Generation of sub-picosecond electron bunches by strong terahertz fields for high gradient electron acceleration and ultrafast electron diffractive imaging

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Ultrabright electron sources based on accelerators have helped greatly to create the new field of atomically-resolved structural dynamics by enabling direct observation of atomic motions governing structural transitions [Siwick’03, Dwyer’06, Zewail’06]. The frontiers of temporal resolution have moved into the femtosecond regime in order to capture the fastest nuclear motions involved in chemical and biological reactions. Therefore, very short electron bunches tightly synchronized to appropriate pump and probe lasers with few femtosecond precision are needed. A primary challenge in achieving short bunches is Coulombic repulsion which causes bunch spreading before reaching the sample. Repulsion can be overcome by sacrificing bunch charge [Baum’13]. To maintain sufficient signal-to-noise calls for increased repetition rate which may become incompatible with irreversible or semi-reversible samples, especially organic samples. Alternatively, a rebunching cavity [Oudheusden’10] can be used, but this requires high electromagnetic field gradients. Usually, modern accelerators are driven by microwave signals with multi-centimeter length scales and have acceleration gradients below 100 MV/m limited by field-induced breakdown of materials. By increasing acceleration field frequencies to the THz regime, higher field gradients and smaller devices can be used, increasing compressive forces and reducing time-scales for self-repulsion [Siwick’03] allowing for shorter bunches. THz pulses can also be optically generated, eliminating timing jitter plaguing microwave devices at the few hundred fs level [Gao’12, Chatelain’12]. Supporting infrastructure is also reduced, making devices more accessible to the scientific community.Here, we address these issues and push the resolution frontier by developing THz-driven accelerator technology which enables the construction of a compact terahertz-based ultrafast electron diffractometer (THz-UED) with sub-100 fs resolution. The groundwork for this project has been laid by demonstrations from both German and Russian teams of highly efficient generation of THz pulses [Bodrov'13, Wu’14, Vicario'14, Vicario'15, Wu'16, Ahr'17] as well as development of theory of high-gradient accelerator structures [Kuzikov'10, Kuzikov'16] and proof-of-principle demonstrations of THz-based photoguns [Huang’16] and LINACs [Nanni’15]. Recent work by the German team indicates that strong-field THz radiation can be used to accelerate, compress, focus and diagnose electron bunches with durations and intrinsic laser synchronization in the 1 - 100 fs range [Fallahi'16, Zhang'17]. Practical electron guns for THz-UED with sub-100 fs temporal resolution will be developed and used to enhance resolution beyond current capabilities, such as phonon involvement in the insulator-metal phase transition of Vanadium-Oxide [O'Callahan'14] and ultrafast dynamics in proteins and DNA, thought to occur on a 10 fs time scale.