Using quadrupole scan measurements we show laser-wakefield accelerated electrons to have a normalized transverse emittance of 0:21 þ0:01 À0:02 mm mrad at 245 MeV. We demonstrate a multishot and a single-shot method, the mean emittance values for both methods agree well. A simple model of the beam dynamics in the plasma density downramp at the accelerator exit matches the source size and divergence values inferred from the measurement. In the energy range of 245 to 300 MeV the normalized emittance remains constant.Laser-wakefield acceleration (LWFA) [1,2] can deliver ultrarelativistic electron beams in a compact setup with unique features [3][4][5][6]. It is receiving particular attention as a source or driver for ultrashort x-ray beams [7,8] and for its potential for realizing a tabletop free-electron laser (FEL) [9]. The electron bunch duration has recently been measured to be only a few femtoseconds long [10,11] which results in peak beam currents on the order of kiloamperes. An essential parameter for the performance of x-ray sources, FELs, or linear colliders is the transverse electron beam emittance. Previous emittance measurements of LWFA electron beams have used the pepperpot method [12][13][14] giving normalized emittances of $2:2 mm mrad with single shots down to the resolution limit of 1:1 mm mrad. As these measurements are not spectrally resolved, they rely on a low energy spread to give a meaningful normalized emittance. For LWFA beams which fluctuate in energy and energy spread, a simultaneous measurement of the spectrum is required. This technique is also limited to electron energies that can be sufficiently scattered by the pepper-pot mask; to date, measurements of a 508 MeV beam have been carried out [15]. Experiments characterizing the betatron radiation emitted by the electron beam while it is in the plasma suggest the beam size there to be & 1 m [16,17], which in combination with a divergence measurement give an estimated emittance of <0:5 mm mrad [18]. However, inferring the emittance from the electron beam size in the plasma and its downstream divergence in the vacuum can be unreliable as this neglects the plasma-vacuum density transition at the accelerator exit; here the decreasing strength of the plasma focusing forces result in an increase in beam size and decrease in divergence [13]. This publication reports on direct measurements of the emittance of LWFA electrons that are both energy resolved and that include the beam transport of the density downramp at the accelerator exit. This is achieved by analyzing their beam size around a focus using a quadrupole lens scan method [19].The transverse phase space of an electron beam is often specified using the Twiss parameters , , , and the natural emittance ". These parameters describe the volume and orientation of the particle distribution in phase space. The beam size at a particular position ðs 1 Þ is related to the Twiss parameters at s 0 by [20] ðs 1 Þ 2 ¼ M 2 11 ðs 0 Þ À2M 11 M 12 ðs 0 Þþ M 2 12 ðs 0 Þ: (1)Here M ij refers to the ij eleme...

The observation and manipulation of electron dynamics in matter call for attosecond light pulses, routinely available from high-order harmonic generation driven by few-femtosecond lasers. However, the energy limitation of these lasers supports only weak sources and correspondingly linear attosecond studies. Here we report on an optical parametric synthesizer designed for nonlinear attosecond optics and relativistic laser-plasma physics. This synthesizer uniquely combines ultra-relativistic focused intensities of about 10 20 W/cm 2 with a pulse duration of sub-two carrier-wave cycles. The coherent combination of two sequentially amplified and complementary spectral ranges yields sub-5-fs pulses with multi-TW peak power. The application of this source allows the generation of a broad spectral continuum at 100-eV photon energy in gases as well as high-order harmonics in relativistic plasmas. Unprecedented spatio-temporal confinement of light now permits the investigation of electric-fielddriven electron phenomena in the relativistic regime and ultimately the rise of next-generation intense isolated attosecond sources.The development and proliferation of intense lasers with sub-two optical-cycle duration during the past decade has allowed to create the tools and techniques for the observation and control of electronic motions in all forms of matter; a field nowadays known as attosecond physics 1 . These techniques have meanwhile provided direct time-domain access to a wide range of electron phenomena with a sub-fs resolution, such as miniscule delays in photo-emission timing 2,3 , charge migration in molecules 4, 5 and solids 6,7 , as well as collective electron motion in extreme laser-plasma interactions 8 . Powerful few-cycle laser pulses have traditionally been produced via chirped-pulse amplification (CPA) in titanium-doped sapphire (Ti:Sa) in conjunction with spectral broadening in gas-filled hollow-core fibres (HCF) 9 . CPA-based lasers have achieved peak powers beyond 1 PW, but only with pulse durations extending to about ten optical cycles or longer 10,11 . Spectral broadening in HCFs provides octave-spanning spectra, but the approach is still limited to pulses with a few millijoules in energy 12,13 . Due to these restrictions few-cycle-driven attosecond sources based on high-harmonic generation (HHG) in gas targets generally suffer from a low intensity, constituting a major limitation to pushing the frontiers of the field. Upscaling few-cycle-driven HHG to higher driving pulse energies [14][15][16] allows the generation of intense isolated attosecond pulses for time-resolved nonlinear optics experiments in the extreme-ultraviolet (XUV) spectral

No abstract