We have conducted an experiment in which the temperature and the wavelength dependent emissivity of a shocked surface has been measured. In the past, only the thermal emission from the shocked surface has been measured. The lack of knowledge of the emissivity as a function of wavelength leads to uncertainty in converting the measured emission spectrum into a surface temperature. We have developed a technique by which we are able to calculate both the emissivity of the shocked surface over a range of relevant wavelengths and the temperature of the surface. We use a multi-channel spectrometer in combination with a pulsed light source having a known spectrum of infrared radiation. Two separate techniques using a pulse of reflected radiation are employed and described. Both give the same result: An initially polished molybdenum surface that is shocked and partially released has a temperature of 1040 degrees Kelvin and a wavelength ((lambda) ) dependent emissivity of 0.16 ((lambda) =1.2micrometers ), 0.10 ((lambda)
High-power ultra-wideband pulses with equivalent center frequences of 10 GHz require picosecond switching times of high electric fields. We are developing a high-pressure gas switch that is designed to store several joules of energy. The switch has an overvoltaged gap that is pulse charged to operating voltages before the electron avalanche causes the voltage to collapse. Depending on the gas pressure (designed for up to 100 atm) and the electric field, the voltage collapse time can be hundreds of picoseconds down to a few picoseconds. Uniform development of the avalanche is initiated by photoionization from a bare spark or an ultraviolet laser. The pulse-power charging system is designed to charge the transmission line in a few nanoseconds. The stored energy that would flow as a post-pulse component is to be minimized. Tests of the repetition rate for the switch are part of the development. Data in this paper indicate the achieved gap electric field, the voltage collapse time, and the launched pulse power and energy down a transmission line.
The resonance at which high-efficiency operation of virtual cathode oscillators is obtained occurs when the beam frequency equals the reflex frequency to within 2%. This tolerance limit in the frequency ratio implies that cathode closure in the anode-cathode gap is not acceptable. The authors have developed and tested a 6-cm2 cathode that will operate longer than 1 microsecond(s) at 300 A/cm2 without significant closure. As yet, the full-scale (>80 cm2) cathode has not worked quite as well. In many tests, the cathode will operate in the emission-limited temperature/field (T/F) mode for approximately 300 ns, and then transition into explosive emission with a relatively slow ($OM0.5 cm/microsecond(s) ) closure rate. The current density was 45 to 90 A/cm2. High-power rf-emission tests have not been run under conditions where the diode stays open and in resonance for the duration of the rf pulse at a current density of 250 A/cm2, which is required for 3-GHz operation; that test remains the focus of continuing research. Long (600-ns) duration rf pulses have been obtained at low power. The data base on microwave generation at lower power also has been extended and has shown that high-efficiency resonances will occur when a multiple of the reflex frequency equals the beam frequency. This allows greater flexibility in the design and scaling of microwave device.