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...