# Multi-GeV Laser Wakefield Electron Acceleration with PW Lasers
## 4 Perspective of LWFA with PW Lasers
The nanoparticle insertion method can be promising as an injection method for achieving 100 GeV by LWFA because nanoparticles can induce an electron bunch with a sufficient charge at extremely low plasma density close to 10\({}^{16}\) electrons/cm\({}^{3}\) while making negligible effects on laser propagation due to its tiny size, much smaller than the laser wavelength. A nanoparticle in plasma medium for LWFA can induce a highly localized injection, leading to an electron beam with a small emittance. A numerical study showed that a nanoparticle in plasma could facilitate a controllable injection to produce a high-quality 5-GeV electron beam with a 0.5-PW laser pulse [44]. Furthermore, a recent experimental study demonstrated nanoparticle-assisted laser wakefield acceleration with a nanoparticle-mixed helium gas jet [45]. Although controlling precisely the location of nanoparticles in plasma is challenging, the nanoparticle injection method can be a promising method to realize a 100-GeV electron beam with 10-PW-class lasers. An alternative way to increase the energy gain of LWFA is to use an intense two-color laser pulse [46]. A recent numerical study with particle-in-cell simulations showed the feasibility of all-optical staging of LWFA using a two-color laser pulse train: a fundamental laser pulse induces an electron injection, and the subsequent second harmonic pulse accelerates the injected electron bunch to high energy. The theoretical study showed the possibility of achieving 10 GeV with a-few-PW lasers and a-few-centimeter-long plasmas. It was suggested that the two-color scheme might achieve 100 GeV with the near-future state-of-the-art lasers having power in the range of 10 PW. In addition to control of injection and increasing the laser power, other technical challenges of plasma media and laser controls need to be addressed. The structure of the acceleration medium needs careful consideration, thus shaping the density profile over long distances is required. For example, the use of density up-ramp medium [47] or multi-jets configuration [48] has been recently employed, albeit they have been done in a low laser power regime and with short distance. The careful shaping of the profile can produce beams with energy spread below 1% [49]. We, thus, foresee that longitudinal control of the plasma density profile over a wide density range (10\({}^{14}\)-10\({}^{19}\) cm\({}^{-3}\)) is a necessity for improving the energy and quality of electron beams produced with multi-PW lasers. While longitudinal control of the density profile seems the major challenge for improving PW-laser-based acceleration, advances in various guiding methods and technologies will provide additional improvement of the acceleration process. Besides the challenge of producing stable long-distance channels [50], curved channel technology will provide a useful method to control the directionality of electron beams and laser pulses [51]. In addition, recent theoretical and numerical studies proposed to overcome the dephasing length of LWFA, so-called phase-locked [52] or dephasingless [53] LWFA, by adapting the superluminal velocity of focal spot movement [54], which can be a way to maximize electron energy for given laser power. LWFA is considered a promising electron acceleration technology that may overcome the limitations of the current RF linear accelerators, despite the drawbacks such as the bulky systems of lasers with peak powers beyond PW and sophisticated acceleration processes. Upcoming laser systems having a peak power beyond 10 PW have the potentials to enhance the electron energy more than an order of magnitude, even close to TeV electron energy, which can initiate a new horizon of fundamental physics.