Studies of high frequency wave excitation in fast and slow wave vacuum devices
Antonsen, Thomas M
THz and millimeter-wave length radiation is widely used in imaging, detection and plasma heating. Vacuum electronic devices are the most efficient sources of high power radiation in the THz and millimeter range. Efforts to increase power by increasing current, are hampered by self-field effects and instabilities. Two examples of these effects are considered: the reduction in bunching efficiency in orotrons (a slow wave device), and the excitation of backward wave instabilities, in gyrotrons (a fast wave device). The goal of producing THz radiation from miniature electron beam devices has refocused interest in orotrons. The efficiency of these devices improves with increasing current density. However, with increasing current density, self-fields become more important. Here, the theory of self-fields in a planar orotron is developed. We find that the parameters of the grating, which provides the slow wave fields that interact with the beam, also affect the self-fields, which give rise to the slow space charge wave. Thus, optimization of the grating parameters requires consideration of their impact on the dispersive properties of the slow space charge wave. We present a sample structure design appropriate for a planar orotron. Heating plasma to fusion temperatures will require multi megawatts of continuous wave (CW) power. Gyrotrons are the sources of choice for this heating. However, the maximum CW power from a single gyrotron is about 1.5MW. Efforts to increase gyrotrons power have led to instabilities in the electron beam. Here, the starting conditions for excitation of backward waves in the beam tunnel between the electron gun and the cavity of a high-power gyrotron are studied. The excitation of these waves leads to electron energy spread that spoils the beam quality and, hence, degrades the gyrotron efficiency. The suppression of these modes by a resistive coating on the wall of a smooth beam tunnel is examined. The guiding magnetic field and the tunnel wall radius vary along the axis, so the theory is essentially the small-signal theory of a gyro-backward-wave oscillator (gyro-BWO) with tapered parameters. The velocity spread and space charge of the beam will affect the interaction between the electron beam and backward wave. We find that space charge significantly lowers backward wave start currents and suppression of the space charge effect is key to operating at higher currents.