TST, Vol. 8, No. 2, PP. 41-49
(Invited paper) High power terahertz production from relativistic electron beams
George R. Neil * and Gwyn P. Williams (retired)
Thomas Jefferson National Accelerator Facility
* Email: neil@jlab.org
(Received June 5, 2015)
Abstract: Relativistic electron beams are highly suitable as generators of high power terahertz radiation. The intensity of the radiation is greatly enhanced by the g4 scaling of synchrotron emission. In addition it is possible to take advantage of collective effects. For wavelengths longer than an electron bunch the electric fields coherently add and the power scales in quadrature. Given that typical electron bunches in accelerator-driven relativistic electron beams contain on the order of 109 electrons, this enhancement factor can be significant. For single bunches of electrons undergoing transverse acceleration such as bending in a static magnetic field, the radiation is broadband and the enhancement extends in wavelength from the beampipe dimensions down to the electron pulse length [1].
Collective effects also can be enhanced in devices such as free electron lasers (FELs) where the electrons undergo microbunching at the optical (in this case terahertz) wavelength and the radiation becomes narrowband and fully coherent [2]. FELs have been around since 1977 providing not only a test bed for the physics of optics and electron/photon interactions but as a workhorse of scientific research. Recent extensions in average current by the application of energy recovery to the accelerator [2, 3] further enhance the power capability of these systems. The characteristics that have driven the development of these sources are the desire for high peak and average power, high micropulse energies, wavelength tunability, timing flexibility, and wavelength production unavailable from more conventional laser sources. Operation of FELs in the FIR to THz regime poses special challenges which have been and are being addressed at a number of facilities around the world. This talk will review the mechanisms of radiation production by relativistic electron beams both as broadband collective sources and coherent sources such as FELs. We will give status and examples of linacs and FELs operating in this regime and discuss future efforts. Applications for use of the radiation have evolved from simple imaging to complex pump probe tests of insulator/metal transitions and energy flow in organic molecules. We will also discuss the technologies for generating and controlling the IR/FIR/THz radiation and mention some of the unique applications of such sources.
Keywords: Terahertz, Free electron laser, Accelerator
doi: 10.11906/TST.041-049.2015.06.05
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TST, Vol. 8, No. 2, PP. 50-57
(Invited paper) Experimental and theoretical investigations on coaxial gyrotron with two electron beams
Diwei Liu 1*, 2, Yang Yan 1, 2, Sheng Yu 1, 2, Wenjie Fu 1, 2, and Shenggang Liu 1, 2
1 Terahertz Science and Technology Center, School of Physical Electronics,University of Electronic Science and Technology of China, Chengdu, Sichuan, China, 610054
2 Cooperative Innovation Center of Terahertz Science, Chengdu, Sichuan, 610054, China
*1 Email: dwliu@uestc.edu.cn
(Received 15 June 2015)
Abstract: The coaxial gyrotron with two electron beams is investigated in this paper. The results of the linear and nonlinear theory calculation show that CGTB has some distinguished advantages, such as improved mode competition and enhanced output power, so CGTB may be capable of providing 2-4 MW continuous-wave output power at 170 GHz to meet the demand of ITER Program. The results of the numerical calculation and PIC simulation show that CGTB can operate at two different frequencies simultaneously. In addition, the power of the high harmonic can be enhanced due to the nonlinear coupling between two electron beams. The prototype of CGTB is fabricated and the verification experiment is being conducted in progress.
Keywords: Terahertz, Coaxial gyrotron with two electron beams, Dual-frequency operation.
doi: 10.11906/TST.050-057.2015.06.06
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TST, Vol. 8, No. 2, PP. 58-68
(Invited paper) Terahertz wave generation based on laser-induced microplasmas
Fabrizio Buccheri * and Xi-Cheng Zhang
The Institute of Optics, University of Rochester, 275 Hutchison Road, Rochester, NY, 14627 USA
* Email: fbuccher@ur.rochester.edu
(Received June 17, 2015)
Abstract: Ambient air can be used as a THz wave emitter and/or THz sensor when ionized by femtosecond laser fields. The integration of such a plasma source and detector in terahertz time-domain techniques allows spectral measurements covering the elusive terahertz gap, further increasing the impact of those scientific tools in the study of the four states of matter. We report on a new paradigm for implementing THz plasma techniques. Specifically, we replace the use of elongated plasmas, with lengths ranging from a few mm to several cm, with sub-mm size plasmas, which we will refer to as microplasmas, obtained by focusing laser pulses with high numerical aperture microscope objectives (NA > 0.4). Those microplasmas have in fact unique properties compared to any other THz source and sensor, with the potential of enabling new and exciting applications. Specifically, a microplasma requires orders of magnitude less laser pulse energy to be created, enabling plasma-based terahertz technique to be implemented with low energy ultrafast lasers. Moreover, they offer a generation, or detection, volume with sub-wavelength size (1 THz = 300 mm), which could be exploited to implement near-field THz plasma techniques. In this paper we describe the experimental study of the terahertz emission from a laser-induced plasma of submillimeter size. One of the interesting phenomena is that the main direction of THz wave emission is almost orthogonal to the laser propagation direction, unlike that of elongated plasmas. Perhaps the most important achievement is that we have demonstrated that laser pulse energies lower than 1 μJ are sufficient to generate measurable terahertz pulses from ambient air. This significant decrease in the required laser energy will make plasma-based terahertz techniques more accessible to the scientific community.
Keywords: Terahertz spectroscopy, Plasmas, Photoionization, Ultrafast nonlinear optics.
doi: 10.11906/TST.058-068.2015.06.07
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TST, Vol. 8, No. 2, PP. 69-84
(Invited paper) Electron beam excitation of surface plasmon polaritons and transformation into electromagnetic radiation
Shenggang Liu 1*, 2, Min Hu 1, 2, Renbin Zhong 1, 2, Xiaoxing Chen 1, 2, Ping Zhang 1, 2, Sen Gong 1, 2, Tao Zhao 1, 2
1 Terahertz Science and Technology Center, School of Physical Electronics,University of Electronic Science and Technology of China, Chengdu, Sichuan, China, 610054
2 Cooperative Innovation Center of Terahertz Science, Chengdu, Sichuan, 610054,
*1 Email: liusg@uestc.edu.cn
(Received 16 June, 2015)
Abstract: The results of many years’ research show that only by means of electronics (both vacuum and semiconductor electronics) or only by means of photonics, the whole Terahertz gap could not be covered. Therefore, to look for a way to remove the difficulties is certainly a big challenge. Recently, a novel physical phenomenon has been found: in a structure of nano-scale metal film with dielectric medium loading, the Surface Plasmon Polaritons excited by electron beam can be transformed into enhanced electromagnetic radiation. Based on this physical phenomenon, a concept of combining the electronics and photonics to generate electromagnetic radiation comes up. In this paper, we show that this concept leads to a novel approach, which can not only cover the whole THz gap, but also generate enhanced coherent radiation from THz to ultraviolet.
Keywords: Electron beam, Surface plasmon polaritons, Terahertz source, Graphene
doi: 10.11906/TST.069-084.2015.06.08
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