Experimental Observation of Proton Bunch Modulation in a Plasma at Varying Plasma Densities.

We give direct experimental evidence for the observation of the full transverse self-modulation of a long, relativistic proton bunch propagating through a dense plasma. The bunch exits the plasma with a periodic density modulation resulting from radial wakefield effects. We show that the modulation is seeded by a relativistic ionization front created using an intense laser pulse copropagating with the proton bunch. The modulation extends over the length of the proton bunch following the seed point. By varying the plasma density over one order of magnitude, we show that the modulation frequency scales with the expected dependence on the plasma density, i.e., it is equal to the plasma frequency, as expected from theory.

Experimental Observation of Plasma Wakefield Growth Driven by the Seeded Self-Modulation of a Proton Bunch.

We measure the effects of transverse wakefields driven by a relativistic proton bunch in plasma with densities of 2.1×10^{14} and 7.7×10^{14} electrons/cm^{3}. We show that these wakefields periodically defocus the proton bunch itself, consistently with the development of the seeded self-modulation process. We show that the defocusing increases both along the bunch and along the plasma by using time resolved and time-integrated measurements of the proton bunch transverse distribution. We evaluate the transverse wakefield amplitudes and show that they exceed their seed value (<15 MV/m) and reach over 300 MV/m. All these results confirm the development of the seeded self-modulation process, a necessary condition for external injection of low energy and acceleration of electrons to multi-GeV energy levels.

Acceleration of electrons in the plasma wakefield of a proton bunch.

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 acceleration1-5, 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 pulse6-9 or electron bunch10,11 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 energies5,12. 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 stage13. Long, thin proton bunches can be used because they undergo a process called self-modulation14-16, 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 CERN17-19 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 stage20 means that our results are an important step towards the development of future high-energy particle accelerators21,22.

Model for computing superconfiguration temperatures in nonlocal-thermodynamic-equilibrium hot plasmas.

A model is presented where the level-population densities in quasi-steady-state hot dense plasmas are described by means of large nonrelativistic superconfigurations (SC's), whose configuration populations follow a decreasing-exponential law versus energy (Boltzmann like) for a temperature depending on the SC. Two systems of linear equations are obtained. The first one yields the average-state population densities of the SC's. Using these results, the second system yields the SC temperatures. In this model, a very large number of atomic levels is accounted for in a simple way, thus yielding the configuration populations and, hence, the ionic distribution and average charge. It also yields accurate simulations of the spectra, which are of the essence for emissivity and absorption calculations. It opens a way to time-dependent calculations.

X-ray emission of a xenon gas jet plasma diagnosed with Thomson scattering.

We present the results of a benchmark experiment aimed at validating recent calculation techniques for the emission properties of medium and high-Z multicharged ions in hot plasmas. We use space- and time-resolved M-shell x-ray spectroscopy of a laser-produced gas jet xenon plasma as a primary diagnostic of the ionization balance dynamics. We perform measurements of the electron temperature, electron density, and average charge state by recording simultaneous spectra of ion acoustic and electron plasma wave Thomson scattering. A comparison of the experimental x-ray spectra with calculations performed ab initio with a non-local-thermodynamic-equilibrium collisional-radiative model based on the superconfiguration formalism, using the measured plasma parameters, is presented and discussed.

L-band x-ray absorption of radiatively heated nickel.

Absorption of L-M and L-N transitions of nickel has been measured using point projection spectroscopy. The x-ray radiation from laser-irradiated gold cavities was used to heat volumetrically nickel foils "tamped with carbon" up to 20 eV. Experimental spectra have been analyzed with calculations based on the spin-orbit split arrays statistical approach and performed for each ionic species Ni5+ to Ni11+. Using a least-squares fit, this method provides an ion distribution broader than at local thermodynamic equilibrium, which is explained by spatial and temporal temperature gradients. A major improvement in the simulation of the absolute value of transmission is obtained with a resolved transition array statistical calculation that reproduces the experimental spectrum with the nominal areal mass density by taking into account the saturation of narrow lines.