Abstract
The fundamental idea of Laser Wakefield Acceleration (LWFA) is reviewed. An ultrafast intense laser pulse drives coherent wakefields of relativistic amplitude with the high phase velocity robustly supported by the plasma. The structures of wakes and sheaths in plasma are contrasted. While the large amplitude of wakefields involves collective resonant oscillations of the eigenmode of the entire plasma electrons, the wake phase velocity ~ c and ultrafastness of the laser pulse introduce the wake stability and rigidity. When the phase velocity gets smaller, wakefields turn into sheaths. When we deploy laser ion acceleration or high density LWFA in which the phase velocity of plasma excitation is low, we encounter the sheath dynamics. A large number of world-wide experiments show a rapid progress of this concept realization toward both the high energy accelerator prospect and broad applications. The strong interest in this has driven novel laser technologies, including the Chirped Pulse Amplification, the Thin Film Compression (TFC), the Coherent Amplification Network, and the Relativistic Compression (RC). These in turn have created a conglomerate of novel science and technology with LWFA to form a new genre of high field science with many parameters of merit in this field increasing exponentially lately. Applications such as ion acceleration, X-ray free electron laser, electron and ion cancer therapy are discussed. A new avenue of LWFA using nanomaterials is also emerging, adopting X-ray laser using the above TFC and RC. Meanwhile, we find evidence that the Mother Nature spontaneously created wakefields that accelerate electrons and ions to very high energies.
Highlights
1.1 The basic philosophyConventional accelerators are by and large based on the single particle interaction of charged particles with the externally imposed accelerating fields (Chao et al 2014)
The radiation pressure acceleration (RPA) regime (Esirkepov et al 2004) with increased laser pressure (a0 >> 1) was proposed in which the laser ponderomotive force is so large to move electron charge to pull ions together (Esirkepov et al 2004) We recently showed that Coherent Acceleration of Ions by Laser (CAIL) and RPA satisfy the same physical condition for the optimal target thickness as a function of the laser intensity and similar physical dynamics (Tajima et al 2017; Magee and Necas 2019)
It is further noted that while the linearly polarized (LP) laser irradiation process is well described such as the maximum energies by the CAIL, when the polarization is switched to the circular polarization (CP), the energy spectrum of the accelerated ions show a quasi-monoenergy feature (Henig and Steinke 2009)
Summary
Problem 1: Breakdown (spark) → E ∼ MeV∕cm → f-center of metal Probem 2: Transverse EM fields in a metallic tube → E∥ needed → vph > c → Introduce slow-wave structure, but more breakdown Problem 3: E∥∕E⊥ ∼ k⊥∕k∥ ≪ 1 → Small accelerating field No “bang” tolerated (Fermi 1954). Nonlinear force: ∼ N2 Solution: Plasma → Already broken down → E ∼ ∞ : GeV/cm, later TeV/cm Solution: longitudinal wakefield → Mainly E∥ ( E⊥ small) → vph < c → Wakefield: vgr ∼ 0. Plasma loves “bang” → laser (Tajima and Dawson 1979; Tajima 1985; Mourou et al 2006). → Compatible “marriage” Even relativistically strong laser “bang” OK it struts the plasma with Relativistic backbone
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