# Enhanced Proton Acceleration from Laser Interaction with a Tailored Nanowire Target
## 1 Introduction  
With the rapid development of ultra-intense short pulse laser, acceleration of charged particles by laser plasma interaction has become an active area of research due to its prospective application in many areas, such as proton radiography [1; 2], tumor therapy [3], isochoric heating [4], tabletop ion accelerators [5], and fast ignition in inertial confinement fusion [6; 7]. To obtain high-quality and high-energy ion beams, many acceleration mechanisms have been developed over the past few decades, such as target normal sheath acceleration (TNSA) [8; 9], radiation pressure acceleration (RPA) [10; 11; 12], and collisionless shock acceleration (CSA) [13; 14]. Among these mechanisms, TNSA is easy to implement and thus it has been studied widely using both simulations and experiments [15; 16]. In TNSA, electrons at the front surface of the solid target are heated and accelerated by the incident laser pulse. The hot electrons propagate forward and easily penetrate through the target, establishing an intense charge separation electric field in the vacuum space near the rear of the target. The induced electric field can reach several teravolts per meter, strong enough to ionize and accelerate the hydrogen atoms from water or hydrocarbon contaminants attached on the target surface. Obviously, the energy and quality of accelerated ions are determined by hot electrons coming from the target front, which are determined by the intensity and absorption of the incident laser. Due to the technological limitations, increasing the laser intensity is very challenging and not practical in experiments, thus improving the energy conversion efficiency from the laser to electrons is of prime importance for TNSA. To achieve this goal, many schemes involving targets with a structured front surface have been proposed, including cone targets [17], double-layer targets [18; 19], foam targets [20], nano-structuredtargets [21; 22; 23], and micro-tube targets [24; 22]. Unlike a planar foil target, the laser pulse is not directly reflected by the sharp density profile, but propagates in the complex geometries of the structured front and interacts for an extended distance with the inner volume of the target. On the other hand, the laser pulse interacting with complex target geometries may benefit the laser absorption via oblique incidence, which provides additional hot electron generation by vacuum heating [25]. The rapid growth of surface micro-structure technologies [26; 27] has also facilitated the fabrication of such targets, and many proposed designs have been realized in experiments. [PERSON] et al. investigated the generation of high-quality electron beams from laser interaction with Si microwire arrays [28]. [PERSON] et al. demonstrated enhanced laser-driven proton acceleration in nanowire targets with different parameters. [PERSON] et al. studied the optimization of nanowire targets on proton acceleration efficiency [29]. All these works suggest that the performance of nano-structured targets is superior to that of targets with flat surfaces in absorbing laser energies and accelerating hot electrons and energetic ion beams, and ion beams with much higher energies can be generated. Our study aims to further improve the laser absorption and hot electron generation in a nano-structured target, by tailoring its front surface to modulated geometries [30]. This novel target, known as a corrugated nanowire target (CNT) originates from a planar nanowire target (PNT) and its structure resembles that of the PNT, except that its front surface is tailored so it has a corrugated profile, as shown in Figure 1. Compared to the PNT, the CNT has a larger interaction surface, and more electrons can interact with the laser pulse at the region near the front surface. Furthermore, the laser pulse is obliquely incident in each cavity of the tailored front surface in contrast to its normal incidence in the PNT. This benefits the laser plasma interaction by vacuum heating [25], leading to additional heating of electrons.