Synchrotron radiation, pair production, and longitudinal electron motion during 10-100 PW laser solid interactions

Abstract

At laser intensities above 10 23 W/cm 2 the interaction of a laser with a plasma is qualitatively different to the interactions at lower intensities. In this intensity regime solid targets start to become relativistically underdense, gamma-ray production by synchrotron emission starts to become an important feature of the dynamics and, at even higher intensities, electron-positron pair production by the non-linear Breit-Wheeler process starts to occur. Previous work in this intensity regime has considered ion acceleration 1 , 2 , identified different mechanisms for the underlying plasma physics of laser generation of gamma-rays 3 , 4 , 5 considered the effect of target parameters on gamma-ray generation 6 and considered the creation of solid density positronium plasma 3 . However a complete linked understanding of the important new physics of this regime is still lacking. In this paper, an analysis is presented of the effects of target density, laser intensity, target preplasma properties and other parameters on the conversion efficiency, spectrum and angular distribution of gamma-rays by synchrotron emission. An analysis of the importance of Breit-Wheeler pair production is also presented. Target electron densities between 10 22 cm −3 and 5×10 24 cm −3 and laser intensities covering the range between 10 21 W/cm 2 (available with current generation laser facilities) and 10 24 W/cm 2 (upper intensity range expected from the ELI facility) are considered. It is found that peak efficiency of conversion of laser energy into gammaray energy is achieved when the target density is 0.1 times the relativistically corrected critical density and that higher efficiency is obtained at higher laser intensity. Target front surface preplasmas of sufficient length are found to increase the efficiency of gamma-ray production compared with striking overdense solid targets directly in a fashion comparable to that in Nakamura et al. 6 by ensuring that the laser pulse interacts with a plasma of the optimum density. However maximum laser conversion to gamma-rays is achieved by striking a uniform target of the optimum density or a preplasma of sufficient length that the preplasma is nearly a uniform slab. It is found that the efficiency of Breit-Wheeler pair production is not related to either relativistic transparency or efficiency of gamma-ray production but is maximized by striking a high density target with the most intense laser possible. A qualitative model for this behaviour is presented. An analysis of the behavior of the longitundal motion of electrons as target density and laser intensity change is presented and it is found that there are two distinct regimes separated by the ratio of the relativistically corrected plasma frequency to the laser frequency. It is found that the transition between the two regimes is related to the structure of the electron phase space at the head of the laser.

Efficient ion acceleration and dense electron–positron plasma creation in ultra-high intensity laser-solid interactions

“…However, this process is expected to happen on a timescale an order of magnitude longer than those considered. Moreover, in [55], the screening has been and resulting low absorption due to QED effects has been seen in simulations with similar parameters to those explored here. As a test, we have conducted a 2D simulation for initial target and laser parameters in regime III.…”

Brilliant petawatt gamma-ray pulse generation in quantum electrodynamic laser-plasma interaction

“…In the QED laser-plasma interaction regime, a promising mechanism for production of γ -ray photons is the nonlinear synchrotron radiation252627282931 of ultrarelativistic electrons in the laser fields, i.e., 30. It is shown that γ -ray photons with the maximum energy extending to 100 s MeV can be generated by irradiating a solid target with an ultraintense laser262728.…”

“…In the QED laser-plasma interaction regime, a promising mechanism for production of γ -ray photons is the nonlinear synchrotron radiation252627282931 of ultrarelativistic electrons in the laser fields, i.e., 30. It is shown that γ -ray photons with the maximum energy extending to 100 s MeV can be generated by irradiating a solid target with an ultraintense laser262728. However, for laser interaction with steep solid density targets, where no preplasmas exist, the γ -ray emission occurs only in the small skin-depth region2262728, therefore, the conversion efficiency from laser to γ -rays is still low, and the peak brightness of γ -rays is also limited.…”

“…It is shown that γ -ray photons with the maximum energy extending to 100 s MeV can be generated by irradiating a solid target with an ultraintense laser262728. However, for laser interaction with steep solid density targets, where no preplasmas exist, the γ -ray emission occurs only in the small skin-depth region2262728, therefore, the conversion efficiency from laser to γ -rays is still low, and the peak brightness of γ -rays is also limited. Recently, a self-matching resonance acceleration scheme3233 in near-critical plasmas by circularly polarized laser pulses has been explored, which can generate much denser relativistic electron beams than the case of direct laser acceleration with linearly polarized lasers3435.…”

Identifying the electron–positron cascade regimes in high-intensity laser-matter interactions

“…We have simulated cascades using the particle-in-cell (PIC) code EPOCH, [32], which includes the strong-field QED processes described above [33]. 1D and 2D simulations have been performed in order to show where in I 24 -n 0 space cascades and dense pair plasmas will develop, as has previously been done for gamma-ray emission by nonlinear Compton scattering only [34]. In doing so, we outline expectations for future experiments using ultra-high intensity lasers and provide an approximate model for predicting the production of a dense pair plasma over a wide range of possible experimental conditions.…”