We report experimental observation of narrow-band coherent Cherenkov radiation driven by a subpicosecond electron bunch traveling along the axis of a hollow cylindrical dielectric-lined waveguide. For an appropriate choice of dielectric wall thickness, a short-pulse beam current profile excites only the fundamental mode of the structure, producing energetic pulses in the terahertz range. We present detailed measurements showing a narrow emission spectrum peaked at 367 + or - 3 GHz from a 1 cm long fused silica capillary tube with submillimeter transverse dimensions, closely matching predictions. We demonstrate a 100 GHz shift in the emitted central frequency when the tube wall thickness is changed by 50 microm. Calibrated measurements of the radiated energy indicate up to 10 microJ per 60 ps pulse for an incident beam charge of 200 pC, corresponding to a peak power of approximately 150 kW.
Various projects under study require an angular-momentum-dominated electron beam generated by a photoinjector. Some of the proposals directly use the angular-momentum-dominated beams (e.g., electron cooling of heavy ions), while others require the beam to be transformed into a flat beam (e.g., possible electron injectors for light sources and linear colliders). In this paper we report our experimental study of an angular-momentum-dominated beam produced in a photoinjector, addressing the dependencies of angular momentum on initial conditions. We also briefly discuss the removal of angular momentum. The results of the experiment, carried out at the Fermilab/NICADD Photoinjector Laboratory, are found to be in good agreement with theoretical and numerical models.
Microbunching of a relativistic electron beam into a helix is examined analytically and in simulation. Helical microbunching is shown to occur naturally when an e beam interacts resonantly at the harmonics of the combined field of a helical magnetic undulator and an axisymmetric input laser beam. This type of interaction is proposed as a method to generate a strongly prebunched e beam for coherent emission of light with orbital angular momentum at virtually any wavelength. The results from the linear microbunching theory show excellent agreement with three-dimensional numerical simulations.
Some of the equations in the manuscript appear incorrectly. Given the definition of the undulator fields, the total relativistic factor is related to the longitudinal factor z via 2 ¼ 2 z ð1 þ K 2 =2Þ. As such, the correct numerical value of the undulator parameter should read K= ffiffiffi 2 p ¼ 0:65 in the captions in Figs. 2 and 3.The correct harmonic field expansion in Eq. (6) iswith the exponential e Çi inside the brackets and acted upon by the derivative operators for h > 2.The bunching factor in Eq. (13) should be b l ðzÞ ¼ 1 2n 0 R fðr ? Þd 2 r ? Z n 1 ðrÞe Àil d 2 r ? ;(2) which yields b ¼ 1=2 for complete bunching at the l ¼ 0 e-beam mode in the linear model, not unity as mentioned in the manuscript. These unfortunate errors do not affect the numeric results, or the conclusions of the Letter.PRL 105, 269907 (2010)
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