Tanaka Takashi, Kida Yuichiro, Hashimoto Satoshi, Miyamoto Shuji, Togashi Tadashi, Tomizawa Hiromitsu, Gocho Aoi, Kaneshima Keisuke, Tanaka Yoshihito
RIKEN SPring-8 Center, Koto 1-1-1, Sayo, Hyogo 679-5148, Japan.
Japan Synchrotron Radiation Research Institute, Koto 1-1-1, Sayo, Hyogo 679-5198, Japan.
Phys Rev Lett. 2023 Oct 6;131(14):145001. doi: 10.1103/PhysRevLett.131.145001.
In seeded free electron lasers (FELs), the temporal profile of FEL pulses usually reflects that of the seed pulse, and, thus, shorter FEL pulses are available with shorter seed pulses. In an extreme condition, however, this correlation is violated; the FEL pulse is stretched by the so-called slippage effect in undulators, when the seed pulse is ultimately short, e.g., few-cycles long. In a previous Letter, we have proposed a scheme to suppress the slippage effect and reduce the pulse length of FELs ultimately down to a single-cycle duration, which is based on "chirped microbunching," or an electron density modulation with a varying modulation period. Toward realization of FELs based on the proposed scheme, experiments have been carried out to demonstrate its fundamental mechanism in the NewSUBARU synchrotron radiation facility, using an ultrashort seed pulse with the pulse length shorter than five cycles. Experimental results of spectral and cross-correlation measurements have been found to be in reasonable agreement with the theoretical predictions, which strongly suggests the successful demonstration of the proposed scheme.
在种子注入自由电子激光器（FEL）中，FEL脉冲的时间分布通常反映种子脉冲的时间分布，因此，种子脉冲越短，可获得的FEL脉冲就越短。然而，在极端情况下，这种相关性会被打破；当种子脉冲最终很短，例如只有几个周期长时，FEL脉冲会因波荡器中所谓的滑动效应而展宽。在之前的一篇快报中，我们提出了一种抑制滑动效应并将FEL的脉冲长度最终减小到单周期持续时间的方案，该方案基于“啁啾微聚束”，即具有变化调制周期的电子密度调制。为了基于所提出的方案实现FEL，已经在NewSUBARU同步辐射装置中进行了实验，以使用脉冲长度短于五个周期的超短种子脉冲来演示其基本机制。已经发现光谱和互相关测量的实验结果与理论预测合理吻合，这有力地表明了所提出方案的成功演示。
