We have demonstrated single-shot continuously time-resolved MeV ultrafast electron diffraction using a static single crystal gold sample. An MeV high density electron pulse was used to probe the sample and then streaked by an rf deflecting cavity. The single-shot, high quality, streaked diffraction pattern allowed structural information within several picoseconds to be continuously temporally resolved with an approximately 200 fs resolution. The temporal resolution can be straightforwardly improved to 100 fs by increasing the streaking strength. We foresee that this system would become a powerful tool for ultrafast structural dynamics studies.

A Compact Linear Collider prototype traveling-wave accelerator structure fabricated at Tsinghua University was recently high-gradient tested at the High Energy Accelerator Research Organization (KEK). This X-band structure showed good high-gradient performance of up to 100 MV=m and obtained a breakdown rate of 1.27 × 10 −8 per pulse per meter at a pulse length of 250 ns. This performance was similar to that of previous structures tested at KEK and the test facility at the European Organization for Nuclear Research (CERN), thereby validating the assembly and bonding of the fabricated structure. Phenomena related to vacuum breakdown were investigated and are discussed in the present study. Evaluation of the breakdown timing revealed a special type of breakdown occurring in the immediately succeeding pulse after a usual breakdown. These breakdowns tended to occur at the beginning of the rf pulse, whereas usual breakdowns were uniformly distributed in the rf pulse. The high-gradient test was conducted under the international collaboration research program among Tsinghua University, CERN, and KEK.

High-intensity trains of electron bunches with tunable picosecond spacing are produced and measured experimentally with the goal of generating terahertz (THz) radiation. By imposing an initial density modulation on a relativistic electron beam and controlling the charge density over the beam propagation, density spikes of several-hundred-ampere peak current in the temporal profile, which are several times higher than the initial amplitudes, have been observed for the first time. We also demonstrate that the periodic spacing of the bunch train can be varied continuously either by tuning launching phase of a radio-frequency gun or by tuning the compression of a downstream magnetic chicane. Narrow-band coherent THz radiation from the bunch train was also measured with μJ-level energies and tunable central frequency of the spectrum in the range of ∼0.5 to 1.6 THz. Our results pave the way towards generating mJ-level narrow-band coherent THz radiation and driving high-gradient wakefield-based acceleration.

Undesirable electron field emission (a.k.a. dark current) in high gradient RF photocathode guns deteriorates the quality of photoemission current and limits the operational gradient. To improve the understanding of dark current emission, a high-resolution (∼100 µm) dark current imaging experiment has been performed in an L-band photocathode gun operating at ∼100 MV/m of surface gradient. Dark current from the cathode has been observed to be dominated by several separated strong emitters. The field enhancement factor, β, of selected regions on the cathode has been measured. The post scanning electron microscopy (SEM) and white light interferometer (WLI) surface examinations reveal the origins of ∼75% strong emitters overlap with the spots where rf breakdown have occurred.Electrons can tunnel through a surface barrier modified by the presence of an electric field, resulting in a field emission (FE) current [1][2][3][4]. While the existence of this physical phenomenon allows the operation of field emission electron sources [5][6][7][8], it has a negative (parasitic) impact on the performance of vacuum resonator-based dc and rf systems such as traveling wave tubes, photocathode guns, and particle accelerators [3,[9][10][11][12]. The troublesome field emission current is referred to as dark current. It is an incoherent source of electrons that impacts the energy budget of a device, and is a source of undesired secondary electrons and ions [9,13,14]. Historically, dark current has been considered to be a trigger of breakdown in vacuum devices which may interrupt the normal operation of the device and even jeopardize the entire facility [3].To date many questions surrounding FE still remain, especially in the rf case which limit the improvement of electron sources and high gradient accelerators for TeVscale linear colliders [15] and compact X-ray electron sources [16,17]. For example, a large discrepancy exists between emitter properties obtained through direct observation using advanced surface analysis tools and those indirectly obtained from fitting the experimental data to the Fowler-Nordheim (F-N) equation [3,18]; the temporal evolution of the FE area under high electromagnetic fields is mostly unknown [19]; and empirical methods and procedures to suppress or enhance dark current lack theoretical support. All these questions result from the lack of a means for in situ high-resolution FE observation. In earlier FE studies under a dc field, emitter mapping with better than 1 µm resolution has been achieved by scanning an anode along the cathode [20][21][22]. However, imaging the field emitters at high resolution while they are emitting under an rf field is extremely challenging due to the wide emitting phase (the timing with respect to the applied rf field) and energy spread range of the dark current [13,[23][24][25]. In this Letter, we present observations of in situ dark current emission in a high gradient photocathode gun using a dedicated dark current imaging beamline. The study was conducted at Argonne Wakefie...

No abstract