High Power Laser Science and Engineering, (2016), Vol. 4, e1, 4 pages.
© The Author(s) 2016. This is an Open Access article, distributed under the terms of the Creative Commons Attribution licence (http://creativecommons.org/
licenses/by/4.0/), which permits unrestricted re-use, distribution, and reproduction in any medium, provided the original work is properly cited.
doi:10.1017/hpl.2015.35
Generating quasi-single-cycle THz pulse from
frequency-chirped electron bunch train and a
tapered undulator
Zhuoran Ma
1,2
, Zhe Wang
1,2
, Feichao Fu
1,2
, Rui Wang
1,2
, and Dao Xiang
1,2
1
Key Laboratory for Laser Plasmas (Ministry of Education), Department of Physics and Astronomy,
Shanghai Jiao Tong University, Shanghai 200240, China
2
IFSA Collaborative Innovation Center, Shanghai Jiao Tong University, Shanghai 200240, China
(Received 30 July 2015; revised 18 September 2015; accepted 3 December 2015)
Abstract
We propose a proof-of-principle experiment to test a new scheme to produce a single-cycle radiation pulse in free-
electron lasers (FELs). Here, a few α-BBO crystals will be first used to produce an equally spaced laser pulse train.
Then, the laser pulse train illuminates the cathode to produce a frequency-chirped electron bunch train in a photocathode
rf gun. Finally, the frequency-chirped electron bunch train passes through a tapered undulator to produce a quasi-single-
cycle THz pulse. This experiment should allow comparison and confirmation of predictive models and scaling laws, and
the preliminary experimental results will also be discussed.
Keywords: electron bunch train; single-cycle THz pulse; tapered undulator
1. Introduction
Fast time-dependent phenomena are typically studied with
a pump–probe technique in which the dynamics are initiated
by a pump laser and then probed by a delayed pulse. Because
the temporal resolution depends on the duration of the pump
and probe beams, there is continuous interest in the free-
electron laser (FEL) community to produce radiation pulses
with shorter and shorter duration to meet the demands of the
studies of faster and faster processes.
There are many ways to reduce the pulse duration in
FELs. For instance, one may just reduce the beam charge to
produce a very short electron bunch that naturally produces
a very short FEL pulse
[1–3]
. Alternatively, one may use the
so-called ‘slotted foil’ in the center of a chicane to spoil the
transverse emittance for most of the portions of the electron
beam such that an ultrashort x-ray pulse is produced only
from the short slices of unspoiled beam that passes cleanly
through the slot
[4–6]
. Instead of using a slotted foil for the
selection of a short beam slice for lasing, one may also use
a few-cycle laser to manipulate the electron beam energy
or divergence distribution for producing an ultrashort FEL
pulse
[7–14]
. Comprehensive reviews and comparisons of
various methods can be found in Refs. [15, 16].
Correspondence to: Dao Xiang, 800 Dongchuan Rd, Minhang District,
Shanghai, China. Email: dxiang@sjtu.edu.cn
However, in all these methods, the FEL slippage length
limited the shortest duration that can be obtained in FELs.
For SASE FELs, because a long undulator is needed for
lasing the minimal pulse duration is typically on the order
of 100 as. For seeded FELs, because the beam is prebunched
one can use a short undulator to produce intense radiation
and the pulse duration may be pushed to tens of attoseconds
(for instance, an isolated radiation pulse with about 20 as
duration has been predicted in Ref. [13]).
Recently, a novel scheme based on coherent emission from
a chirped microbunch passing through a strongly tapered
undulator has been proposed to produce a single-cycle ra-
diation pulse in order to counteract the slippage effect
[17]
.
Before applying this idea to large scientific facilities, we feel
it is necessary to perform a proof-of-principle experiment to
demonstrate its underlying physics. The main purpose of this
paper is to discuss a proof-of-principle experiment to test this
idea at THz wavelength range.
2. Methods
In this section, we discuss how one may generate a quasi-
single-cycle THz pulse by superposition of chirped radiation
pulse trains. The idea is to adjust the separation of the
electron microbunch (which determines the separation of
the radiation pulse train) and the tapering of the undulator
1
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