Abstract
An electron beam passing through a tube of small inner diameter which is lined on the inside with a dielectric layer will radiate energy in the THz range due tothe interaction with the boundary. The resonant enhancement of certain frequencies is conditioned by structure parameters such as tube radius and the permittivity and thickness of the dielectric layer. In low-loss structures narrow-band radiation is generated which can be coupled out by suitable antennas. For higher frequencies, the coupling to the resistive outer metal layer becomes increasingly important. The losses in the outer layer prohibit reaching higher frequencies with narrow-band conditions. Instead, short broad-band pulses can be generated with still attractive power levels. In the first section of the paper, a general theory of the impedance of a two-layer structure is presented and the coupling to the outer resistive layer is discussed. Approximate relations for the radiated energy, power and pulse length for a set of structure parameters are derived and compared with numerical results in the following section. Finally, the first numerical result of the out-coupling of the radiation by means of a Vlasov antenna and estimates of the achieved beam quality are presented.
Highlights
X-ray free-electron lasers (XFELs) are the brightest, tunable sources of short X-ray pulses available for basic scientific research
A suitable THz source has to be synchronized to the XFEL facility with a low temporal jitter and it has to be able to deliver THz
We present first a generalized description of the round tube impedance; the pure dielectric layer and the metallic layer case are treated as limiting cases of this general form
Summary
X-ray free-electron lasers (XFELs) are the brightest, tunable sources of short X-ray pulses available for basic scientific research. The high beam energy of XFEL facilities (> 10 GeV) can require a total undulator length of 10 m with peak fields of up to 7.3 T and a period length of 1 m to comply with only a portion of the requested THz parameters While such an undulator appears to be technically feasible with state-of-the-art superconducting technology, the cost and complexity of such a device is not attractive. In this paper we consider another option which has not yet been discussed in detail with respect to the broad user requirements of XFEL pump–probe experiments, i.e. the utilization of superradiant THz radiation which is created by electron beams passing through vacuum pipes which are coated on the inside with a layer of, for example, a dielectric material (Lemery et al, 2019). First numerical result concerning the out-coupling of the radiation by means of a Vlasov antenna and first estimates of the achieved beam quality are presented
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