# Betatron Radiation and Bremsstrahlung in the Interaction of Intense Laser Pulse with Solid Target
## 1 Introduction  
Depending on their energy and other properties, high-energy X-rays from the interaction of high-intensity lasers with solid targets have a wide range of applications in the basic sciences [1], medical imaging [2; 3], defense, and other industries [4; 5; 6], etc. In laser-plasma interaction (LPI), the properties of the low-density transparent preplasma produced by the laser prepulse can play important roles, especially in X-ray generation [1]. If the preplasma is sufficiently long, a considerable amount of laser energy will be deposited in the preplasma, producing relativistic electrons. The latter can generate X- and even \(\gamma\)-rays through inverse-Compton scattering [7; 8] and/or betatron radiation [9], which can be regarded as synchrotron radiation, during their motion in low-density preplasma region. Knowledge of the intensity and spectral distribution of these high-energy radiations are useful for diagnosing the preplasma as well as the laser-driven relativistic electrons. Moreover, with higher (\(I_{L}\gtrsim 10^{22}\) W/cm\({}^{2}\)) laser intensity, radiation reactions in the above radiation processes can become important [10; 11; 12; 13], thereby providing an experimental platform for verifying the existing assumptions/theories of radiation reaction [13]. As the laser reaches the target's critical-density region, its energy will be absorbed by the affected electrons through collisionless mechanisms, such as vacuum and \(j\times B\) heating, and accelerating them to relativistic energies [14; 15; 16; 17; 18]. As they collide with the target ions and atoms, the energetic electrons can generate bremsstrahlung as well as characteristic X-rays. High-energy X-rays can easily pass through solid matter and are thus useful as a backlighter for diagnosing the content and structure of dense matter [19; 20; 21; 22; 23; 24; 25; 26; 27]. Bremsstrahlung from laser-target interactions are also of basic scientific interest. For example, [PERSON] et al. recently experimentally considered \(>\)70 keV X-ray conversion efficiency with the ARC laserat the National Ignition Facility [28], [PERSON] et al. considered laser-driven photon dose and bremsstrahlung spectrum using various metal targets [29],and [PERSON] et al. considered laser-driven plasma and bremsstrahlung characterization [30], etc. The laser intensities and pulse lengths in these experiments are typically less than \(10^{19}\) W/cm\({}^{2}\) and greater than 1 ps. Recently, there has been much theoretical and experimental interest in the energy conversion efficiency of the radiation mechanisms [31; 32; 33; 34; 35; 36; 37; 38; 39], as well as their dependence on the laser and target parameters [40; 41; 42; 43]. For example, [PERSON] et al. [40] found that if the laser intensity exceeds \(\sim 10^{22}\) W/cm\({}^{2}\), synchrotron radiation dominates. [PERSON] et al. [41] found that, for 1 um thick Al and Au targets, synchrotron radiation dominates if \(I_{\rm L}\geq 10^{21}\) W/cm\({}^{2}\) and \(I_{\rm L}\geq 10^{22}\) W/cm\({}^{2}\), respectively. [PERSON] et al. [42] found that, in general, the conversion efficiency of bremsstrahlung increases with the target thickness and the laser intensity, and the conversion efficiency of nonlinear inverse Compton scattering increases with the laser intensity [43]. These studies suggest that synchrotron radiation dominates for high-laser intensities (\(I_{\rm L}\geq 10^{22}\) W/cm\({}^{2}\)) and/or thin targets. More recently, [PERSON] et al. [44] simulated bremsstrahlung X-ray generation in laser-solid interactions for use as light sources in high-energy density science. A phenomenological model where the temperature scales with the laser duration and plasma density scalelength is presented for the transition from superponderomotive to ponderomotive temperatures. However, the above studies involve bremsstrahlung driven by highly intense lasers.