# Optically Switchable MeV Ion/Electron Accelerator
## 1 Introduction  
The invention of chirped pulse amplification [1], for which the 2018 Nobel Prize in Physics was awarded, introduced the era of multi-petawatt lasers [2] and led to new regimes of light-matter interaction. The most striking feature of intense laser interaction with solid targets is the emission of a variety of intense radiation types [3], including electrons, ions, x-rays, and positrons. The relatively small scale of these lasers earned these machines the moniker \"tabletop accelerators\" and triggered research ranging from small portable machines [4] to large facilities [5] and accelerators at the energy frontier [6]. The relatively small size and cost of laser accelerators is often promoted as their main advantage [7]. Another appealing characteristic is their ability to transport the beam over optical mirrors for most of the way [8] and generate the particle beam close to the interaction point. Consequently, radiation shielding of laser accelerators is simpler than that of conventional accelerators with comparable energies [9]. The ultrashort nature of laser-accelerated particle bunches has also been deemed advantageous for applications [10] and research [11]. This capability is further reflected by the possibility of synchronizing the particle emission with another laser or particle pulse, with sub-ps temporal jitter [12, 13]. The lion's share of laser-particle acceleration research focuses on acceleration of ions and electrons. One distinct experimental difference between these two types of research isthat electron acceleration usually involves gaseous targets, whereas most ion acceleration experiments are conducted by irradiation of solids. Laser acceleration of ions to MeV level energies was introduced more than 2 decades ago [14; 15] with the target-normal sheath acceleration (TNSA) [16] mechanism. Higher laser intensities and improved pulse contrast promoted more robust acceleration mechanisms, such as the breakout afterburner [17], which rely on opaque plasma becoming relativistically transparent, and radiation pressure acceleration [18] in which electrons are compressed to a highly dense layer that in turn accelerates ions. In all these experiments, the highest energy ions are generated from sub-\(\upmu\)m-thick foil targets. Several methods for replenishing such targets at a high rate were devised in the form of, e.g., thin sheets of liquid ethylene glycol [19] or hydrogen, which solidify when injected into vacuum [20], or with automatically positioned [21] micro-machined foil targets [22]. Gas targets are naturally easier to refresh. High-energy ion beams resulting from collisionless shockwaves induced in nearly critical gas targets were demonstrated by either using long-wavelength laser pulses [23] or with very high-density gas [12; 24; 25; 26]. Tailoring the plasma profile around a solid foil target to enhance the emission of TNSA ions was also recently investigated [27]. Gaseous targets are a common choice for laser generation of high-quality electron beams. For the past 2 decades, the laser wakefield community focused on optimizing the accelerated beam quality for higher particle energy [28], sharper energy spectrum [29], higher charge [30], and improved repeatability. A few early works identified an electron acceleration technique from solid foil targets, referred to as \"the exploding foil method (EXFM)\" [31]. With EXFM, low-energy light preceding the main pulse turns the foil into an expanding plume of plasma. Owing to the expansion of the plasma, the electron density falls below the critical value and becomes transparent to the main pulse, which arrives tens of ns later. The main pulse forms self-guided laser wakefield structures [32], which generates ultra-collimated, multi-MeV beams of electrons [33]. Compared to modern wakefield electron acceleration schemes, EXFM seemed non-competitive on maximum energy and a peaked spectrum.