Narrow bandwidth emission from a mirrorless, far infrared, 13CH3F laser

Frequency tuning and linewidth measurements are reported for a pulsed, mirrorless, kilowatt-power-level, far-infrared (FIR) ¹³ CH₃F laser operating at 245 GHz. The pump laser is an etalon tunable, single-mode CO₂ TEA laser. The FIR frequency spectrum was measured with 2.5-MHz resolution on individual 100-ns laser output pulses using harmonic mixing techniques. The linewidth of the amplified spontaneous emission was found to be surprisingly narrow, about 15 MHz. Frequency tuning of the FIR laser, as the pump laser frequency is tuned, is nonlinear, possibly due to inhomogeneous broadening of the gain by the K-level substructure of the rotational states. These results indicate that heterodyne receivers capable of single-shot frequency measurements can be important tools for investigating the properties of Raman FIR lasers     

A high-voltage modulator for high-power RF source research

The design, construction, and operating results of a high-voltage modulator system capable of generating 700-kV, 2.5-μs pulses at 5 p.p.s. into a load of 900 Ω are presented. The modular is used to energize a variety of high power microwave devices requiring voltage stability and reproducibility. Voltage ripple is less than 0.2% during the 1.0-μs flat top, with a shot-to-shot voltage variation of less than 0.1%. The primary circuit consists of two seven-stage tunable Rayleigh-type pulse-forming networks (PFNs) connected in parallel with a total impedance of 2.25 Ω, a total capacitance of 0.56 μF, and a total inductance of 2.8 μH. The PFN is charged by a highly stable 80-kV capacitor charging power supply (0.1% RMS voltage ripple) at a rate of 10 KJ/s. The total energy stored (1.5 kJ) is released through an ITT F-187 thyratron into a 20:1 pulse transformer, which generates 700-kV, 2.5-μs pulses. By changing the transformer, it was possible to obtain 250-kV, 1.70-kA pulses for driving low-impedance relativistic magnetron diodes. The flat-top voltage generated by the modulator is highly desirable for driving RF sources requiring high-quality electron beams, such as free-electron lasers (FELs) and cyclotron autoresonance masers (CARMs). The modulator performance in the relativistic magnetron and CARM experiments is described     

Simulation of noise-power ratio with the large-signal codeCHRISTINE

This paper describes simulations of the noise-power ratio (NPR) for a helix traveling wave tube (TWT) performed with the large-signal, one-dimensional (1-D), multifrequency code CHRISTINE. The results obtained with this code are in better agreement with measured values than are the more traditional values calculated by power series. We conclude that NPR simulations with large-signal codes have the potential to shorten the design phase of TWTs by eliminating the need for repeated build-test cycles to meet a required NPR     

Modeling the National Ignition Facility neutron imaging system

Numerical modeling of the neutron imaging system for the National Ignition Facility (NIF), forward from calculated target neutron emission to a camera image, will guide both the reduction of data and the future development of the system. Located 28 m from target chamber center, the system can produce two images at different neutron energies by gating on neutron arrival time. The brighter image, using neutrons near 14 MeV, reflects the size and symmetry of the implosion "hot spot." A second image in scattered neutrons, 10-12 MeV, reflects the size and symmetry of colder, denser fuel, but with only ～1%-7% of the neutrons. A misalignment of the pinhole assembly up to ±175?μm is covered by a set of 37 subapertures with different pointings. The model includes the variability of the pinhole point spread function across the field of view. Omega experiments provided absolute calibration, scintillator spatial broadening, and the level of residual light in the down-scattered image from the primary neutrons. Application of the model to light decay measurements of EJ399, BC422, BCF99-55, Xylene, DPAC-30, and Liquid A suggests that DPAC-30 and Liquid A would be preferred over the BCF99-55 scintillator chosen for the first NIF system, if they could be fabricated into detectors with sufficient resolution.     

Vacuum electronics

This paper explores the recent history and diversity of this remarkable technology, with emphasis on recent advances in the more traditional device types (traveling-wave tube and klystron), as well as more recent innovations such as the microwave power module, inductive output amplifier, fast-wave devices, ultrahigh-power sources, and RF vacuum microelectronics. These advances can be credited to a combination of device innovation, enhanced understanding gained through improved modeling and design, the introduction of superior materials and sub-assembly components and the development of advanced vacuum processing and manufacturing techniques     

Far-infrared raman laser with high intensity laser pumping

Theoretical and experimental results are presented for a pulsed far-infrared (FIR) molecular gas laser with high intensity laser pumping. In these FIR lasers, high intensity pumping is found to produce stimulated Raman emission at very large offsets (up to 30 GHz) from resonance with the intermediate state. A theoretical, density matrix model is developed for these lasers to account for simultaneous Raman emission on rotational levels in the ground and excited vibrational states (double Raman resonance). This theoretical approach is necessary in the case of off-resonant, high intensity pumping. Theory predicts the FIR emission frequency, the FIR laser gain, and the pump threshold intensity as a function of pump laser frequency. Experimental results are obtained onP-,Q-, andR-branch transitions in¹²CH3F and¹³CH3F using a single-mode, grating tuned CO2TEA pump laser with an intensity of up to 40 MW/cm². Good agreement is obtained between theory and experiment for the observed values of FIR emission frequency and pump threshold intensity. These results indicate that a widely tunable (150-1200 mum), pulsed FIR CH3F laser could be constructed with a tunable, multiatmospheric CO2pump laser of modest power (about 2-5 MW).     

A tunable far infrared laser

A continuously tunable far infrared (FIR) laser has been demonstrated; experimental results are presented. A high-pressure (10-12 atm) continuously tunable CO2TE laser is used to pump Raman transitions in CH3F; the generation of continuously tunable radiation in the250-300 mum wavelength range is reported. Accurate frequency and bandwidth measurements have been made and the FIR bandwidth in superradiant emission isapprox4-5GHz. Consequently, the generation of frequency tunable, subnanosecond pulses in the FIR appears feasible. The generation of tunable laser radiation from 150 to 1000 μm by stimulated Raman scattering should be possible using higher pump intensity and/or other gases.     

Free-electron lasers with electromagnetic standing wave wigglers

A detailed analysis of the electromagnetic standing wave wiggler for free-electron lasers (FEL's) is conducted for both circular and linear wiggler polarizations, following a single-particle approach. After determination of the unperturbed electron orbits in the wiggler field, the single-particle spontaneous emission spectrum and subsequently the gain in the low gain Compton regime (using the Einstein coefficient method) are explicitly calculated. This analysis results in a clear understanding of the resonance conditions and the coupling strength associated with each resonance of this type of FEL. In particular, a striking feature obtained from this investigation is that the electromagnetic standing wave wiggler FEL, under certain circumstances, exhibits a rich harmonic content. This harmonic content is caused by the presence of both the forward and backward wave components of the standing wave wiggler field. In addition, the nonlinear self-consistent equations for this type of FEL are also presented, permitting further investigation of it by the theoretical techniques and numerical codes developed for conventional FEL's.     

Principles of gyrotron powered electromagnetic wigglers for free-electron lasers

The operation of free-electron lasers (FEL's) with axial electron beams and high-power electromagnetic wiggler fields such as those produced by high-power gyrotrons is discussed. The use of short wavelength electromagnetic wigglers in waveguides and resonant cavities can significantly reduce required electron beam voltages, resulting in compact FEL's. Gain calculations in the low- and high-gain Compton regime are presented, including the effects of emittance, transverse wiggler gradient, and electron temperature. Optimized scaling laws for the FEL gain and the required electromagnetic wiggler field power are discussed. Several possible configurations for FEL's with electro-magnetic wigglers powered by millimeter wavelength gyrotrons are presented. Gyrotron powered wigglers appear promising for operation of compact FEL's in the infrared regime using moderate energy (<10 MeV) electron beams.     

Characteristics and applications of fast-wave gyrodevices

Gyrodevice oscillators and amplifiers (or gyro-oscillators and gyro-amplifiers) are being utilized in a variety of applications where high power levels are required at millimeter-wave frequencies. Gyro-oscillators, developed primarily for magnetic fusion research applications, have achieved power levels near 1 MW for pulse durations in excess of 1 s at frequencies above 100 GHz. Continued work on these devices should enable them to achieve continuous-wave operation at multimegawatt power levels at frequencies in the 100-GHz to 200-GHz range, thereby meeting the requirements of planned magnetic fusion experiments. The development of gyro-oscillators for fusion experiments has led to the utilization of the devices in several industrial applications, such as ceramic sintering and metal joining. Activities in this area involve adapting the oscillators to the industrial environment where reliability, efficiency, and ease of operation are paramount. Gyro-amplifiers are being developed for applications requiring phase coherence and instantaneous bandwidth, such as in linear accelerators and millimeter-wave radar. Impressive results from X-band to W-band already suggest the promise of these devices. Potential new applications and novel gyrodevice design approaches continue to attract the attention of researchers around the world