Length Of Slow Wave Structure Research Articles

Experimental study on performance change in F-band surface-wave oscillator due to slow-wave structure length

A surface-wave oscillator (SWO) is a high-power terahertz-wave source based on a slow-wave structure (SWS). A cylindrical corrugated waveguide is used as the SWS. The length of SWS is an important factor in the oscillation. Experiments of the SWO with various length structures are conducted. An instability responsible for the SWO changes depending on the SWS length. For the 200-period SWS, the radiation is due to the absolute instability and depends on the beam current like the temporal growth rate. By contrast, for the 80-period SWS, the radiation is due to the global instability having the current dependence like the spatial growth rate. Compared to the 80-period SWS, the 200-period SWS can improve the radiation power more than seven times in the backward wave oscillation (BWO) region. The maximum BWO power reaches up to about 10 kW and the maximum electric efficiency of 1.5% is obtained.

Study on Operation of a Surface-Wave Oscillator Around the $\pi$ -Point Region

We numerically and experimentally studied the operation of a W-band (75–110 GHz) surface-wave oscillator driven by the electron beam energy of less than 100 keV. Two numerical methods of the real wavenumber analysis and the saddle point analysis are studied to examine the operation of the oscillator. We analyzed the Cherenkov instability for the interaction of the space charge modes and the structure modes EH01. We showed that the instabilities are of absolute instabilities when the oscillator operates around the $\pi $ -point region. In the experiment, radiations due to the absolute instability are detected around the $\pi $ -point region when the length of the slow-wave structure (SWS) is short enough. On the other hand, the absolute instability generates no meaningful radiation in the $\pi $ -point region for the long length of SWS.

A compact P-band coaxial relativistic backward wave oscillator with only three periods slow wave structure

A compact P-band coaxial relativistic backward wave oscillator (BWO) with only three periods slow wave structure (SWS) is investigated both theoretically and numerically. The characteristics of the coaxial SWS are analyzed when the SWS is changed from the structure with only outer conductor ripple to the structure with both inner and outer conductor ripples. It is found that the existence of the inner conductor ripple can reduce the period length of coaxial SWS to maintain the same operating frequency of the BWO and can largely increase the temporal growth rate and the spatial growth rate of the device. Then, the effects of SWS period numbers on the generation of the microwave in the P-band relativistic BWO are studied by PIC simulations. The results show that three periods SWS cannot only make the device more compact but also has a wide region of single-frequency operation and relatively large efficiency and output power in a wide range of the diode voltage. Typical simulation results show that, with a 585 kV and 7.85 kA electron beam guided by a 0.8 T solenoidal field, the microwave of 1.65 GW is generated at the frequency of 900 MHz, and the interaction efficiency is about 36%. Compared with the conventional P-band coaxial relativistic BWO with five periods SWS, the axial length of the SWS is reduced by about one half, which is only 38.4 cm, and the saturation time of the microwave signal is reduced by about 10 ns.

On the Relative Merits of Travelling-Wave and Resonant Operation of Linacs

Discussion of the relative merits of so-called standing wave vis-a-vis travelling wave operation of linear accelerator waveguide is complicated by various considerations. In the first instance, standing wave should be distinguished from resonant operation. Standing wave operation is exactly the same as travelling wave, excepting that the waveguide is terminated by a total reflection of power instead of a matched load. It is probable that more accelerators are operated standing wave than travelling wave; formerly, medical and industrial accelerators were built with detuned or short-circuited terminations since there is no net energy gain (or loss) from the reflected wave. The only serious drawback to the scheme was that with long pulses the output power of an RF source is often determined by the VSWR of its load and that an RF window is possibly subjected to increased field stress, depending on its relative location with respect to the standing wave pattern. In resonant operation a length of slow wave structure is terminated, theoretically at reflection planes of symmetry; the discrete modes of resonance consist of two oppositely directed travelling wave ensembles, one of which will provide a space harmonic of an intended phase velocity.