Abstract
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[
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. [
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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 radiation pulse in order to counteract the slippage effect[
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 (which determines the frequency chirp of the radiation pulse) such that all the radiation pulses will coherently add up at only one radiation cycle while destructively interfering at all other cycles[
When an electron microbunch passes through an undulator with strength
We first study what kind of radiation pulse one may get for a bunch train with uniform separation and an untapered undulator. Figure
Now, we consider the case when the undulator is tapered, e.g.,
Finally, we consider the case where the bunch train is also chirped, e.g., the separation of adjacent bunches linearly changes along the longitudinal direction. Analysis shows that when the chirp of the bunch train matches that of the tapered undulator (in this case the bunch separation increases from bunch head to bunch tail), constructive interference only occurs for one cycle while those at other cycles are destructive (Figure
Note that the pulse width is eventually determined by the bandwidth of the radiation pulse. Thus, a bunch train with strong chirp and a strongly tapered undulator are required to get a truly single-cycle radiation pulse. It should also be noted that to match the tapering of the undulator, a bunch train with a strong energy chirp instead of a frequency chirp may be used as well[
3. Generation of a chirped bunch train
The electron bunch train can be produced with a laser pulse train in a photocathode rf gun, taking the advantage of the promptness of photoemission. Furthermore, because the time-of-flight of electrons from cathode to the exit of the gun depends on rf phase, a frequency-chirped electron bunch train can be produced by properly choosing the laser launching phase. Figure
Figure
It is worth mentioning that in addition to generating bunch trains, the
4. Proposed experiment
We propose to conduct a proof-of-principle experiment to test this novel scheme for the generation of a single-cycle radiation pulse at Shanghai Jiao Tong University where a photocathode rf gun is available[
5. Summary and outlook
In conclusion, we have proposed a proof-of-principle experiment to test the recently proposed novel scheme to produce a single-cycle radiation pulse in FELs. The underlying physics of this method and the feasibility of the experiment at THz wavelength have been discussed based on calculations and simulations. A frequency-chirped electron bunch train has been successfully produced in our photocathode rf gun. In future, we will build a strongly tapered undulator to produce a quasi-single-cycle THz radiation. We anticipate that this proof-of-principle experiment will mark a great step toward generation of shorter and shorter radiation pulses in FELs.
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