New Types of Dissipative Streaming Instabilities

Two new, previously unknown types of dissipative streaming instabilities (DSI) are substantiated. They follow from new approach, which allows solving in general form the classical problem of an initial perturbation development for streaming instabilities (SI). SI is caused by relative motion of the streams of plasma components. With an increase in level of dissipation SI transforms into a DSI. The transformation occurs because dissipation serves as a channel for energy removal for the growth of the negative energy wave of the stream. Until recently, only one type of DSI was known. Its maximal growth rate depends on the beam density nb and the collision frequency ν in the plasma as ∼nb/ν. All types of conventional beam-plasma instabilities (Cherenkov, cyclotron, etc.) transform into it. The solution of the problem of the initial perturbation development in systems with weak beam-plasma coupling leads to a new type of DSI. With an increase in the level of dissipation, the instability in these systems transforms to the new DSI. Its maximal growth rate is ∼nb/ν. The second new DSI develops in beam-plasma waveguide with over-limiting current of e-beam. Its growth rate ∼nb/ν. In addition, the solutions of abovementioned problem provide much information about SI and DSI, significant part of which is unavailable by other methods.

On the spectra of natural waves in a plasma waveguide in the presence of collisions

The dispersion characteristics of surface and evanescent waves in metal-dielectric-plasma-dielectric-metal structure in the presence of collisions are investigated analytically and numer ically. In the absence of absorption, when the electron density passes through the doubled critical value, a rearrangement of the eigenwave structure, associated with the appearance of surface waves, occurs. A rearrangement also occurs in an absorbing plasma, but the numbers of reconnecting modes depend on the size of the structure and the ratio of the electron collision frequency to the field frequency. Correct consideration of this process is necessary for the analytical analysis of the field structure in plasma reactors, the design of plasma antennas, and the solution of other problems of plasma electrodynamics.

THz Quantum Cascade Laser Operating Frequency Comb Over Near-full-current Dynamic Range

We report a terahertz quantum cascade laser frequency comb (THz QCL FC) with low threshold current density, high power, and wide current dynamic range. The active region design with the semi-insulated surface plasma waveguide is beneficial to optimize the gain dispersion value and temperature stability. At 10K, the comb with 3-mm-long and 150-µm-wide is capable of emitting 22mW with the threshold current density Jth = 64.4 A·cm−2. The total spectral emission is of about 300 GHz centered around 4.6 THz. Without any extra dispersion compensation measures, the intermode beatnote map reveals stable frequency comb operating within a current dynamic range more than 97% and the narrowest beatnote linewidth is 7.2 kHz. The stable FC operation of a free-running THz QCL makes our device an ideal source for further development of dual-comb spectroscopy.

Quasi-phase-matched laser wakefield acceleration of electrons in an axially density-modulated plasma channel

Quasi-phase matching in corrugated plasma channels has been proposed as a way to overcome the dephasing limitation in laser wakefield accelerators. In this study, the phase-lock dynamics of a relatively long electron bunch injected in an axially-modulated plasma waveguide is investigated by performing particle simulations. The main objective here is to obtain a better understanding of how the transverse and longitudinal components of the wakefield as well as the initial properties of the beam affect its evolution and qualities. The results indicate that the modulation of the electron beam generates trains of electron microbunches. It is shown that increasing the initial energy of the electron beam leads to a reduction in its final energy spread and produces a more collimated electron bunch. For larger bunch diameters, the final emittance of the electron beam increases due to the stronger experienced transverse forces and the larger diameter itself. Increasing the laser power improves the maximum energy gain of the electron beam. However, the stronger generated focusing and defocusing fields degrade the collimation of the bunch.

A Photogenerated Silicon Plasma Waveguide Switch and Variable Attenuator for Millimeter-Wave Applications

This paper reports the design, fabrication, and measurement of a millimeter-wave solid-state ?pi-match waveguide switch using bulk silicon micromachining. A photogenerated plasma within a silicon post is utilized as the switching element within the waveguide channel. Not only does this isolate the switch bias network from the RF signal path, but allows for tuning of the OFF-state isolation with increasing optical power for application as a variable attenuator. A measured OFF-state isolation greater than 25 dB up to 40 GHz is reported, with a measured extracted ON-state insertion loss of 0.52 dB at 35 GHz, and less than 0.88 dB across the entire band from 30-40 GHz. The proposed switch illustrates the significant potential for photogenerated silicon plasma switching of high-performance bulk micromachined millimeter-wave waveguides.

A Photogenerated Silicon Plasma Waveguide Switch and Variable Attenuator for Millimeter-Wave Applications

This paper reports the design, fabrication, and measurement of a millimeter-wave solid-state ?pi-match waveguide switch using bulk silicon micromachining. A photogenerated plasma within a silicon post is utilized as the switching element within the waveguide channel. Not only does this isolate the switch bias network from the RF signal path, but allows for tuning of the OFF-state isolation with increasing optical power for application as a variable attenuator. A measured OFF-state isolation greater than 25 dB up to 40 GHz is reported, with a measured extracted ON-state insertion loss of 0.52 dB at 35 GHz, and less than 0.88 dB across the entire band from 30-40 GHz. The proposed switch illustrates the significant potential for photogenerated silicon plasma switching of high-performance bulk micromachined millimeter-wave waveguides.