Ashwini Sawant

Orbital Angular Momentum (OAM) of Rotating Modes Driven by Electrons in Electron Cyclotron Masers

The well-defined orbital angular momentum (OAM) of rotating cavity modes operating near the cutoff frequency excited by gyrating electrons in a high-power electron cyclotron maser (ECM)—a gyrotron—has been derived by photonic and electromagnetic wave approaches. A mode generator was built with a high-precision 3D printing technique to mimic the rotating gyrotron modes for precise low-power measurements and shows clear natural production of higher-order OAM modes. Cold-test measurements of higher-order OAM mode generation promise the realization towards wireless long-range communications using high-power ECMs.

Study on statistical breakdown delay time in argon gas using a W-band millimeter-wave gyrotron

In this study, we investigated plasma initiation delay times for argon volume breakdown at the W-band frequency regime. The threshold electric field is defined as the minimum electric field amplitude needed for plasma breakdown at various pressures. The measured statistical delay time showed an excellent agreement with the theoretical Gaussian distribution and the theoretically estimated formative delay time. Also, we demonstrated that the normalized effective electric field as a function of the product of pressure and formative time shows an outstanding agreement to that of 1D particle-in-cell simulation coupled with a Monte Carlo collision model [H. C. Kim and J. P. Verboncoeur, Phys. Plasmas 13, 123506 (2006)].

Accurate identification of whispering gallery mode patterns of gyrotron with stabilized electro-optic imaging system

The precise field pattern measurement and analysis of a typical whispering gallery mode excited in a gyrotron are important to understand the interaction physics of the gyrotron. We precisely analyzed the characteristic of a whispering gallery mode, rotating TE6,2 mode, by a photonic-assisted W-band (75–110 GHz) electro-optic imaging measurement system. The whispering gallery mode in the W-band region diverges fast in free space as it propagates from the radiation port. Therefore, scanning the field patterns of a device-under-test should be performed as close as possible to identify the devices characteristics. We successfully accomplished visualizing highly accurate field patterns of a rotating and mixed whispering gallery mode based on the measured electric field magnitude and phase by using dual optical fiber-scale electro-optic (EO) probes. We observed the distorted fields when the typical open-ended waveguide and a general EO probe were used in the extremely near-field zone, whereas a very precise f...

Competition-free second harmonic mode THz orbital angular momentum gyrotron with dual-mode operation by a perturbed cavity

We propose a dual-mode orbital angular momentum (OAM) scheme at 400 GHz that uses a gyrotron. A conventional gyrotron with two modes of operation has a second harmonic resonance that competes with the fundamental mode. We used a perturbed cavity to reduce the fundamental mode competition in a second harmonic gyrotron by allowing higher-order axial mode operation. Based on a simulation using an in-house self-consistent gyrotron code, we demonstrated that the operating dual second harmonic modes TE6,4(+)/TE12,2(+) of the OAM source in the perturbed cavity at around 400 GHz can be excited without competition from the fundamental mode TE4,2(−).

Generation and Validation of Topological Charges of High-Power Gyrotron Orbital Angular Momentum Beams From Phase Retrieval Algorithm

We present a phase coefficient optimization technique for determining the vortex charge of a high-power orbital angular momentum beam. This high-power vortex beam is generated by transmitting a Gaussian-like beam emitted by a gyrotron with an output power of 25 kW through a spiral phase plate (SPP), which introduces a vorticity at the center of the beam characterized by the geometrical parameters of the SPP. One rigorous, intensity-measurement-based, phase retrieval technique, known as the Gerchberg–Saxton algorithm, does not converge to a correct solution because of the presence of a phase singularity in the high-power vortex beam. Here, we introduce a new phase retrieval algorithm that chooses an appropriate initial phase estimate. This technique yields successful vortex charge determination for both low- and high-power vortex beams. The retrieved intensity profiles show 99.51% and 99.27% intensity regeneration at measurement planes with optimized initial phase estimates for the low- and high-power beams, respectively.

Measuring the carrier lifetime by using a quasi-optical millimeter- and THz-wave system

The existing method for contactless measurement of the photoconductivity decay time is limited in terms of sample selection according to the injection level or doping density. To solve this problem and improve the measurement sensitivity, we developed a quasi-optical photoconductivity decay (QO-PCD) technique based on millimeter- and terahertz-wave technology. A semi-insulating silicon (Si) wafer was used in a proof-of-concept experiment with the proposed QO-PCD system to find the initial excess carrier density and carrier lifetime based on the Drude–Zener model with a single decay function. The initial excess carrier density and carrier lifetime were measured to be 1.5 × 1015 cm−3 and 30.6 μs, respectively, in semi-insulating Si wafer (460 μm thickness). A 2D areal measurement of the decay time of the Si wafer was experimentally obtained. The proposed QO-PCD technique can provide more reliable and sensitive carrier lifetime measurement data for semiconductor wafers, which may impact the fields of photovo...

Dual band operation of a photonic band gap cavity for 400 GHz gyrotron

We introduce the design of a photonic bandgap (PBG) cavity for a 400 GHz gyrotron which can support a dual mode operation. The PBG cavity generates TE6,4 and TE12,2 mode at 399.98 GHz and 402.30 GHz respectively. The simulations of the cavity is performed by recently developed in house gyrotron simulation code, UGDT, which confirms few kWs of output power with 14/17 kV, 3A electron beam.

A Method for Pulse Shortening of High-Power Millimeter Waves Using Plasma Breakdown

We report an efficient pulse shortening technique of a high-power millimeter source using plasma breakdown. A W-band gyrotron with a frequency of 95 GHz and an output power of 30 kW was used for the demonstration. The pulse shortening technique was based on plasma breakdown phenomena that do not need any change in the high-power source and its modulator. The pulse shortening of a factor of 20 was demonstrated. This method may provide a simple and compact system for controlling the RF pulse length without modifying the main millimeter-wave system.