M.R. Lopez

Folded waveguide traveling-wave tube sources for terahertz radiation

Microfabricated folded waveguide traveling-wave tubes (TWTs) are potential compact sources of wide-band, high-power terahertz radiation. We present feasibility studies of an oscillator concept using an amplifier with delayed feedback. Simulations of a 560-GHz oscillator and experimental evaluation of the concept at 50 GHz are presented. Additionally, results from various fabrication methods that are under investigation, such as X-ray lithography, electroforming, and molding (LIGA), UV LIGA, and deep reactive ion etching are presented. Observations and measurements are reported on the generation of stable single-frequency oscillation states. On varying the feedback level, the oscillation changes from a stable single-frequency state at the threshold to multifrequency spectra in the overdriven state. Simulation and experimental results on amplifier characterization and dynamics of the regenerative TWT oscillator include spectral evolution and phase stability of the generated frequencies. The results of the experiment are in good agreement with the simulations.

Projection ablation lithography cathode for high-current, relativistic magnetron

Initial results are presented of an innovative cathode operating in a relativistic magnetron powered by an accelerator with parameters: −0.3 MV, 1–10 kA, and 0.5 μs pulse length. This cathode is fabricated by ablating a pattern on the cathode using a KrF laser. This projection ablation lithography (PAL) cathode has demonstrated fast current turn-on and microwave startup times have decreased from an average of 193 to 118 ns. The pulselength of 1 GHz microwave oscillation has increased from a 144 ns average to 217 ns. With these improvements in microwave startup and pulse length, the microwave power has approximately remained the same compared to the previously used cloth cathodes. A new triple-azimuthal emission region is tested as means of prebunching the electrons (“cathode priming”) into the three spokes desired for pi mode operation in a six-cavity magnetron. The Tri-PAL cathode priming results in the fastest startup and highest efficiency of relativistic magnetron microwave generation.

Cathode effects on a relativistic magnetron driven by a microsecond e-beam accelerator

Experiments have been performed on a relativistic magnetron driven at e-beam accelerator peak parameters: voltage = -0.4 MV, current = 16 kA, and pulselength = 0.5 /spl mu/s. The magnetron is a six-vane device operating at about 1 GHz with extraction from two cavities. For equal power in both extraction waveguides, the peak microwave power of this device is between 200 and 300 MW. Microwave pulse-shortening limits pulselengths to the range of 10-100 ns. Time-frequency analysis of microwave emission indicates operation at about 1.03 GHz, close to the pi mode frequency identified from cold tests and the three-dimensional MAGIC code. Two cold cathodes were tested: 1) an emitting aluminum knob in the vane region with no endcap and 2) an extended cathode with a graphite fiber emission region in the vanes and endcap outside the vanes. Electron endloss current has been measured for the two cathodes. With no endcap, the cathode exhibited endloss current fraction up to 50% of the total; with one endcap, the cathode reduced the endloss current fraction to as little as 12%. Both cathodes produced peak total-electronic efficiency in the range of 14%-21%.

X-ray power and yield measurements at the refurbished Z machine

Advancements have been made in the diagnostic techniques to measure accurately the total radiated x-ray yield and power from z-pinch implosion experiments at the Z machine with high accuracy. The Z machine is capable of outputting 2 MJ and 330 TW of x-ray yield and power, and accurately measuring these quantities is imperative. We will describe work over the past several years which include the development of new diagnostics, improvements to existing diagnostics, and implementation of automated data analysis routines. A set of experiments on the Z machine were conducted in which the load and machine configuration were held constant. During this shot series, it was observed that the total z-pinch x-ray emission power determined from the two common techniques for inferring the x-ray power, a Kimfol filtered x-ray diode diagnostic and the total power and energy diagnostic, gave 449 TW and 323 TW, respectively. Our analysis shows the latter to be the more accurate interpretation. More broadly, the comparison demonstrates the necessity to consider spectral response and field of view when inferring x-ray powers from z-pinch sources.

Radio-frequency plasma cleaning for mitigation of high-power microwave-pulse shortening in a coaxial gyrotron

Results are reported demonstrating that radio-frequency (rf) plasma cleaning is an effective technique for mitigating microwave-pulse shortening (i.e., lengthening the pulse) in a multimegawatt, large-orbit, coaxial gyrotron. Cleaning plasmas were generated by 50 W of rf power at 13.56 MHz in nitrogen fill gas in the pressure range 15–25 mTorr. Improvements in the averaged microwave energy output of this high-power-microwave device ranged from 15% to 245% for different initial conditions and cleaning protocols. The mechanism for this improvement is believed to be rf plasma sputtering of excess water vapor from the cavity/waveguide and subsequent removal of the contaminant by cryogenic vacuum pumps.

Limiting current in a relativistic diode under the condition of magnetic insulation

The maximum emission current density is calculated for a time-independent, relativistic, cycloidal electron flow in a diode that is under the condition of magnetic insulation. Contrary to conventional thinking, this maximum current is not determined by the space charge limited condition on the cathode, even when the emission velocity of the electrons is assumed to be zero. The self electric and magnetic fields associated with the cycloidal flow are completely accounted for. This maximum current density is confirmed by a two-dimensional, fully electromagnetic and fully relativistic particle-in-cell code.

The spatial location of laser-driven, forward-propagating waves in a National-Ignition-Facility-relevant plasma

Ion acoustic and electron plasma waves, associated with backward-propagating stimulated Brillouin scattering and stimulated Raman scattering, have been diagnosed in a long-scale-length, nearly homogenous plasma with transverse flow. Thomson scattered light from a probe beam is employed to show that these waves are well localized in space and for a time much shorter than the laser pulse duration. These plasma conditions are relevant to hohlraum design for the National Ignition Facility inertial confinement fusion laser system. [R. Sawicki et al., Fusion Technol. 34, 1097 (1998)].

THz radiation using compact folded waveguide TWT oscillators

Microfabricated folded waveguide traveling wave tubes are a promising solution to the generation of compact, wideband, high power terahertz radiation. We present feasibility studies of an oscillator concept using an amplifier with recirculated feedback. Simulations of a 560 GHz oscillator and experimental verification of the principle at 50 GHz are presented. Observation and measurements are reported on the generation of stable single frequency oscillation states. On varying the feedback level, the oscillation changes from a single frequency state at threshold to multifrequency spectra in overdriven state. Investigations on dynamics of the regenerative TWT oscillator will be presented. Additionally, preliminary results of experiments to fabricate 100-400 GHz folded waveguide TWT circuits will be reported.

RF and magnetic priming of relativistic magnetrons

Summary form only given. Research is underway to investigate two techniques for priming of relativistic magnetrons for rapid startup and reduced mode competition: 1) RF priming experiments with a 2 MW magnetron signal 2) Magnetic-priming simulations by an azimuthally-varying axial magnetic field. Experiments utilize the MELBA-C (Titan) 6-vane, relativistic magnetron which operates with parameters: V=-300 kV, I=1-10 kA, e-beam pulselength=0.5 /spl mu/sec, microwave power=100-500 MW, microwave frequency in L-band: 1-1.3 GHz. The ceramic insulator enables operation down to 8.5 E-8 Torr. The RF priming source is a 2 MW, 2.2 /spl mu/sec, pulsed magnetron from AFRL operating at 1.3 GHz. The microwaves are injected into 1 of the 3 open coupling slots in the MELBA-C relativistic magnetron. Magnetic priming consists of imposing N/2 azimuthal variations in the axial magnetic field of an N-vane magnetron. Such optimal magnetic priming has been demonstrated in low voltage experiments and high voltage simulations to cause rapid startup of magnetrons by pre-bunching the electrons into the N/2 electron spokes desired for the pi-mode. A highly idealised model of magnetic priming uncovered a parametric instability, which draws electrons into N/2 spokes that extend to the anode even in the absence of RF fields.

Investigations of folded waveguide TWT oscillators for THz radiation

In this paper we are investigating folded waveguide traveling wave tubes (FWGTWT). These FWGTWTs combine the power density and high frequency advantages of vacuum microelectronics with the powerful microfabrication methods of MEMS and solid state electronics. Experimental, theoretical and computer simulation investigations of a FWGTWT oscillator have been performed, including microfabrication tests of a variety of methods.