Experiments on the formation of an intense helical electron beam under conditions of picking-up of the electrons reflected from the magnetic mirror

We propose a method of increasing the pitch factor and decreasing the spread of rotational velocities of helical electron beams (HEBs) formed in nonadiabatic magnetron-injection guns of gyroresonant devices. The method is based on the effect of a special diaphragm mounted at the starting point of the transport channel on the process of formation of the laminar electron beam. The diaphragm located at one of the trajectory minima has such a diameter that it cannot be bent around by electrons with minimum rotational velocities. Such electrons land on it, whereas the remaining electrons pass further, moving in an increasing magnetic field. Then the electrons with the maximum rotational velocities reflect from the magnetic mirror adiabatically and land on the other side of the same diaphragm. Thus, the electron beam in the cavity contains electrons with a smaller resulting spread of rotational velocities. In the region of the HEB formation, the accumulation of the space charge of reflected electrons is eliminated, and the shielding of the electric field at the cathode is reduced, which eventually leads to an increase in the HEB pitch factor. Using such a diaphragm in the regime of current limitation by the space charge, the HEB with a high pitch factor (about 1.4) and a velocity spread acceptable for gyrotron applications (lower than 30%) was formed.