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...

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...

Increasing the dimensions of high power microwave devices is an efficient method to improve the power capacity. However, an overmoded structure usually results in mode competition and a low beam-wave conversion efficiency. In this paper, a multi-mode operation mechanism is used to avoid mode competition and increase the efficiency. The calculation results of nonlinear theory of beam-multimode interaction show that the optimized conversion efficiency is up to 48% when TM01 mode, TM02 mode, and TM03 mode are all considered. As only the TM01 mode, TM02 mode, or TM03 mode is taken into account independently, the corresponding efficiency is 38%, 22%, or 20%. Based on this, a multi-mode relativistic backward wave oscillator is proposed with the ratio of the mean diameter of the slow wave structure (SWS) to the wavelength of the output microwave to be 3.5. The non-uniform SWS is used to increase the beam-wave conversion efficiency, and a combined reflector is adopted to reflect partial of the mixed microwave modes and make the device compact. The particle-in-cell simulations show that as the diode voltage is 1.1 MV, the beam current is 22.8 kA, and the external magnetic field is 0.76 T, the conversion efficiency is 45%, and the output microwave of 11.3 GW is the mixed modes of TM01 mode, TM02 mode, and TM03 modes with the corresponding power ratio of 74%, 7%, and 19%, respectively.

An X-band dual-mode relativistic backward wave oscillator (RBWO) operating at low magnetic field is presented in this paper. Three new design principles are introduced in the device. First, the electron beam interacts with TM01 mode and TM02 mode simultaneously, rather than with a fixed single mode. Second, the device outputs with mixed modes, rather than with a pure mode. Third, an internal reflector inserted into the annular cathode, rather than a long resonant reflector before the slow-wave structure, is adopted to reflect part of the backward wave. Accordingly, the beam–wave interaction efficiency is increased significantly and the whole device is very compact. The particle in cell simulation results reveal that at a magnetic field of 0.64 T, the output microwave power is 4.8 GW and the conversion efficiency is up to 44%. In the experiment, at a guiding magnetic field of 0.66 T, a microwave pulse with power of 4.6 GW, frequency of 9.96 GHz, pulse duration of 42 ns, and efficiency of 42% was generated when the diode voltage was 880 kV and beam current was 12.5 kA, which agree well with the simulation results. Furthermore, as the diode voltage was 1.17 MV, a highest microwave power of 7.6 GW was achieved. This is a record of high efficiency and high power of microwave generation in an X-band RBWO operating at low magnetic field.

The design and preliminary results for a C-band relativistic backward wave oscillator (RBWO), which is magnetically well insulated, are presented. Under an external magnetic field of 0.36 T, the RBWO generated high power microwave radiation with a power of 3.3 GW and a frequency of 4.37 GHz for a diode voltage of 870 kV and a beam current of 13.5 kA. The electric field on the surface of the cathode holder was below the emission threshold, and an inlaid graphite cathode was designed to suppress the shunting current in the diode area. The device, operating with a low magnetic field and diode insulation, is a promising candidate for use as a permanent magnet package in high power microwave systems.