  Purpose: To describe a concept of a compact electron accelerator for external
radiation therapy with variable energy in the range of 6 - 20 MeV, based on
linotron principle. Methods: Beam dynamics simulation using the CST and MAD-X
code. Various optimization methods of multi-parameter problem. Results: Our
accelerator differs from the Reflexotron in a number of essential details: a
much more compact and more efficient C-band accelerating structure, optimized
for the high capture efficiency, narrow energy and phase spectra, and low
transverse emittance; magnetic mirror with fixed field based on rare-earth
permanent magnets; three-electrode electron gun with off-axis placement of the
cathode with a current regulated in the range of two orders of magnitude. These
improvements allow the possibility to: adjust the accelerated beam current in a
wide range in accordance with the required energy; reduce parasitic losses of
the beam current and the associated parasitic radiation; eliminate the risk of
setting an erroneous energy value; significantly reduce the dimensions of the
accelerator and simplify its operation. Conclusions: We presented the results
of calculation of the electron accelerator for external radiation therapy in
the energy range of 6 - 20 MeV. The accelerator is based on the principle of
double beam acceleration in the same accelerating structure, which allows to
control the beam energy in a wide range, reduce RF power consumption and the
dimensions of the accelerator, and, therefore, reduce its cost. The results can
be used to develop the design of the accelerator on the platform of the KLT-6
complex created by ROSATOM.
