Preplasma has an important effect on the interaction between laser and matter, especially in solid targets, which produces different laser absorption mechanisms in sub-relativistic laser states, such as resonance absorption^[1^] and vacuum heating (the Brunel mechanism)^[2^]. Based on these studies, several interesting results have been obtained recently, such as the Brunel-like mechanism^[3^], high harmonic generation^[4^], and vacuum electron acceleration^[5^]. With the gargantuan laser powers projected to be realized in the developing petawatt (PW) facilities ELI in Europe^[6^–9^] and SULF in China^[10^], the interaction between lasers and plasmas is poised to occur in the ultrarelativistic state^[11^]. As is commonly understood, in the quantum electrodynamics (QED)-dominated regime, when an ultraintense laser interacts with a target, γ-rays can be generated by synchrotron radiation arising from high-energy electrons traveling in a strong, background electromagnetic field^[12^,13^]. Previous studies focused on the function of the preplasma before a dense target^[14^,15^], scanning different parameters to obtain the optimal γ-ray source. The other efficient method to generate γ-ray flare is to make accelerated electrons interact with a reflected laser, a method called all-optical Compton backscattering, in which electrons may be accelerated by a wakefield^[16^,17^] or pondermotive force^[18^] and laser is reflected by a dense plasma mirror. In these cases, underdense gas or nanoparticles^[19^] are required before the solid targets in their plans. The γ-rays generated in different regimes have lent themselves to many applications, such as dense matter tomography^[20^], photonuclear reactions^[21^], and laboratory astrophysics^[22^]. Previous research has shown that an ultraintense laser interacting with a plasma can emit γ-rays in different directions, depending on the plasma density^[11^,12^,23^] and the corresponding physical mechanisms at play.

As mentioned previously, the preplasma scale length plays an important role in the ultraintense laser matter interactions, which is the object of the present article. Via two-dimensional (2D) simulation and theoretical analysis, we found that the preplasma affects the γ-ray generation markedly. With increasing preplasma scale length, the angular distribution of the γ-rays tends to proceed with small angle. Moreover, the conversion efficiency of laser to γ-ray reaches its highest at an appropriate scale length. This shows that piled plasma formed in preplasma has similar effects to the reflecting mirror in an all-optical Compton backscattering scenario. The mirror is usually made of a solid target and reflects the laser. We propose a simple 2D analytical model to explain the simulation results, which is in good agreement with our simulation. It indicates that a single solid target with appropriate preplasma could also be robust to generating a γ-ray source.

The nonlinear QED effect^[24^] must be considered under ultraintense laser conditions, which mainly contains two important processes: γ-ray generation through synchrotron radiation and electron–positron pair creation by the Breit–Wheeler (BW) process^[25^]. These two processes can be simply described as

 1

 (1)

γ-ray generation is controlled by the parameter:

 (2)

In a comparatively low-density plasma of near-critical density ( ${n}_{\mathrm{e}}\sim {n}_{\mathrm{c}},\,{n}_{\mathrm{c}}=\frac{\omega_\mathrm{L}^2{\varepsilon}_0{m}_{\mathrm{e}}}{e^2}$ is the critical density), the laser penetrates the plasma and a proportion of the electrons become trapped in the plasma channel owing to the combined effects of the radiation reaction and the self-generated magnetic field^[27^]. Brilliant forward collimated γ-rays can be generated by relativistic electron bunches through synchrotron radiation, in a cone with an angle 1/ $\gamma$ ( $\gamma$ is the Lorentz factor of the electron) around the direction of electron motion.

When the plasma is no longer transparent (the plasma density is close to the penetration threshold value n_e $\;\sim\,$ n_th, in which n_th satisfies

 1

In the transition region with n_thec* (n_c* is the relativistically corrected critical density, which depends on the relativistic effect as ${n}_{\mathrm{c}}^{\ast}\sim \left\langle \gamma \right\rangle {n}_{\mathrm{c}}$ and the energy distribution between the degrees of freedom in the plasma^[23^,31^]), the longitudinal laser ponderomotive force can come to a balance with the restoring electrostatic force. The electrons mainly oscillate in the transverse direction. Thus, transversely emitted γ-rays at laser frequency are produced by the transversely oscillating electron synchrotron emission (TOEE)^[11^].

When the density is greater than n_c*, the laser pulse can only penetrate the target to several skin depths, after which it is reflected backwards. A small number of electrons, accelerated by the ponderomotive force within several skin depths, counterpropagate with the reflected pulse, and emit forward γ-ray bursts, a process known as skin-depth emission (SDE)^[11^,12^].

The four traditional mechanisms mentioned previously occur specifically in a uniform plasma with sharp edge. If a preplasma exists that covers a broad range from underdense to solid density, the γ-ray generation is not simply the superimposition of the four traditional mechanisms. The preplasma will alter the electron acceleration and laser reflection process, which then alters the features of the γ-rays. This represents a new phenomenon, as described in the following.

Here, a 2D particle-in-cell (PIC) simulation code OPIC^[32^] was used to investigate the process of an ultraintense laser (10²² W/cm²) irradiating a solid CH target with different preplasma scale lengths. A QED model is included in the code, which takes into account QED effects of the synchrotron radiation of γ-rays and the Breit–Wheeler electron–positron pair production using a Monte Carlo algorithm. Meanwhile, the radiation reaction effect is also taken into account in our simulation, which is important in the QED regime^[33^–36^]. It presents high-energy electrons that partially lose their energy through emitting gamma photons in laser field.

In the simulation, a box of dimensions 25 m × 10 m ( $x\times y$ ) is divided into 2500 grids in the x direction and 1000 in the y direction. A p-polarized laser pulse, with a peak intensity of $5.5\times {10}^{22}\ \mathrm{W}/{\mathrm{cm}}^2\left({a}_0=200\right)$ and wavelength of λ = 1 m, propagates from the left boundary. It has a transverse Gaussian profile with a full width at half maximum (FWHM) radius of r = 2λ and a sine square temporal profile a $\,={a}_0{\sin}^2\left(\mathrm{\pi} t/20{T}_{\mathrm{L}}\right)$ , in which ${T}_{\mathrm{L}}$ is the laser period. The target is composed of CH₂, which consists of a uniform main target and a preplasma before the main target, as shown in Figure 2. The uniform main target is located from 18λ to 20λ, with an electron density 200n_c ( ${n}_{\mathrm{c}}={\omega}_{{\rm L}}^2{\varepsilon}_0{m}_{\mathrm{e}}/{e}^2$ is the critical density). The target is fully ionized, and the initial temperatures of ions and electrons are 30 eV and 1 keV, respectively. Each cell of plasma is represented by 50 pseudoelectrons, 25 pseudoprotons, and 16 pseudocarbons. The scale lengths L of the exponential preplasma ( $L={\left(\mathrm{d}\ \ln n/\mathrm{d}x\right)}^{-1}$ ) are set to L = 4λ (2–18 m in the x direction), L = 2λ (10–18 m), L = 0.5λ (16–18 m), and L = 0. The cutoff electron density is about 3.6n_c to minimize the computational cost. We also simulated a larger preplasma region, in which the cutoff density of electrons was below n_c. There is no difference between the simulation results for the same scale length parameters, which means that the contribution of electrons to γ-ray generation in the <3.6n_c attenuate plasma region can be ignored.

The sharpest boundary case of L = 0 presents typical characteristics of the SDE mechanism. The gamma ray and energetic electrons are generated in laser skin layer from the region of x>18λ, which is the dense plasma of 200n_c, seen in Figure 4(a). The energetic electron have similar forward and transverse momentum (as shown in Figures 4(e) and 4(i)), which makes the electrons and gamma photons split into two large angles around ±45° in the forward direction, as shown in Figures 5(a) and 5(e).

When increasing the preplasma scale length slightly to L = 0.5λ, the energetic electrons are born from two different regions: dense plasma of 200n_c (x > 18λ) and preplasma <200n_c (x < 18λ), marked by a red dashed line in Figure 4(b). Most energetic electrons are produced in dense plasma of 200n_c via an SDE-like mechanism. In addition to those accelerated in skin depth similar to SDE, electrons are also accelerated in preplasma. Therefore, the forward-moving electrons have higher momentum and numbers owing to the longer acceleration length, as shown in Figures 4(f) and 4(j). The forward-moving energetic electrons interact with the backward laser field reflected by the skin layer of dense plasma, and produce brighter forward γ-rays. Some energetic electrons are generated in preplasma of x < 18λ, which exhibit obvious features of the RESE mechanism. The electrons have backward and transverse momentum. The backward reinjected electrons, reflected by the space-charge field of dense plasma, counterpropagate in the incident laser field, and emit a burst of backward γ-rays in preplasma, as can be seen in Figure 5(f). Thus, the angular profiles of energetic electrons and γ-rays present two forward ±45° peaks (SDE-like) and one backward peak (RESE), as shown in Figures 5(b) and 5(f).

If further increasing the preplasma to L = 2λ scale length, the generation region of γ-rays has changed apparently: the majority of γ-rays generated in underdense preplasma below $\sim 74{n}_{\mathrm{c}}$ (x < 16λ) and the minority in dense preplasma of 74n_c–200n_c (16λ < x < 18λ), which is illustrated in Figure 4(c) and Figure 6(a). The behaviors of electrons in those two regions are significantly different as presented in Figures 4(g) and 4(f). In underdense preplasma (<74n_c), electrons are mainly going forward accompanied by transverse oscillation, which emit a forward γ-ray beam. However, in the dense preplasma region of 74n_c–200n_c, electrons only oscillate in transverse direction with little forward motion, which generates the typical transverse γ-ray emission of the TOEE mechanism. Therefore, the angular distribution of γ-rays is the superposition of the dominated forward γ-rays in the underdense region and transverse γ-ray emission by TOEE in dense plasma, as shown in Figures 5(c) and 5(g). As can be found in Figure 6, the piled plasma by laser ponderomotive force produces a dense plasma mirror in preplasma, which plays the same function as an additional plasma mirror^[37^] in double target scheme. The backward laser field is mainly reflected by the preplasma region instead of the main target because there is no laser field behind the piled plasma. Meanwhile, the piled electron density shown in Figure 6(b) is almost reaches 200 ${n}_{\mathrm{c}}$ in the position of the laser axis and the main target shows little alteration. The reflected laser (pink line in Figure 6(b)) serves as a background electric field to maximum γ-ray generation. Thus, the forward γ-rays are generated in the laser pathway through Compton backscattering with a laser reflected by the piled plasma mirror, which presents a relatively collimated emission angle compared with shorter cases.

Increasing the preplasma scale length to L = 4 $\lambda$ , the characteristics of electrons and γ-rays are similar to the case of L = 2 $\lambda$ . From the Figures 4(d), 4(h), and 4(l), the majority of electrons are accelerated forward, emitting γ-rays in the underdense region density below $74{n}_{\mathrm{c}}$ (x < 12λ) through Compton backscattering. Minor γ-rays are contributed by TOEE from transversely oscillating electrons in the dense plasma of 74n_c–120n_c (12λ < x < 16λ). The electrons and γ-rays are further collimated as illustrated in Figures 5(d) and 5(h). The expanding angles have changed from ±45° to nearly ±25° owing to increasing scale length. Although the angles are still quite large compared with other schemes^[38^,39^], the present simple layout is easier to perform in experiments such as the radiography of dense objects^[40^].

A simple analytical model is now presented to explain the continuous enhanced and collimated γ-ray emission with increasing preplasma scale length. When a linear-polarized laser penetrates the plasma, electrons are pushed forward by the ponderomotive force, which can be written as^[41^]

 1

As can be found in Figure 8, with increasing scale length, the conversion efficiency first increases owing to the longer acceleration length, then reaches a saturated region, and finally decreases slightly at various laser parameters. The saturation phenomenon is a natural result of the piled plasma mirror. The γ-rays are the combined effect of forward electron acceleration and backward laser field reflected by piled plasma. At small scale length, the short acceleration length limits the electron energy and the γ-ray emission. At very long scale length, the plasma accumulation and the piled plasma mirror are not formed in time owing to over-attenuated preplasma, which decreases the reflected laser field and the γ-ray emission, although it has more energetic electrons. Thus, the optimal scale length of γ-ray generation is the balance between acceleration of electrons and laser reflection.

From Figure 8 we also find the positive correlation between the optimal scale length and laser pulse duration. A shorter laser pulse needs to be reflected by piled plasma early to serve as the background field for the accelerated electrons before the laser energy is depleted. On the other hand, longer laser duration can accelerate electrons fully before the laser reflected because the longer pulse contains more energy to accelerate electrons and act as a background field. Meanwhile, the formation time of piled plasma mirror and the scale length are proportionate. Therefore, the optimal scale length for a long pulse is nearly five times that of a short pulse (as shown in Figure 8 for a = 200), which is a multiple of the duration time.

In summary, we have investigated the influence of preplasma scale length on γ-ray emission in the interaction between an ultraintense laser and solid targets via 2D PIC simulation. We find that the preplasma will enhance and collimate the γ-ray emission significantly. It exhibits the formation of piled plasma mirror, which has the same function as plasma mirror in an all-optical Compton backscattering scenario. The forward-accelerated electrons interact with the backward laser field reflected by the piled plasma in the preplasma region and produce the γ-ray emission. A solid target with appropriate preplasma could be robust to generating a useful γ-ray source, which is easier for experimental operation. The feature may be slightly different in an actual 3D situation, but the primary physical characterization will be maintained^[15^]. The current study may be conducted in PW facilities such as SULF and ELI, and is helpful for future experiments, especially for bright collimated γ-ray source generation, and it may also be used to diagnose the preplasma according to the features of the emitted γ-rays.