Fig. 1. Schematic of traditional γ-ray generation mechanisms at uniform plasma with different density (blue backgrounds represent plasma densities; black circles are electrons; purple represents the gamma photons; small red arrows show the moving direction of the electrons and gamma photons; light red arrows are the laser; and yellow arrows are the space charge force). (a) Low-density plasma n_en_c, forming a plasma channel. (b) Plasma density is close to the penetration threshold value n_en_th, showing the RESE process. (c) Transition region with n_th<n_e<n_c* of the TOEE mechanism. (d) High density of n_e>n_c* of the SDE process.

Fig. 2. Schematic of the simulation setup and γ-ray generation mechanisms for a solid target with preplasma. Electrons are accelerated in the preplasma, and then interact with the reflected laser through the piled preplasma, emitting bright gamma radiation.

Fig. 3. Temporal evolution of electron numbers with at preplasma scale length of 0, 0.5λ, 2λ, and 4λ (the characteristic times of the maximum electron numbers are marked with arrows). The laser pulse profile at x = 18λ is also provided for the case of L = 0.

Fig. 4. (a)–(d) Spatial density distributions of the electrons (black) and gamma photons (red) at corresponding characteristic times. (e)–(l) Phase space distribution of electron momentum (e)–(h) P_x and (i)–(l) P_y at the characteristic time. There are four cases: (a), (e), (i) L = 0; (b), (f), (j) L = 0.5λ (preplasma region 16–18 µm); (c), (g), (k) L = 2λ (10–18 µm); (d), (h), (l) L = 4λ (2–18 µm). The colorbars represent the electron density.

Fig. 5. The angular energy distributions of (a)–(d) energetic electrons and (e)–(h) and gamma photons at corresponding characteristic times (all units are MeV): (a), (e) L = 0; (b), (f) L = 0.5λ; (c), (g) L = 2λ; (d), (h) L = 4λ. The colorbars represent the lg N of electrons or gamma photons (N represents their density).

Fig. 6. (a) The electron density (black to white colorbar) and laser field E_y (red to blue colorbar) distribution of the preplasma scale length L = 2 condition at characteristic time; (b) lineplot in the position of y = 5 µm for the laser field E_y, reflected laser field bp (bp = ), and electron density (blue is the current distribution and green is the initial profile).

Fig. 7. The conversion efficiency of laser energy to (a) electrons and (b) γ-rays at various preplasma scale lengths.

Fig. 8. The conversion efficiency from laser energy to γ-rays at different scale lengths and laser parameters. The conversion efficiencies are saturated at longer scale lengths (d is the laser pulse width (FWHM) and T = 3.3 fs is the laser period for ).