Young-Min Shin

System Design Analysis of a 0.22-THz Sheet-Beam Traveling-Wave Tube Amplifier

The primary constituents of a 0.22-terahertz (THz) sheet-beam traveling-wave tube (TWT) amplifier, composed of a staggered double grating array waveguide, have been designed for broadband THz operation (~ 30%) using the fundamental passband (TE-mode). Currently, we are looking into the possibility of a pulsed low-duty test of this device as a proof of principle (POP) and have been making efforts to construct the system. The optimally designed input coupler has ≤ 1 dB insertion loss at 0.22 THz with ~ 75 GHz (34%) 1-dB matching bandwidths. A thin mica RF window provides a coupling bandwidth spanning multiple octaves. The collector is designed to have a jog for collecting the spent beam along the RF path coupled to the output RF window. Computer simulations show that the collector hybridized with a WR-4 window has ~ 60 GHz matching bandwidth with ~ - 0.5 dB insertion loss at 0.22 THz. The hybrid periodic permanent-magnet design combined with the quadrupole magnet (PPM-QM), intended for low-duty pulse operation in a proof-of-concept experiment, allows the elliptical sheet beam from an existing gun (25 : 1 aspect ratio) to unoptimized gun to have 73% beam transmission. The POP pulsed test is designed to be matched to our existing system, which will thereby tolerate beam transmission. However, a proper gun for the sheet-beam tunnel of the designed circuit will provide much better transmission. In our prior works, we successfully proved at W-band that the magnet design provided >; 99% beam transmission of a 10:1 aspect ratio sheet beam. Most of the TWT circuit components have been designed, and currently, a full simulation modeling effort is being conducted.

Particle-in-cell simulation of x-ray wakefield acceleration and betatron radiation in nanotubes

Though wakefield acceleration in crystal channels has been previously proposed, x-ray wakefield acceleration has only recently become a realistic possibility since the invention of the single-cycled optical laser compression technique. We investigate the acceleration due to a wakefield induced by a coherent, ultrashort x-ray pulse guided by a nanoscale channel inside a solid material. By two-dimensional particle in- cell computer simulations, we show that an acceleration gradient of TeV/cm is attainable. This is about 3 orders of magnitude stronger than that of the conventional plasma-based wakefield accelerations, which implies the possibility of an extremely compact scheme to attain ultrahigh energies. In addition to particle acceleration, this scheme can also induce the emission of high energy photons at ~O(10-100) MeV. Our simulations confirm such high energy photon emissions, which is in contrast with that induced by the optical laser driven wakefield scheme. In addition to this, the significantly improved emittance of the energetic electrons has been discussed.

Principle of Terahertz Radiation Using Electron Beams

This part introduces high power THz coherent radiation sources that take advantage of free electron beams. Following a description of characteristics on vacuum electron devices (VEDs), fundamental radiation principle of beam-wave interaction is explained with specifying their types and applications. Conventional high power microwave VEDs such as klystrons, TWTs, gyrotrons, and FELs are described in their technical perspectives with brief overview of device characteristics. Addressing technical challenges on up-conversion-to-THz of conventional approach, this part explores the state-of-the-art micro-VEDs considered for modern THz applications such as communication, imaging, sensing, spectroscopy, and so on, which are combined with modern microfabrication technologies. Novel MEMS techniques to microminiaturize RF components such as electron gun and RF interaction circuits are also presented.

Staggered double vane traveling wave tube amplifier (SDVTWTAs) with multi-staged phase velocity matching

The electron beam co-propagating with slow waves in a staggered double vane array (SDVA) efficiently amplifies millimeter and sub-millimeter waves over a wide spectrum. Our theoretical and numerical analyses show that the power amplification in the fundamental passband is enhanced by the extended beam-wave phase-matching. Particle-in-cell (PIC) simulations on the SDVA slow wave structure, designed with 10.4 keV and 50 - 100 mA sheet beam, indicate that maintaining beam-wave synchronization along the entire length of the circuit improves the gain by 7.3 % leading to a total gain of 28 dB, corresponding to 62 Watts saturated power at middle of operating band, and a 3-dB bandwidth of 7 GHz with 10.5 % at V-band (73.5 GHz center frequency) with saturated peak power reaching 80 watts and 28 dB at 71GHz. These results also show a reasonably good agreement with analytic calculations based on linear gain theory.

Commissioning test of S-band klystron/photo-gun system for femto-second sub-MeV electron beam R&D

A timely stable femto-second electron beam system is being constructed for pump-probe time-resolved experiments and applications. A pulsed S-band klystron is installed and fully commissioned with 5.5 MW peak power in a 2.5 μs pulse length and 1 Hz repetition rate. A single-cell RF photo-gun is designed to produce with 0.16 - 1.6 pC electron bunches in a photo-emission mode within a 600 fs ∓ 3 ps at 0.5 - 1 MeV. The measured RF system jitters are within ± 1 % in magnitude and ± 0.2° in phase, which would induce 3.4 keV and 0.25 keV of ΔEg, corresponding to 80 fs and 5 fs of Δte, respectively. The beam brightness could be improved by replacing a conventional photo-cathode with field emission array or carbon nanotube tips as they limit the emission area, while providing high current emission. PIC simulations indicate that our designed bunch compressor reduces the TOA-jitter by about an order of magnitude. The transport and focusing optics of the designed beamline with the bunch compressor enables an energy spread within 10-4 and a bunch length (electron probe) within > 50 - 100 fs.

X-ray Wakefield Acceleration and Betatron Radiation in Nanotubes

We investigate X-ray laser pulse induced wakefield acceleration in a nanotube inside a solid material via 2D particle-in-cell simulations. Meanwhile, QED betatron radiation, improved emittance of the energitc electrons has been discussed.

Electron Lens for the Fermilab Integrable Optics Test Accelerator

The Integrable Optics Test Accelerator (IOTA) is a research machine currently being designed and built at Fermilab. The research program includes the study of nonlinear integrable lattices, beam dynamics with self fields, and optical stochastic cooling. One section of the ring will contain an electron lens, a low-energy magnetized electron beam overlapping with the circulating beam. The electron lens can work as a nonlinear element, as an electron cooler, or as a space-charge compensator. We describe the physical principles, experiment design, and hardware implementation plans for the IOTA electron lens.

Development of a ultra-short pulse electron beam source for advanced accelerator/radiation source research at Northern Illinois University

We are currently developing an ultra-short pulsed RF-gun by means of a series of comprehensive modeling and design processes. A complete system will be available for applications such as ultra-fast electron diffraction (UED) system, high power coherent radiation sources (THz/X-Ray), and advanced accelerators that normally require a high brightness electron beam source. All the cavity features, including input coupler, are engineered with EM and particle-in-cell simulators (HFSS, ASTRA, and CST) for beam energy from 0.5-1 MeV (safety limited) to be powered by our S-band (2.856 GHz) klystron (max. power = 5.5 MW). The klystron (Thales TH2163) and modulator system (ScandiNova K1 turnkey system) were successfully installed and fully commissioned. Performance tests of the klystron system show peak output power > 5 MW, as per operation specifications. At the quasi-relativistic energies, the electron source is capable of generating 100fC-100 pC electron bunch with sub-ps pulse duration. Our current approach on RF-gun design is focused on preserving time of flight (TOF) of the electron bunch from the cathode to the interaction chamber. A system that is inherently time-stable can be more useful as an analytic tool in the laboratory. A stabilized system can also be used to collect more data at some desired value of time delay, such as to better resolve a fast transition. Particle-in-cell simulations have shown that the electron bunch undergoes fast RF acceleration, rapidly reaching the desired energies, which can be controlled by tuning RF injection phase and input driving power. Engineering design and beam optics assessment of the photoinjector gun is currently underway, which is scheduled to be fully installed and commissioned in 2014.

Electron cloud density analysis using microwave cavity resonance.

We report on a method to detect an electron cloud in proton accelerators through the measurement of the phase shift of microwaves undergoing controlled reflections with an accelerator vacuum vessel. Previous phase shift measurement suffered from interference signals due to uncontrolled reflections from beamline components, leading to an unlocalized region of measurement and indeterminate normalization. The method in this paper introduces controlled reflectors about the area of interest to localize the measurement and allow normalization. This paper describes analyses of the method via theoretical calculations, electromagnetic modeling, and experimental measurements with a bench-top prototype. Dielectric thickness, location and spatial profile were varied and the effect on phase shift is described. The effect of end cap aperture length on phase shift measurement is also reported. A factor of ten enhancement in phase shift is observed at certain frequencies.