Observation of Narrow-Band Terahertz Coherent Cherenkov Radiation from a Cylindrical Dielectric-Lined Waveguide

Abstract

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.

Surface impedance formalism for a metallic beam pipe with small corrugations

“…Our theory allows for direct comparison with metallic tubes lined with a thin dielectric layer, another approach for generating terahertz radiation [15]. Indeed, the main characteristic of the radiation-its frequency-is expressed by Eq.…”

“…(17) through the effective values of and . For a pipe with wall corrugations these parameters are calculated as described in this paper, for the dielectric structure is just the electric permeability for the dielectric (e.g., ¼ 3:8 for the fused silica used in [15]), and is typically close to 1.…”

“…A metallic pipe with wall corrugations is of special interest in light of recent proposals to use pipes with corrugated surfaces for the generation of terahertz radiation [13], and for energy dechirping of electron bunches in free electron lasers [14] (note that the use of a metallic pipe with a thin dielectric layer has also been studied for these purposes; see, e.g., Refs. [15,16]). Following these papers, we will focus on the case of azimuthally symmetric structures with small rectangular corrugations (in longitudinal view) as shown in Fig.…”

Terahertz radiation source based on self-wake beam bunching

“…Grating structures can be used to excite a THz Smith-Purcell radiation [18]. Finally, one can use coherent Cherenkov radiation (CCR) [19,20]. The length of the THz extraction structure controls the resulting bandwidth (less than 1% is attainable).…”

Tailoring of Highly Intense THz Radiation Through High Brightness Electron Beams Longitudinal Manipulation

“…This method makes the SPARC_LAB THz source flexible, allowing both broadband and narroband THz generation, taking advantage from the laser comb technique integrated in the velocity bunching regime [12]. This versatility is not possible in both undulator-based and dielectric waveguide THz sources [13,14], where the broadband emission cannot be achieved. In addition, unlike other ways of bunch train generation, for instance chirped-pulse beating [15] and dispersive mask techniques [16], the laser comb scheme permits a full control of both the temporal bunch inter-distance and the amplitude pulse shape.…”