Cold Test Validation of Novel Slow Wave Structure for High-Power Backward-Wave Oscillators

Chipengo, Ushemadzoro; Nahar, Niru K.; Volakis, John L.

doi:10.1109/tps.2016.2559163

Cited by 17 publications

(9 citation statements)

References 22 publications

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“…This approach is also utilized in [36], for instance, to retrieve the dispersion of a slow-wave structure. As such, the dispersion relation takes the general form…”

Section: B Reflection Measurementsmentioning

confidence: 99%

Experimental Demonstration of Degenerate Band Edge in Metallic Periodically Loaded Circular Waveguide

Othman¹,

Pan

Atmatzakis

et al. 2017

IEEE Trans. Microwave Theory Techn.

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Abstract-We experimentally demonstrate for the first time the degenerate band edge (DBE) condition, namely the degeneracy of four Bloch modes, in loaded circular metallic waveguides. The four modes forming the DBE represent a degeneracy of fourth order occurring in a periodic structure where four Bloch modes, two propagating and two evanescent, coalesce. It leads to a very flat wavenumber-frequency dispersion relation, and the finitelength structure's quality factor scales as N 5 where N is the number of unit cells. The proposed waveguide in which DBE is observed here is designed by periodically loading a circular waveguide with misaligned elliptical metallic rings, supported by a low-index dielectric. We validate the existence of the DBE in such structure using measurements and we report good agreement between full-wave simulation and the measured response of the waveguide near the DBE frequency; taking into account metallic losses. We correlate our finding to theoretical and simulation results utilizing various techniques including dispersion synthesis, as well as observing how quality factor and group delay scale as the structure length increases. Moreover, the reported geometry is only an example of metallic waveguide with DBE: DBE and its characteristics can also be designed in many other kinds of waveguides and various applications can be contemplated as high microwave generation in amplifiers and oscillators based on an electron beam interaction or solid state devices, pulse compressors and microwave sensors.Index Terms-Degenerate band edge (DBE), slow-wave structures, cavity resonators.

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“…This approach is also utilized in [36], for instance, to retrieve the dispersion of a slow-wave structure. As such, the dispersion relation takes the general form…”

Section: B Reflection Measurementsmentioning

confidence: 99%

Experimental Demonstration of Degenerate Band Edge in Metallic Periodically Loaded Circular Waveguide

Othman¹,

Pan

Atmatzakis

et al. 2017

IEEE Trans. Microwave Theory Techn.

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“…[10][11][12][13][14][15][16][17][18] However, a very few experiments have been carried out to actually generate high power microwaves with an electron beam passing through an MTM structure. [19][20][21][22][23] At MIT, in our previous experiment, 19 we built a structure with two MTM plates loaded in a waveguide with dimensions below the cut-off of the TM 11 mode.…”

Section: Introductionmentioning

confidence: 99%

High power long pulse microwave generation from a metamaterial structure with reverse symmetry

Stephens

Mastovsky

et al. 2018

Physics of Plasmas

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Experimental operation of a high power microwave source with a metamaterial (MTM) structure is reported at power levels to 2.9 MW at 2.4 GHz in full 1 μs pulses. The MTM structure is formed by a waveguide that is below cutoff for TM modes. The waveguide is loaded by two axial copper plates machined with complementary split ring resonators, allowing two backward wave modes to propagate in the S-Band. A pulsed electron beam of up to 490 kV, 84 A travels down the center of the waveguide, midway between the plates. The electron beam is generated by a Pierce gun and is focused by a lens into a solenoidal magnetic field. The MTM plates are mechanically identical but are placed in the waveguide with reverse symmetry. Theory indicates that both Cherenkov and Cherenkov-cyclotron beam-wave interactions can occur. High power microwave generation was studied by varying the operating parameters over a wide range, including the electron beam voltage, the lens magnetic field, and the solenoidal field. Frequency tuning with a magnetic field and beam voltage was studied to discriminate between operation in the Cherenkov mode and the Cherenkov-cyclotron mode. Both modes were observed, but pulses above 1 MW of output power were only seen in the Cherenkov-cyclotron mode. A pair of steering coils was installed prior to the interaction space to initiate the cyclotron motion of the electron beam and thus encourage the Cherenkov-cyclotron high power mode. This successfully increased the output power from 2.5 MW to 2.9 MW (450 kV, 74 A, 9% efficiency).

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“…Equations (17,27) are the instability conditions: the electron beam loses energy γ(z) < γ(0), gains transverse momentum p ⊥ , and the field amplitude is the highest at the entrance |A(0)| > |A(z)| because of the backward wave. We now introduce the linear current represented as…”

Section: Beam Interaction With Antisymmetric Mode: Cherenkov-cycmentioning

confidence: 99%

“…This is a promising approach for vacuum electron devices since the subwavelength dimensions of MTMs can lead to compact microwave sources smaller than conventional devices such as coupled-cavity traveling-wave tubes or klystrons 15 . Different interaction circuits formed as MTMloaded waveguides have been designed and tested at low microwave power levels 10,16,17 . High power experiments of MTM sources with relativistic electron beams have been reported 5,6,18 .…”

Section: Introductionmentioning

confidence: 99%

Linear theory of instabilities generated by an electron beam in a metamaterial-loaded waveguide

Shapiro

Temkin

2019

Physics of Plasmas

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We present the linear theory of the starting current of Cherenkov-cyclotron and Cherenkov instabilities generated by an electron beam passing through a metamaterial-loaded waveguide. Effective medium theory is used to represent the metamaterial structure properties. The theory predicts that the instabilities compete, with the Cherenkov-cyclotron mode dominating at lower magnetic field and the Cherenkov instability at higher magnetic field. The theoretical results are compared to results from recent experiments at MIT using a 490 kV, 84 A electron beam in magnetic fields of 300 G to 1500 G. For an effective medium model fitted to the MIT experimental parameters, theory predicts that the Cherenkov-cyclotron mode will dominate below 780 G and the Cherenkov mode above 780 G, in good agreement with experimental observations of switching between these modes at 750 G. The analytical theory allows a better understanding of the mode competition and the dependence of the instabilities on key parameters such as voltage, current and magnetic field.

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Cold Test Validation of Novel Slow Wave Structure for High-Power Backward-Wave Oscillators

Cited by 17 publications

References 22 publications

Experimental Demonstration of Degenerate Band Edge in Metallic Periodically Loaded Circular Waveguide

Experimental Demonstration of Degenerate Band Edge in Metallic Periodically Loaded Circular Waveguide

High power long pulse microwave generation from a metamaterial structure with reverse symmetry

Linear theory of instabilities generated by an electron beam in a metamaterial-loaded waveguide

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