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High-energy particle accelerators have been crucial in providing a deeper understanding of fundamental particles and the forces that govern their interactions. To increase the energy of the particles or to reduce the size of the accelerator, new acceleration schemes need to be developed. Plasma wakefield acceleration, in which the electrons in a plasma are excited, leading to strong electric fields (so called 'wakefields'), is one such promising acceleration technique. Experiments have shown that an intense laser pulse or electron bunch traversing a plasma can drive electric fields of tens of gigavolts per metre and above-well beyond those achieved in conventional radio-frequency accelerators (about 0.1 gigavolt per metre). However, the low stored energy of laser pulses and electron bunches means that multiple acceleration stages are needed to reach very high particle energies. The use of proton bunches is compelling because they have the potential to drive wakefields and to accelerate electrons to high energy in a single acceleration stage. Long, thin proton bunches can be used because they undergo a process called self-modulation, a particle-plasma interaction that splits the bunch longitudinally into a series of high-density microbunches, which then act resonantly to create large wakefields. The Advanced Wakefield (AWAKE) experiment at CERN uses high-intensity proton bunches-in which each proton has an energy of 400 gigaelectronvolts, resulting in a total bunch energy of 19 kilojoules-to drive a wakefield in a ten-metre-long plasma. Electron bunches are then injected into this wakefield. Here we present measurements of electrons accelerated up to two gigaelectronvolts at the AWAKE experiment, in a demonstration of proton-driven plasma wakefield acceleration. Measurements were conducted under various plasma conditions and the acceleration was found to be consistent and reliable. The potential for this scheme to produce very high-energy electron bunches in a single accelerating stage means that our results are an important step towards the development of future high-energy particle accelerators.

The shock-induced transition from graphite to diamond has been of great scientific and technological interest since the discovery of microscopic diamonds in remnants of explosively driven graphite. Furthermore, shock synthesis of diamond and lonsdaleite, a speculative hexagonal carbon polymorph with unique hardness, is expected to happen during violent meteor impacts. Here, we show unprecedented in situ X-ray diffraction measurements of diamond formation on nanosecond timescales by shock compression of pyrolytic as well as polycrystalline graphite to pressures from 19 GPa up to 228 GPa. While we observe the transition to diamond starting at 50 GPa for both pyrolytic and polycrystalline graphite, we also record the direct formation of lonsdaleite above 170 GPa for pyrolytic samples only. Our experiment provides new insights into the processes of the shock-induced transition from graphite to diamond and uniquely resolves the dynamics that explain the main natural occurrence of the lonsdaleite crystal structure being close to meteor impact sites.

The scope of applications that require intense and ultrafast THz fields has been increasing during the last years. Applications such as terahertz time-domain spectroscopy [1], the study of carrier dynamics in semiconductors [2], electric field gating of interlayer charge transport in superconductors [3], or THz assisted attosecond pulse generation [4] benefit from higher pulse energies than currently available, and so there is keen interest in scaling the peak power of the THz generation schemes. More recently, high peak power THz sources have been proposed for charged particle acceleration, undulation, deflection and spatiotemporal arbitrary manipulation too [5].There are different methods for generating high peak field THz pulses. Among them, difference frequency generation (DFG) and optical rectification (OR) are the most common. Sell et al. demonstrated that it is possible to use DFG between two parametrically amplified pulse trains to generate phase locked terahertz transients with peak electric fields of 10 8 MV/cm and center frequencies continuously tunable from 10 to 72 THz [6]. However, such methods typically exhibit fairly low photon conversion efficiencies due to the Manley-Rowe limit and are also restricted to high THz frequencies approaching the mid-IR spectral region due to limitations imposed by the phase matching condition in the DFG medium, such as GaSe or AgGaS 2 . Optical rectification, on the other hand, has been widely implemented to generate pulses at low THz frequencies [7]. Because the nonlinear process can be cascaded, over 100% of photon conversion efficiency has been demonstrated [8,9]. Of the common nonlinear materials used for OR, ZnTe presents the problem of free carrier absorption, limiting the total efficiency [10]. Lithium niobate presents multiple advantages such as large d eff , high damage threshold, low THz absorption, and large bandgap, but it requires tilted pulse front pumping techniques to achieve

The LCLS beam provides revolutionary capabilities for studying the transient behavior of matter in extreme conditions. The particular strength of the Matter in Extreme Conditions instrument is that it combines the unique LCLS beam with high-power optical laser beams, and a suite of dedicated diagnostics tailored for this field of science. In this paper an overview of the beamline, the capabilities of the instrumentation, and selected highlights of experiments and commissioning results are presented.