We propose and demonstrate a Terahertz (THz) oscilloscope for recording time information of an ultrashort electron beam. By injecting a laser-driven THz pulse with circular polarization into a dielectric tube, the electron beam is swept helically such that the time information is uniformly encoded into the angular distribution that allows one to characterize both the temporal profile and timing jitter of an electron beam. The dynamic range of the measurement in such a configuration is significantly increased compared to deflection with a linearly polarized THz pulse. With this THz oscilloscope, nearly 50-fold longitudinal compression of a relativistic electron beam to about 15 fs (rms) is directly visualized with its arrival time determined with 3 fs accuracy. This technique bridges the gap between streaking of photoelectrons with optical lasers and deflection of relativistic electron beams with radio-frequency deflectors, and should have wide applications in many ultrashort electron beam based facilities.The ability to characterize the time information of an ultrashort electron beam including both the temporal profile and arrival time is crucial for optimizing and enhancing the performance of many electron beam based scientific facilities such as free-electron lasers (FELs [1-3]), ultrafast electron diffraction (UED [4,5]) and microscopy (UEM [6-9]), laser-driven and beam-driven advanced accelerators [10-15], etc. In accelerator community, radio-frequency (rf) deflecting cavities have been widely used to measure the temporal profile of relativistic electron beams with energy ranging from MeV to GeV (see, e.g. [16][17][18]). However, the information of beam arrival time with respect to an external laser as required in a pump-probe experiment can not be directly measured with an rf deflector. In attosecond science community, streaking of photoelectrons with optical lasers has become a standard technique for characterizing the complete information of attosecond pulses [19]. Recently, this technique has been adapted to characterize femtosecond x-ray pulses in FELs with the streaking imprinted by farinfrared and THz pulses [20][21][22][23]. However, this technique doesn't apply to a relativistic electron beam, as dictated by Lawson-Woodward theorem [24]. Very recently, THz streaking of keV and MeV electrons [25][26][27] in a sub-wavelength metallic structure has been used to measure both the temporal profile and arrival time of electron beams. However, the small aperture used to enhance THz field may significantly limit the number of useful electrons. Furthermore, with the streaking imprinted by a linearly polarized THz pulse, the beam receives sinusoidal angular streaking and thus the measurement has a rather limited dynamic range (time window where the measurement is accurate) comparable to about one quarter of the wavelength.In this Letter, we demonstrate a laser-driven THz oscilloscope that allows one to record the complete time information of an ultrashort electron beam with both large dynamic range and high tem...
Continuous cropping obstacles have increasingly become an important phenomenon affecting crop yield and quality. Its harm includes the deterioration of soil basic physical and chemical properties, changes of soil microbial community structure, accumulation of autotoxins, weakness of plant growth, and aggravation of diseases and pests. In this review, the evolutionary trend of soil microbial structure driven by continuous cropping was generalized, while drivers of these changes summed up as destruction of soil microbial living environment and competition within the community. We introduced a microorganism proliferation and working model with three basics and a vector, and four corresponding effective measures to reshape the structure were comprehensively expounded. According to the model, we also put forward three optimization strategies of the existing measures. In which, synthetic microbiology provides a new solution for improving soil community structure. Meanwhile, to ensure the survival and reproduction of soil microorganisms, it is necessary to consider their living space and carbon sources in soil fully. This review provided a comprehensive perspective for understanding the evolutionary trend of the soil microbial community under continuous cropping conditions and a summary of reshaping measures and their optimization direction.
We propose and demonstrate a novel method to reduce the pulse width and timing jitter of a relativistic electron beam through THz-driven beam compression. In this method the longitudinal phase space of a relativistic electron beam is manipulated by a linearly polarized THz pulse in a dielectric tube such that the bunch tail has a higher velocity than the bunch head, which allows simultaneous reduction of both pulse width and timing jitter after passing through a drift. In this experiment, the beam is compressed by more than a factor of four from 130 fs to 28 fs with the arrival time jitter also reduced from 97 fs to 36 fs, opening up new opportunities in using pulsed electron beams for studies of ultrafast dynamics. This technique extends the well known rf buncher to the THz frequency and may have a strong impact in accelerator and ultrafast science facilities that require femtosecond electron beams with tight synchronization to external lasers.
PACS numbers:Ultrashort electron beams with small timing jitter with respect to external lasers are of fundamental interest in accelerator and ultrafast science communities. For instance, such beams are essential for laser and THz-driven accelerators ([1-4]) where the beam energy spread and beam energy stability largely depend on the electron bunch length and injection timing jitter, respectively. For MeV ultrafast electron diffraction (UED [5-12]) where ultrashort electron beams with a few MeV energy are used to probe the atomic structure changes following the excitation of a pump laser, the temporal resolution is primarily limited by the electron bunch length and timing jitter. Similar limitations exist for x-ray free-electron lasers ) too, since the properties of the x-ray pulses depend primarily on that of the electron beams. Therefore, one of the long-standing goals in accelerator and ultrafast science communities is to generate ultrashort electron beams with small timing jitter.Photocathode rf gun is the leading option for producing high brightness ultrashort electron beam for FEL and MeV UED (see, e.g. [16,17]). Due to space charge effect, the electron beam pulse width is broadened and therefore bunch compression is typically needed to reduce the pulse width. Bunch compression requires first a mechanism to imprint energy chirp (correlation between an electron's energy and its longitudinal position) and then sending the beam through a dispersive element such that the longitudinal displacement of the electrons is changed in a controlled way for reducing the pulse width. For MeV beam, this is typically achieved by first sending the beam through a rf buncher cavity at zero-crossing phase where the bunch head (t < 0) is decelerated while the bunch tail (t > 0) is accelerated. This imprints a negative chirp h = dδ/cdt < 0 in the beam longitudinal phase space, where δ is the relative energy difference of an electron with respect to the reference electron and c is the speed of light. Then the electron beam is sent through a drift after which the electrons at the bunch tail...
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