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RISS 인기검색어
- 검색키워드
- 저자 : Ashwini Sawant (검색결과 1 건)
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Development of a Second Harmonic Dual Mode Gyrotron for Generation of Orbital Angular Momentum Beam
Ashwini Sawant Graduate School of UNIST 2019 국내박사
This thesis presents the idea of the existence of the orbital angular momentum in gyrotron modes and includes the design process of a gyrotron source to demonstrate the generation of the OAM modes. The cavity modes of gyrotrons are rotating modes with no electrical field intensity along the axis. These modes are analogous to an energy packet revolving around an axis and generating an orbital angular momentum of order (l). We have numerically derived the existence of the orbital angular momentum in the gyrotron modes, which is proportional to the azimuthal index of the mode. We have reported the existence of the orbital angular momentum in gyrotron through a low power test carried out by a quasi-optical mode converter to generate a TE6,2 mode. We found out that the phase pattern of the emitted modes is having spiral patterns, which is the easiest way to identify the order of the low power OAM modes. We also presented a new modified phase-retrieval technique to determine the topological charges of the high-power OAM beams. The OAM property of the high-power gyrotron modes can be useful in wireless communication for enhancing the channel capacity and transmission distance. Development of a 190 GHz, dual mode gyrotron has been carried out for this work in UNIST. This gyrotron can operate on TE8,3 and TE11,2 mode to resonate at 188.5 GHz and 190.5 GHz respectively. The gyrotron is having an axial output, where the cavity modes are directly emitted into the free space through an overmoded tapered waveguide section to reduce its divergence. The gyrotron modes are second harmonic modes and generates around 30 kW of power with an efficiency of around 18%. To carry out the design simulations of the gyrotron, a time-dependent numerical code named as UNIST Gyrotron Design Tool (UGDT) has been developed. Second harmonic gyrotron modes are always susceptible to be suppressed by fundamental mode excitation. To avoid this fundamental mode excitation, the cavity opted for the gyrotron is a perturbed cavity with two sinusoidal perturbation at the straight section of the cavity to excite stable higher order axial modes. After a rigorous analysis of the perturbed cavity, a cavity with two sinusoidal perturbations and a raised cosine up-taper has been manufactured. Developed 190 GHz OAM gyrotron has been tested and its performance analysis has been carried out. The generated modes have been recognized based on the frequency measurements using heterodyne frequency measurement system. However, the preliminary results concluded in this thesis suggests the excitation of the desired TE8,3 mode have been observed along with the spurious fundamental mode excitation due to the alignment problem of the vacuum tube in the superconducting magnet. We confirmed this issue with the simulation results and will resolve it in our future work.
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