An Experimental Second-Harmonic Axis-Encircling Beam Gyro-TWT
Despite the bright potential as a broad band source for future radar and communication applications, the area of harmonic gyro-TWTs with axis-encircling beams has been relatively unexplored. A second-harmonic TE21 gyro-TWT has been constructed at UCD that is predicted to produce 50 kW in Ka-band with 20% efficiency, 30 dB saturated gain and 3% bandwidth and is ready for hot testing. The device is driven by an axis-encircling electron beam from a novel Northrop Grumman cusp gun.
Cusp gun TE/sub 21/ second-harmonic Ka-band gyro-TWT amplifier
A second-harmonic TE/sub 21/ gyrotron traveling-wave tube (gyro-TWT) amplifier with an axis-encircling electron beam is being constructed at the University of California-Davis that is predicted to double the efficiency of our previous 200-kW 12%-efficient magnetron injection gun (MIG) TE/sub 21//sup (2)/ gyro-TWT. Both devices employ a sliced mode-selective circuit to suppress the odd-order azimuthal modes. The new device will avoid the loss in efficiency that is due to off-axis electrons interacting with the resulting linearly polarized mode. The amplifier is predicted by our large signal code to produce 50 kW in Ka-band with 20% efficiency, 30 dB saturated gain, and 3% bandwidth. The 70-kV 3.5-A axis-encircling electron beam with /spl upsi//sub /spl perp////spl upsi//sub z/ = 1.2 and /spl Delta//spl upsi//sub z///spl upsi//sub z/ = 7% will be produced by a Cusp electron gun delivered by Northrop Grumman. The amplifier has been designed using linear theory to provide a 30% safety margin from absolute instability. Distributed loss is added to the first 30.5 cm of the circuit to ensure the device is stable to harmonic gyro-BWO. The last 11.5 cm of the circuit is standard copper to avoid attenuating the wave at high power levels.
Mass Loss History of Evolved Stars
We provide high-resolution maps of the circumstellar dust shells of several dozen Asymptotic Giant Branch (AGB) stars using data from the Spitzer space telescope Multiband Imaging Photometer (MIPS) imaged at 70μm. AGB stars are the major contributors of chemical elements such as carbon, oxygen and silicon, which are essential to the existence of life in the universe, through mass loss processes that take place at the surface of the star. We probe the spatial distribution of cold (∼40 K) dust grains in order to trace the history of mass loss from the observed radial density variation in these shells. Our images illustrate that different morphologies may evolve because AGB stars can already interact with the interstellar medium (ISM) that surrounds them even at these early stages of the AGB mass loss history. Relative motion of the star as well as the ISM with respect to the local environment may be of particular importance for shell structure evolution.