G. S. Park

Dynamics of mode competition in a gigawatt-class magnetically insulated line oscillator

An axial mode competition is observed in a 1GHz magnetically insulated line oscillator operating at gigawatt power level with a pulse duration of 130ns. A fast-growing axial mode adjacent to desired π-mode starts up first and hops to the slow-growing and stable π mode. The dynamics of the mode competition is found to be strongly dependent on the time-varying axial velocity of the magnetically insulated electron beam. The experimental observation is verified by the particle-in-cell simulation using a time-frequency analysis.

Yet another tutorial of disturbance observer: robust stabilization and recovery of nominal performance

This paper presents a tutorial-style review on the recent results about the disturbance observer (DOB) in view of robust stabilization and recovery of the nominal performance. The analysis is based on the case when the bandwidth of Q-filter is large, and it is explained in a pedagogical manner that, even in the presence of plant uncertainties and disturbances, the behavior of real uncertain plant can be made almost similar to that of disturbance-free nominal system both in the transient and in the steady-state. The conventional DOB is interpreted in a new perspective, and its restrictions and extensions are discussed.

Reduction of noise in strapped magnetron by electric priming using anode shape modification

Noise reduction in a 2.45GHz strapped magnetron oscillator is experimentally demonstrated by electric priming using anode shape modification. The sideband noise is reduced by approximately 15dB at the nominal operating current and by 28dB at the start-oscillation current; this is due to electron prebunching into the π mode, resulting from the modulation of the drift velocity of the electrons by an azimuthally periodic electric field. In this experiment, a 4.3kV–330mA half-wave rectified input power is employed.

Embedding Internal Model in Disturbance Observer With Robust Stability

The disturbance observer has been widely employed in applications due to its powerful ability for disturbance rejection and robustness under plant uncertainties. However, it rejects the disturbance approximately rather than exactly since it is usually designed without considering structural properties of disturbance. In order to improve the disturbance rejection performance, we propose a design method to embed the internal model of disturbance into the disturbance observer structure. Furthermore, a systematic design procedure is proposed so that one can always design the disturbance observer to guarantee robust stability of the closed-loop system even though uncertain parameters of the plant belong to an arbitrarily large (but bounded) set.

Transmission of gigawatt-level microwave using a beam-rotating mode converter in a relativistic backward wave oscillator

Gigawatt-level circularly polarized radiation was transmitted using a coaxial beam rotating antenna in an X-band relativistic backward wave oscillator. The mode conversion from the TM01 mode to the circularly polarized TE11-like mode was experimentally and numerically shown. The simulated radiation pattern was in good agreement with the measured radiation pattern.

Rejection of polynomial-in-time disturbances via disturbance observer with guaranteed robust stability

The type-k disturbance observer, known as ‘high order disturbance observer’, is a variant of disturbance observer which contains an internal model for polynomial-in-time disturbances so that it can reject those disturbances exactly as well as unmodelled low frequency disturbances approximately. This paper presents a systematic design procedure for the type-k disturbance observer so that the closed-loop system is robustly internally stable and at the same time polynomial-in-time disturbances are rejected exactly. To develop the procedure, we employ the Kharitonov theorem and a root-locus based design tool to deal with the uncertain plant parameters explicitly, and as a result it is guaranteed that one can always find a solution step by step even if the uncertain parameters belong to an arbitrarily large (but bounded) set.

Experimental investigation of millimeter wave folded-waveguide TWT

Folded-waveguide traveling-wave tube (FWTWT) is one of the candidates for high power, broad-band robust millimeter wave sources. The FWTWT has no periodic reactive load. Therefore, the dispersion relation of space harmonics is estimated by simple transform of that of rectangular waveguide. The folded waveguide circuits are designed and fabricated using 12 kV, 150 mA linear electron beam for Ka-band amplifier and oscillator based on the synthesis process. Solenoid focusing system and pulsed electron beam are used to investigate basic properties of FWTWT in the laboratory. Non-stationary beam-wave interactions performance are investigated by MAGIC3D, and one-dimensional code. From initial folded-waveguide BWO experiment, about 20-watt output power at 35 GHz is obtained with 6% linear tunability. The LIGA fabrication is in process for the operation at the higher frequency.

On robust stability of disturbance observer for sampled-data systems under fast sampling: An almost necessary and sufficient condition

Despite increased interest on discrete-time disturbance observer (DOB) in both theory and application, study of robust stabilization via the DOB does not seem to be mature. This is because most of the existing studies on robust stability are based on the small-gain theorem, so that the results are just sufficient conditions and the amount of uncertainty that the DOB-based control system can tolerate is conservative. In this paper, motivated by a recent work on the continuous-time case, we present an almost necessary and sufficient condition for robust stability of the sampled-data system controlled by a discrete-time DOB under fast sampling. In particular, our study clarifies the phenomenon that the sampling process can hamper stability of the DOB-controlled systems by generating additional (and possibly nonminimum-phase) zeros, and explains why the blind discretization of the continuous-time DOB controller using fast sampling may fail. For the robust stabilization against both these extra zeros and plant uncertainty, we also propose a new design methodology for the nominal model and the Q-filter. It turns out that arbitrarily large uncertainty can be compensated by appropriately designing the Q-filters and by fast sampling. A benchmark problem is revisited to illustrate the validity of the proposed analysis and design method.

1.5 octave wideband traveling-wave tube with heavily-loaded helical slow-wave structure

Summary form only given. A 1.5 octave wideband traveling wave tube (TWT) with a helical structure loaded by the thick dielectric support rods has been designed and fabricated for the frequency range of 6-18 GHz. Helical slow-wave structure (SWS) was modeled using three-dimensional HFSS code. The nonresonant perturbation measurement using a thin copper wire with 20 mm diameter was performed to verify the phase velocity and interaction impedance of the helical structure. The performance of TWT was predicted using one-dimensional (1-D) nonlinear theory involving a macro particle beam model. The harmonic effect was considered in this calculation. The measured performance of TWT using a beam voltage 4 kV and a beam current of 120 mA was shown. These results were compared with a 1-D nonlinear theory. The comparison showed that the measured power and gain were less than the predicted one but had a similar trend over the operating frequency range. The 2nd harmonic levels at the low frequency range of 6-8 GHz were nearly 0 dBc. This relatively high 2nd harmonic level might be attributed to the positive dispersion at the low frequency range due to the deformation of a barrel during the assembly process.

When adversary encounters uncertain cyber-physical systems: Robust zero-dynamics attack with disclosure resources

In this paper we address the problem of designing a robust stealthy attack for adversaries to compromise an uncertain cyber-physical system without being detected. We first re-interpret the zero-dynamics attack based on the normal form representation. Then, a new alternative zero dynamics attack is presented for uncertain systems. This alternative employs a disturbance observer and does not require exact system knowledge in order to remain stealthy. The proposed robust zero-dynamics attack needs a nominal model of the system and, in addition, utilizes the input and output signals of the system. The proposed attack illustrates how the adversary is able to use disclosure resources instead of exact model knowledge. A simulation result with a hydro-turbine power system is presented to verify the attack performance.