Cyclotron wave amplification using simultaneous R. F. -coupling and D. C. -pumping

Abstract

This report presents a new wideband cyclotron-wave amplifier using simultaneous rf coupling and dc pumping.A single quadrifilar helix is operated so that its -1 mode is synchronous with the fast cyclotron wave over the full helix passband (ka -I at w -'bc). This coincides with the condition for dc pumping when quadrupolar dc voltages are applied to the four helix wires. The one helix then serves simultaneously as dc pump structure and as slow-wave structure to couple rf energy into and out of the beam waves.A detailed analysis of simultaneous pumping and coupling is developed, using appropriate coupled-mode equations. Expressions for gain, bandwidth, and noise figure are presented in terms of practical tube parameters, especially for the case of a hollow beam in a helix with concentric inner conductor. For the latter case, when the frequency dependence of.all parameters is taken into account, the gain is found to vary by less than 3 db over a frequency range in excess of 4:1.Since the amplification process involves both the slow and fast cyclotron waves, its noise figure depends in part on cathode temperature and is not extremelylow.An experimental tube in S-band using a 1-in.-diameter by 5 -in.-long helix and an (intended) 300-v, 1 0 0 -ma beam was built and tested. An unduly large helix-to-center-conductor gap (0.100 in.) was used because of an erroneous early approximation for the dc pumping fields.As a result of this and of spurious gun oscillations at currents above a few ma, high-gain operation was not obtained.However, the dependence of the low-gain behavior on all available parameters was closely examined and found to be in excellent agreement with computations based on the analysis. Moreover, the broadband nature of the gain was verified with the observation of , 5-db gain over a 4:1 frequency range with all tube parameters fixed.Certain unexpected high-gain operating modes were also observed with different tube parameters, particularly with different dc-pump-voltage configurations. These are not fully understood as yet. They form an interesting possible future extension for this work, in addition to further development of the simultaneous pumping/coupling concept.-

Computational Neuroscience

“…We can observe relatively large synchronized firing of the neurons which is not present in the subnetworks of the corresponding random networks. Such synchronization was also observed experimentally in the spontaneous activity in slices of mouse visual cortex (Mao et al, 2001;Cossart et al, 2003;Ikegaya et al, 2004;Grinstein and Linsker, 2005). A model investigation into the role of network structure in the generation of synchrony suggested that networks with hubs are able to exhibit synchrony peaks (Grinstein and Linsker, 2005).…”