Deglacial rapid sea level rises caused by ice-sheet saddle collapses

The last deglaciation (21 to 7 thousand years ago) was punctuated by several abrupt meltwater pulses, which sometimes caused noticeable climate change. Around 14 thousand years ago, meltwater pulse 1A (MWP-1A), the largest of these events, produced a sea level rise of 14-18 metres over 350 years. Although this enormous surge of water certainly originated from retreating ice sheets, there is no consensus on the geographical source or underlying physical mechanisms governing the rapid sea level rise. Here we present an ice-sheet modelling simulation in which the separation of the Laurentide and Cordilleran ice sheets in North America produces a meltwater pulse corresponding to MWP-1A. Another meltwater pulse is produced when the Labrador and Baffin ice domes around Hudson Bay separate, which could be associated with the /`8,200-year/' event, the most pronounced abrupt climate event of the past nine thousand years. For both modelled pulses, the saddle between the two ice domes becomes subject to surface melting because of a general surface lowering caused by climate warming. The melting then rapidly accelerates as the saddle between the two domes gets lower, producing nine metres of sea level rise over 500 years. This mechanism of an ice /`saddle collapse/' probably explains MWP-1A and the 8,200-year event and sheds light on the consequences of these events on climate

LERF - New Life for the Jefferson Lab FEL

International audienceIn 2012 Jefferson Laboratory's energy recovery linac (ERL) driven Free Electron Laser successful completed a transmission test in which high current CW beam (4.3 mA at 100 MeV) was transported through a 2 mm aperture for 7 hours with beam losses as low as 3 ppm. The purpose of the run was to mimic an internal gas target for DarkLight* - an experiment designed to search for a dark matter particle. The ERL was not run again until late 2015 for a brief re-commissioning in preparation for the next phase of DarkLight. In the intervening years, the FEL was rebranded as the Low Energy Recirculator Facility (LERF), while organizationally the FEL division was absorbed into the Accelerator division. In 2016 several weeks of operation were allocated to configure the machine for Darklight with the purpose of exercising - for the first time - an internal gas target in an ERL. Despite a number of challenges, including the inability to energy recover, beam was delivered to a target of thickness 10¹⁸ cm⁻² which represents a 3 order of magnitude increase in thickness from previous internal target experiments. Details of the machine configuration and operational experience will be discussed

Hard Two-Photon Contribution to Elastic Lepton-Proton Scattering Determined by the OLYMPUS Experiment

The OLYMPUS collaboration reports on a precision measurement of the positron-proton to electron-proton elastic cross section ratio, $\it R_{2 \gamma}$, a direct measure of the contribution of hard two-photon exchange to the elastic cross section. In the OLYMPUS measurement, 2.01 GeV electron and positron beams were directed through a hydrogen gas target internal to the DORIS storage ring at DESY. A toroidal magnetic spectrometer instrumented with drift chambers and time-of-flight scintillators detected elastically scattered leptons in coincidence with recoiling protons over a scattering angle range of $\approx$ 20° to 80°. The relative luminosity between the two beam species was monitored using tracking telescopes of interleaved GEM and MWPC detectors at 12°, as well as symmetric Møller/Bhabha calorimeters at 1:29°. A total integrated luminosity of $4.5 fb^{-1}$ was collected. In the extraction of $\it R_{2\gamma}$, radiative effects were taken into account using a Monte Carlo generator to simulate the convolutions of internal bremsstrahlung with experiment-specific conditions such as detector acceptance and reconstruction efficiency. The resulting values of $\it R_{2\gamma}$, presented here for a wide range of virtual photon polarization 0:456 < $\epsilon$< 0:978, are smaller than hadronic two-photon exchange calculations predict, but are consistent with phenomenological models