One of the most important tasks in high power electronics is the development of RF sources in submillimeter wavelength region. Among a lot of different devices, which could reach, in principle, specified frequency range, one of the most promising is Large Orbit Gyrotrons [l] (LOGs). But efficiency of LOGs is rather smallusually some percents only [2]. So, to obtain high enough value of the output power, it is necessary to use powerful electron beam as the energy source for RF radiation.Recently in the Nagaoka Institute of Technology, M a n generator with accelerating voltage U0 about 400 kV was developed [3]. Combination of such power supply with pulse magnet providing the operating magnetic field 12 T opens possibility in LOG to reach the border of terahertz frequency range with output power above 1-2 megawatt. Below the project of electron beam formation system with mentioned power supply, operation of LOG at third to fifth cyclotron harmonics is discussed.There are some ways to form a suitable axis-encircling electron beam for LOGs. Usually the rectilinear electron beam is formed first. After that it passes through the region with non-adiabatic magnetic field. In such a field, electrons obtain the gyration motion and then come to adiabatically increasing magnetic field, where the helical electron beam (HEB) rotating around the axis with suitable oscillatory energy is formed. In submillimeter wavelength region, the systems with reverse magnetic field are more preferable [4, 51. In the report results of numerical simulation of the gun with cusp magnetic field are presented.For preliminary analytical estimations of the HEB formation, the model of magnetic field with momentary symmetric reverse from value -Bc near cathode to + Bc after cusp region was used. After that, the adiabatic compression of the magnetic field up to value Bo takes place. For given cathode radius rc and pitchfactor in the cavity g=vLdv,p, where vLo, vllo correspondingly oscillatory and longitudinal velocities, clight velocity . ' Here y=l+eUdmoc2 and m, -, is the electron rest mass. rc is free parameter. But, if we use big value of rc, the magnetic field will be too small. It leads to increasing of the sensitivity of the formation system to the influence of the external magnetic field. It is well known that such fields can be about some Gauss at least. So, it would be better to have Bc more than 100 Gauss. Finally, rc=5 mm was chosen. In this case, Bp120 Gauss according to eq. (1).The characteristic feature of the systems with cusp magnetic field is the existence of rather long region with weak magnetic field intensity after the diode part of electron-optic system. In the specified region space charge force F, is much larger than the focusing magnetic force FB. It may cause the big beam ripple and even intersection of electrons by the walls of transportation channel. To diminish the influence of the space charge forces, it is necessary to use as big gradient dBJ& of axial magnetic field as possible. The most natural way to produce big valu...
