Terahertz (THz) radiation promises breakthrough advances in compact advanced accelerators due to the gigavolts-per-meter fields achievable, but the challenge of maintaining overlap and synchronism between beams and short laser-generated THz pulses has so far limited interactions to the few-millimeter scale. We implement a novel scheme for simultaneous group and phase velocity matching of nearly single-cycle THz radiation with a relativistic electron beam for meter-scale inverse free-electron laser interaction in a magnetic undulator, resulting in energy modulations of up to 150 keV using modest THz pulse energies (≤1  μJ). Using this scheme, we demonstrate for the first time the use of a laser-based THz source for bunch-length compression and time-stamping of a relativistic electron beam.

A measurement of the absolute fluorescence yield of the 337 nm nitrogen band, relevant to ultra-high energy cosmic ray (UHECR) detectors, is reported. Two independent calibrations of the fluorescence emission induced by a 120 GeV proton beam were employed: Cherenkov light from the beam particle and calibrated light from a nitrogen laser. The fluorescence yield in air at a pressure of 1013 hPa and temperature of 293 K was found to be Y 337 = 5.61±0.06 stat ±0.21 syst photons/MeV. When compared to the fluorescence yield currently used by UHECR experiments, this measurement improves the uncertainty by a factor of three, and has a significant impact on the determination of the energy scale of the cosmic ray spectrum.Key words: Nitrogen Fluorescence Yield, Air Fluorescence Detection, Ultra-High Energy Cosmic Rays PACS: , 96.50. 96.50.sb, 96.50.sd, 32.50.+d, 34.50.Fa, 34.50.Gb IntroductionA well established technique for the detection of Ultra-High Energy ( 10 18 eV) Cosmic Rays (UHECRs) -first successfully employed by the Fly's Eye [1] and HiRes [2] experiments -is based on nitrogen fluorescence light emission induced by Extensive Air Showers (EAS). Excitation of atmospheric nitrogen by EAS charged particles results in fluorescence emission, mostly in the wavelength range between 300 and 430 nm. This UV light is measured as a function of time and incoming direction by photomultiplier cameras at the focus of large (few m 2 ) mirrors. Fluorescence telescopes measure the longitudinal EAS development in the atmosphere, which provides unique information on the primary cosmic ray's type and a calorimetric measurement of its energy.The fluorescence light yield along the EAS depends on the air pressure, temperature and humidity at the point of emission. In addition, wavelengthdependent atmospheric attenuation affects the light intensity reaching the telescope. Thus, the intensities of the fluorescence bands must be known for atmospheric conditions corresponding to the EAS development in the atmosphere, which ranges between about 2 km and 15 km above sea level. Early measurements of the fluorescence yield include those with low-energy stopped-particles in air by Bunner [3] and with electrons in air by Davidson and O'Neil [4]. A * corresponding author Email address: priviter@kicp.uchicago.edu (P. Privitera). The AIRFLY (AIR FLuorescence Yield) Collaboration has carried out an extensive program of measurements to significantly improve the precision on the fluorescence light yield. The fluorescence emission was studied as a function of the kinetic energy, ranging from keV to GeV, of particle beams at several accelerators [11]. The relative intensities of 34 fluorescence bands in the wavelength range from 284 to 429 nm, together with their pressure dependence, were reported in [12]. The temperature and humidity dependence of the main fluorescence bands was also measured [13]. These measurements have provided the most complete and consistent set of fluorescence yield data for UHECR calibration, establishing the relative ...

We describe an inverse free electron laser (IFEL) interaction driven by a near single-cycle THz pulse that is group velocity-matched to an electron bunch inside a waveguide, allowing for a sustained interaction in a magnetic undulator. We discuss the application of this guided-THz IFEL technique for compression of a relativistic electron bunch and synchronization with the external laser pulse used to generate the THz pulse via optical rectification, as well as a laser-driven THz streaking diagnostic with the potential for femtosecond scale temporal resolution. Initial measurements of the THz waveform via an electro-optic sampling based technique confirm the predicted reduction of the group velocity, using a curved parallel plate waveguide, as a function of the varying aperture size of the guide. We also present the design of a proof-of-principle experiment based on the bunch parameters available at the UCLA PEGASUS laboratory. With a -10 MV m 1 THz peak field, our simulation model predicts compression of a 6 MeV 100 fs electron beam by nearly an order of magnitude and a significant reduction of its initial timing jitter.

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