than to present a particular engineered design. Still, the predicted parameters for an idealized eCRA suggest that practical realizations could emerge to satisfy a range of needs for efficient, compact accelerators for industrial, commercial, and national security applications.