Preamble

Pulsed neutron source from interaction of petawatt femtosecond laser with pitcher-catcher target
Jin-Long Luo,¹ Kai-Yuan Feng,¹ De-Bin Zou,¹,²,† Wei Qi,³ Bo Cui,³ Li-Xiang Hu,¹ Shu-Kai He,³ Zhi-Meng Zhang,³ Zhi-Gang Deng,³,‡ Xiao-Hu Yang,¹ Tong-Pu Yu,¹ and We-Min Zhou³
¹College of Science, National University of Defense Technology, Changsha 410073, China
²Hunan Research Center of the Basic Discipline for Physical States, National University of Defense Technology, Changsha 410073, China
³Science and Technology on Plasma Physics Laboratory, Laser Fusion Research Center, China Academy of Engineering Physics, Mianyang 621900, China

Production of pulsed neutron sources from petawatt-class femtosecond laser interacting with pitcher-catcher targets is studied using comprehensive radiation-hydrodynamics, particle-in-cell, and Monte Carlo simulations. We find that preplasma from laser prepulse ablation is beneficial for increasing neutron yield. A beryllium converter as the catcher layer offers superior performance in improving neutron source quality. For experimental consideration, a novel converter with LiH ceramic wrapping beryllium material is designed to maintain emissions within safety limits and achieve a high retention rate. With a laser intensity of 4 × 10²¹ W/cm², duration of 30 fs, and power of 1.45 PW, one can obtain a forward-peaked neutron source with yield of 1.95 × 10¹⁰, duration of 150 ps, and maximum energy of ∼70 MeV. This high-quality pulsed neutron source can be produced at high repetition rates and is promising for industrial non-destructive testing and related applications.

Keywords: Petawatt femtosecond laser pulse; pitcher-catcher target; preplasma; converter

Introduction

Neutron sources serve as essential tools for interdisciplinary research and are extensively utilized in fields such as material structure analysis, biomedicine, industrial non-destructive testing and safety monitoring, and nuclear energy research [1]. High-flux pulsed neutron sources have emerged as critical platforms for advancing cutting-edge scientific and industrial innovations.

At present, high-flux pulsed neutron sources are mainly produced through spallation reactions, such as at the China Spallation Neutron Source, where 1.6 GeV protons are directed at tungsten or uranium targets [2]. In this process, each proton can generate approximately 20 to 30 neutrons, with output pulse durations ranging from 4 to 30 ns and peak flux up to 10¹⁶–10¹⁷ n/cm²/s [3]. To achieve higher spatiotemporal resolution, there remains a need to produce high-quality neutron sources with higher flux and shorter pulse duration.

The development of chirped pulse amplification (CPA) [4] and optical parametric chirped-pulse amplification (OPCPA) [5] technologies has significantly advanced ultraintense, ultrashort laser systems. Laser pulses with intensities of 10¹⁸–10²³ W/cm² and durations ranging from picoseconds to femtoseconds are now available in laboratories [6, 7]. When such pulses interact with matter, they can generate energetic charged-particle beams and bright secondary radiations with extremely high number densities and short pulse durations in compact spatial scales [8–12]. By inducing different nuclear reactions, it is possible to produce pulsed neutron sources with peak flux >10¹⁸ n/cm²/s, durations from hundreds of picoseconds to even femtoseconds, and focal spots at the centimeter scale [13–16]. Among all laser-based neutron production schemes [17–23], the pitcher-catcher target configuration is very promising for developing compact, high-flux pulsed neutron sources [24–27]. In this scheme, protons or ions (e.g., deuterium ions) are first accelerated when a laser pulse irradiates the pitcher layer. When these ions subsequently strike the catcher layer (i.e., the converter), neutrons are produced through various nuclear reactions [28]. To obtain high neutron yield, sub-picosecond or multi-picosecond petawatt (PW) laser pulses are typically used in experiments because they can deliver tens or hundreds of joules of laser energy [27, 29–34]. In addition, the energy conversion efficiency from laser to ions can reach relatively high levels, up to ∼10% [35, 36].

However, for practical applications, PW femtosecond laser systems might be more advantageous for producing neutron sources, as they can simultaneously achieve high repetition rates and high yields while maintaining device compactness. Recently, many PW-fs laser facilities have been put into operation worldwide [7], providing the possibility to develop compact, high-flux, high-repetition-rate pulsed neutron sources.

For PW-fs laser-driven neutron sources, two key factors require special attention: the preplasma and the converter. Currently, typical PW-fs laser systems can focus laser pulses to intensities ranging from 10²¹ W/cm² to 10²³ W/cm² [7]. Using modern laser cleaning techniques [40], such as the plasma mirror method [37–39], the laser contrast (the intensity ratio between the main pulse and the amplified spontaneous emission pedestal) can reach above 10¹⁰ [41]. However, the prepulse intensity could still exceed 10¹¹ W/cm² [39, 41], and there is a steep temporal rising edge before the main pulse. Consequently, a preplasma with a certain scale length can be induced before the target [42, 43], which may affect laser-driven ion acceleration and subsequent neutron production [44]. Another important factor is the converter. For ion beams with different energy characteristics, the converter must be specially designed to achieve high nuclear reaction cross-section and strong radiation resistance.

In this paper, we investigate neutron production from PW-fs laser pulses interacting with pitcher-catcher targets in the presence of a prepulse. We emphasize the influence of preplasma and converter materials on neutron source quality in detail. We find that the laser prepulse significantly alters the density profile of the pitcher layer. The preplasma from laser prepulse ablation leads to a remarkable increase in neutron yield. Among common converter materials, beryllium is advantageous for high-yield neutron production. Considering experimental safety requirements, we have designed a novel converter structure with LiH ceramic wrapping beryllium material to maintain a high nuclear reaction cross-section and strong irradiation resistance. Under typical PW-fs laser irradiation parameters, one can obtain a forward-peaked neutron pulse with yield >10¹⁰, duration of ∼150 ps, and maximum energy of tens of MeV. Such a high-quality neutron source is promising for industrial non-destructive testing and related applications.

The paper is organized as follows. In Section 2, we introduce the laser and pitcher-catcher target parameters as well as the simulation process. In Section 3, we show preplasma formation from laser prepulse ablation of the pitcher layer and the interaction of the main pulse with the ablated pitcher layer. In Section 4, we present the characteristics of neutron sources produced through nuclear reactions when the accelerated proton and ion beams transport through the converters. Effects of preplasma and converter material are then discussed. In Section 5, we propose a novel converter to optimize neutron production for typical PW-fs laser pulses. Finally, a summary is given in Section 6.

II. Simulation Model and Laser-Target Parameters

We employ a comprehensive approach using the radiation-hydrodynamics (RHD) code FLASH [45], particle-in-cell (PIC) code EPOCH [46], and Monte Carlo (MC) code GEANT4 [47] to investigate preplasma formation, laser-driven ion acceleration, and neutron generation, respectively. A schematic of the interaction model between a laser pulse and a pitcher-catcher target is shown in Fig. 1 [FIGURE:1].

Considering the actual power of current PW-fs laser pulses on target, especially for high-repetition-rate operation, we simulated four different power scenarios: P = 0.18 PW, 0.36 PW, 0.72 PW, and 1.45 PW. These laser pulses, with a wavelength of 0.8 µm and period T₀ = 2.67 fs, are linearly polarized and focused on the front surface of the target at an incidence angle of 5°. Their temporal and spatial profiles follow Gaussian distributions, with a pulse duration of 30 fs and focal spot of 4.8 µm. The corresponding laser intensities are 5 × 10²⁰ W/cm², 1 × 10²¹ W/cm², 2 × 10²¹ W/cm², and 4 × 10²¹ W/cm², with normalized amplitudes a₀ = eE₀/(mₑω₀c) = 15, 22, 31, and 43, respectively. Here, −e and mₑ are the electron charge and mass, E₀ and ω₀ are the electric field amplitude and angular frequency of the laser, respectively, and c is the speed of light in vacuum. For all scenarios, the contrast ratio remains constant at 10¹⁰, as referenced for the SILEX-II laser system [48]. The target consists of a CD₂ pitcher layer with thickness d₀ = 5 µm and a catcher layer (i.e., converter) with thickness d₂ = 3 cm, separated by a distance L₀ = 1 cm. A natural contamination layer behind the pitcher layer is characterized as an ultrathin carbon-hydrogen-oxygen film with thickness d₁ = 20 nm. The ion density ratio of this contamination layer is C:H:O = 2:4:1, corresponding to an electron density of 3 × 10²² cm⁻³ [49]. The converter is designed as a cubic structure with an edge length of 10 cm.

Fig. 1 [FIGURE:1] (Color online). Schematic of a linearly polarized PW-fs laser pulse interacting with a pitcher-catcher target. The pitcher layer is composed of CD₂ material with a C₂H₄O contamination layer behind it. For the catcher layer, three common materials (LiF, LiD, and Be) are chosen for comparison.

The FLASH simulation is performed in a two-dimensional Cartesian coordinate system, with laser deposition achieved through ray-tracing projection [50]. The Courant-Friedrichs-Lewy (CFL) time limit is set to 0.4, and equation-of-state and multigroup opacity tables from PROPACEOS are used. The FLASH code employs an adaptive mesh refinement (AMR) scheme with coarsest and finest mesh sizes of 0.04 µm and 0.026 µm in the axial (x) and radial (y) directions, respectively.

The preplasmas formed in the FLASH simulation are fully modeled in the PIC simulation as the initial density distribution of the pitcher layer. The pitcher and contamination layers are treated as fully ionized due to laser prepulse ablation. In the EPOCH simulation, the simulation box measures 100 µm × 30 µm with 10,000 × 1,500 cells and 36 macroparticles per cell. Open boundary conditions for both particles and fields are used at all boundaries. The pitcher layer is initially located between x = 21 µm and x = 26 µm. Subsequently, all position, momentum, and energy information as well as particle weight data for the ions from the EPOCH simulation are imported into the GEANT4 post-processing program within the MC framework. Note that only ions with momentum pₓ > 0 are considered valid, as ions with pₓ < 0 cannot be incident on the converter. The neutron reaction module is activated to invoke the relevant nuclear reaction cross-section database for simulating neutron generation. In the GEANT4 simulation, the laser-accelerated ion beams can be regarded as a point source since their spatial size is much smaller than that of the converter. To investigate the influence of laser prepulse on neutron production, we also conducted full-process simulations without laser prepulses. The role of converter material is then discussed, with three common materials (LiF, LiD, and Be) chosen for comparison. In these simulations, we vary one parameter at a time while keeping all others constant.

III. Petawatt Femtosecond Laser-Driven Ion Acceleration

Figure 2 [FIGURE:2] shows the electron density distribution of the CD₂ pitcher layer at t = 120 ps for different laser powers P from the FLASH simulations. The preplasma mainly appears within the transverse extent of the laser focal spot region due to prepulse ablation of the pitcher layer. With increasing P, the radiative heat wave becomes stronger, penetrating to greater depth inside the pitcher layer. The peak density of the CD₂ layer is correspondingly increased, and the preplasma before the pitcher layer expands to a larger spatial scale along the −x direction. From the axial density distribution, we observe that the density profile becomes less steep and exhibits disturbances in the high-density region. Overall, the laser prepulse significantly alters the density distribution of the pitcher layer. In particular, the preplasma of a certain scale length that appears in front of the pitcher layer may affect energy absorption of the main laser pulse.

The dynamics of the main laser pulse interacting with the ablated pitcher layer are investigated through PIC simulations. Figure 3 [FIGURE:3] shows the distributions of electron density, spectrum, and longitudinal electric field at t = 60T₀ for cases both with and without preplasma from laser prepulse ablation. Dense oscillating bunches are generated due to betatron-like resonance [51–53] as relativistic self-focusing occurs within the preplasma [43]. These bunches then transport through the high-density region to the rear of the pitcher layer. Most of these electrons move forward, and their energies are much higher than those in the case without preplasma. The temperature of hot electrons even exceeds the Wilks ponderomotive force scaling Tₑ = (√(1 + a₀²/2) - 1)mₑc² [54]. Originating from these "superponderomotive" electrons [55–58], a stronger sheath electric field Eₓ (at least 1.5 times stronger) is thus induced, since its amplitude is proportional to Tₑ and increases with P [59]. We note that the longitudinal electric field in front of the pitcher exhibits a distinct zonal structure, attributed to interference between the incident and reflected laser light from the pitcher layer.

Fig. 2 [FIGURE:2] (Color online). Electron density distribution of the CD₂ pitcher layer nₑ/n_c at t = 110 ps for different laser powers P from the RHD simulations. The green curves show the axial electron density profile along y = 0. Here, density is normalized by the critical plasma density n_c = mₑω₀²/4πe².

Fig. 3 [FIGURE:3] (Color online). Distribution of electron density and spectrum (all, red solid curves), and the normalized longitudinal electric field eEₓ/mₑω₀c and its axial profile (blue curves) at t = 60T₀ for different P in the case with preplasma (w) from PIC simulations. For comparison, the spectra (all, red dashed curves) and axial Eₓ (green curves) in the case without preplasma (w/o) are also shown in panels (e)-(h) and (i)-(l). To distinguish contributions from forward (pₓ > 0, blue curves) and backward (pₓ < 0, green curves) electrons, electron spectra in both directions are presented in panels (e)-(h) for both cases (w, solid curves; w/o, dotted curves). The color bars in panels (a)-(d) represent the logarithm of electron density log(nₑ/n_c).

Figures 4(a)-4(f) show the density and spectral distributions of protons (H⁺) and deuterium (D⁺) ions at t = 100T₀ for different P. Their densities exhibit a layered structure. Owing to their relatively large charge-to-mass ratio [60, 61], protons can be accelerated to higher velocities by the sheath electric field via the target normal sheath acceleration (TNSA) mechanism [62] and thus concentrate at the front. In contrast, the heavier D⁺ ions have lower energy per nucleon and are distributed in the rear. They act as a buffer layer, resulting in small quasi-monoenergetic spikes in the proton spectra [63], as shown in Figs. 4(e)-4(h). Notably, the D⁺ ions are also divided into two layers, a phenomenon that becomes particularly prominent at higher P. The first layer consists of high-energy D⁺ ions originating from acceleration in the sheath electric field established by energetic electrons in the preplasma. The second layer comprises lower-energy D⁺ ions contributed by hot electrons from J × B heating [64, 65] when the main laser pulse interacts with the compressed high-density pitcher layer. The preplasma facilitates ion acceleration, thereby increasing the energy of D⁺ ions and protons by factors of 2 and 1.5, respectively.

Fig. 4 [FIGURE:4] (Color online). Density [(a)-(d)], spectral [(e)-(h)], and angular [(i)-(l)] distributions of protons and D⁺ ions at t = 100T₀ for different P, both with and without preplasma from PIC simulations. The color bars in panels (a)-(d) represent log(nₚ/n_c) and log(n_D/n_c).

The angular distribution of the two ion species dN_i/dθ for both cases is presented in Figs. 4(i)-4(l). The divergence angle of protons (θ_p = 20°) is slightly larger than that of D⁺ ions (θ_D = 12°) due to the strong Coulomb repulsive force from D⁺ ions on the front protons. This repulsion becomes more pronounced at higher P (e.g., 1.45 PW), resulting in a significant increase in θ_p while the proton number decreases substantially at θ = 0°, as shown by the green lines in Figs. 4(k) and 4(l). Regardless of preplasma presence, the divergence angles of both ion species are essentially the same. This is attributed to the thermal diffusion depth [66] L_th = 5.23 × 10⁻³ µm being much smaller than d₀ = 5 µm, where α = 2.28 × 10⁻⁷ m²/s is the thermal diffusion coefficient and τ_prepulse = 120 ps is the laser prepulse duration. During prepulse ablation, heat is confined to the front of the pitcher layer and cannot penetrate it. The CD₂ layer can avoid damage from thermal expansion, and its performance is similar to that without preplasma. It should be noted that the number of protons around θ = 0° is much smaller than that of D⁺ ions because protons are derived only from a very thin contamination layer. Moreover, the electrons contributing to their acceleration mainly come from the preplasma, and their number is far below that of hot electrons from the pitcher layer in the absence of preplasma.

IV. Neutron Production in Converters of Different Materials

When proton and D⁺ ion beams are incident on LiF, LiD, and Be converters, neutrons are produced through different nuclear reaction channels. For the LiF converter, the channels are ⁷Li(d,n)⁸Be and ⁷Li(p,n)⁷Be; for the LiD converter, the channels are D(d,n)³He, D(p,n+p)p, ⁷Li(d,n)⁸Be, and ⁷Li(p,n)⁷Be; and for the Be converter, they are ⁹Be(d,n)¹⁰B and ⁹Be(p,n)⁹B. During nuclear reactions, the neutron yield can be approximately expressed as Y_n ∝ Σ n_t · σ_i · Φ_i [67], where n_t is the target nucleus density, σ_i is the reaction cross-section for the i-th incident ion, and Φ_i is its flux. With increasing P or when preplasma is considered, Y_n is remarkably enhanced due to improved Φ_i, as shown in Figs. 5(a)-5(c). For LiF, LiD, and Be converters, n_t is 6.11 × 10²²/cm³, 1.208 × 10²³/cm³, and 1.235 × 10²³/cm³, respectively. When proton and D⁺ ion energies exceed 2 MeV and 3 MeV, respectively, σ_i for Be material are the highest among all reaction channels [68]. Therefore, it is easily predicted that the Be converter should yield the highest Y_n, followed by the LiD converter, with the LiF converter producing the lowest yield, which is in good agreement with Figs. 5(a)-5(c). Figures 5(d)-5(f) show the neutron spectral distribution for the three converter materials, both with and without preplasma at different P. The maximum neutron energy E_n,max is positively correlated with P. Additionally, the number of neutrons at a given E_n increases because more protons and D⁺ ions are accelerated to higher energies. Among all converters, the Be converter is most beneficial for obtaining high neutron energies, especially for higher P lasers (e.g., P = 1.45 PW).

Fig. 5 [FIGURE:5] (Color online). Neutron yield [(a)-(c)] and spectrum [(d)-(f)] for three converter materials [LiF: (a) and (d); LiD: (b) and (e); Be: (c) and (f)], both with [red columns in (a)-(c) and curves in (d)-(f)] and without [blue columns in (a)-(c) and shaded regions in (d)-(f)] preplasma at different P from MC simulations.

Figure 6 [FIGURE:6] shows the angular distribution of neutrons dN_n/dθ for both cases. Regardless of preplasma presence, the distribution is roughly the same since dN_i/dθ is not significantly different in Figs. 4(i)-4(l). Interestingly, with increasing P, these neutron sources exhibit excellent forward directionality. We count neutrons with P_x > 0 and find they account for 70% ± 5% of the total neutron number, mainly consisting of high-energy neutrons (for the Be converter at P = 1.45 PW, E_n reaches up to ∼75 MeV). This phenomenon can be qualitatively understood through impulse-mass coupling in nuclear reaction dynamics. During the nuclear reaction, the momenta of the incident ion (p_i), target nucleus (p_t), residual nucleus (p_r), and neutron (p_n) must satisfy momentum conservation: p_i + p_t = p_r + p_n. Considering the target nucleus is almost at rest (v_i ≈ 0) and the residual nucleus velocity is extremely small (v_r ≈ 0), we have p_i ≈ p_n. Therefore, the neutron inherits the direction of motion of the incident ion. If v_r ≠ 0, the neutron may deviate from the incident ion direction.

We can quantitatively estimate the proportion of forward neutrons. In the center-of-mass frame, the angular distribution of the nuclear reaction differential cross-section is described by quantum scattering theory [69, 70]. Its Legendre series expansion can be written as [71, 72]:

$$\frac{d\sigma}{d\Omega} = \sum_{\ell=0}^{\infty} (2\ell + 1)P_\ell(\cos\theta_c) \exp[-\ell(\ell+1)/\ell_0^2]$$

where $P_\ell(\cos\theta_c)$ is the Legendre polynomial, $\theta_c$ is the neutron scattering angle, and $\ell$ is the partial wave angular momentum channel. The cutoff parameter $\ell_0 = kR$ is determined by the nuclear geometry, where $k = \sqrt{2\mu E_i}/\hbar$ is the reduced wave number, $\mu$ is the reduced mass, $E_i$ is the incident ion energy, $\hbar$ is the reduced Planck constant, $R = 1.25A^{1/3}$ fm is the nuclear radius, and $A$ is the mass number of the target nucleus.

In the high-energy region where $E_i > E_{th}$, $d\sigma/d\Omega$ exhibits a pronounced forward peak since $\ell_0 = \sqrt{\langle\ell^2\rangle} \propto \sqrt{E_i}$ [73], where $E_{th} = |Q|(1 + m_i/m_t)$ is the nuclear reaction threshold energy, $Q$ is the net energy released during the reaction, and $m_i$ and $m_t$ are the masses of the incident ion and target nucleus, respectively. This forward peak originates from constructive interference of all partial waves with angular momentum up to $\ell \approx \ell_0$ [74]. The number of contributing partial waves increases with energy ($\ell_0 \propto \sqrt{E_i}$), corresponding to grazing collisions with maximum impact parameter $b_{max} \approx R$. The maximum angular momentum is $\ell_{max}\hbar = pb_{max} = \hbar kR$.

Analysis of partial wave contributions reveals that the $\ell = 0$ partial wave, which is isotropic ($P_0(\cos\theta_c) = 1$), accounts for 70% ± 5% of the total reaction cross-section, as obtained from Geant4 simulations. The pronounced forward peak in the differential cross-section is therefore primarily generated by constructive interference of the remaining ∼30% of higher-ℓ partial waves ($\ell \ge 1$).

For low-energy incident ions with $E_i < E_{th}$, multiple Coulomb scattering causes ion trajectories to deviate from the incident direction [75]. The differential cross-section follows the Rutherford scattering formula [76]:

$$\frac{d\sigma}{d\Omega} \propto \left(\frac{q_i q_t e^2}{16\pi\varepsilon_0 E_i}\right)^2 \frac{1}{\sin^4(\theta/2)}$$

We see that low-energy incident ions are responsible for large-angle neutron scattering. Events with small impact parameters could even produce backward neutrons (where the angle between p_n and p_i exceeds π/2). As shown by the red curves in Fig. 6, when P = 0.18 PW, the ion energy is not high and neutron emission is almost isotropic.

Fig. 6 [FIGURE:6] (Color online). Angular distribution of neutrons dN_n/dθ for three converter materials [LiF: (a) and (d); LiD: (b) and (e); Be: (c) and (f)], both with [(a)-(c)] and without [(d)-(f)] preplasma at different P from MC simulations.

The neutron source duration Δt_n is a key quality indicator for practical applications. For instance, a wide duration of Δt_n ∼ µs can satisfy spectral integration requirements for resonance capture, while a short duration of Δt_n < 100 ps is beneficial for achieving high temporal resolution in time-resolved imaging [77]. Figures 7(a)-7(f) show the temporal evolution of the neutron production rate P_n(t) for all conditions. P_n(t) exhibits a Gaussian-like distribution except for the LiF converter, matching precisely the temporal structure of the incident ions. The neutron source duration [full width at half maximum (FWHM)] ranges from 100 ps to 200 ps, close to the requirement for high time-resolved imaging. As P increases or preplasma is considered, P_n(t) also increases since P_n(t) ∝ Φ_i(t) · σ_i[E_i(t)] [67]. For the Be converter, the maximum P_n(t) is 1.4 × 10²⁰ s⁻¹ at P = 1.45 PW, representing an increase of approximately 2.42 times compared to the case without preplasma.

Interestingly, in Figs. 7(a) and 7(d), a two-peak structure in P_n(t) appears for the LiF converter. This can be attributed to the difference in flight time for the two ion species over the same flight distance L₀ = 1 cm. The first peak originates from protons and the second from D⁺ ions. For LiF material, its volumetric reaction rate R is lowest, so the two-peak feature from the flight time difference is retained. However, for Be material with a higher R and broader σ_i, the typical two-peak structure gradually merges into a single peak.

Fig. 7 [FIGURE:7] (Color online). Evolution of neutron production rate P_n(t) for three converter materials [LiF: (a) and (d); LiD: (b) and (e); Be: (c) and (f)], both with [(a)-(c)] and without [(d)-(f)] preplasma at different P from MC simulations. The time t = 0 in the MC simulation corresponds to t = 100T₀ = 0.2667 ps in the PIC simulation.

V. A Wrapped-Type Converter Based on LiH Ceramic Wrapping Beryllium Material

We have previously demonstrated that for typical PW-fs laser pulses, beryllium is very suitable as a converter for neutron production. However, this material is extremely toxic and exists in powdered form. To facilitate handling, a wrapping layer is typically used to isolate the highly active Be material from the experimental environment. The essence of the wrapping structure lies in the decoupled selection of the embedded material (Be) and the wrapping material. The wrapping material should be chosen to have a low neutron absorption cross-section, high mechanical strength, and good environmental stability. For convenience, Al is a common choice for the wrapping material (Al+Be). However, its nuclear reaction cross-section is very low, which is disadvantageous for neutron production. Moreover, bright γ-rays are emitted when intense electron beams transport through it, affecting neutron diagnostics. Optimization of the wrapping layer material is therefore essential.

Considering that LiD has a rapid deliquescence rate (weight gain rate ≈ 5 wt%/h at RH = 50%), while LiF lacks sufficient bending strength (polycrystalline ceramics ≈ 45 MPa) [78], neither is suitable as a wrapping material. To maintain emissions within safety limits and achieve a high retention rate, we designed a novel converter with LiH ceramic wrapping beryllium material (LiH+Be), as shown in Fig. 8 FIGURE:8. LiH ceramic can achieve a density greater than 99% through a temperature isostatic pressing process [79]. The flexural strength exceeds 110 ± 10 MPa, and surface passivation ensures a hydrolysis rate of less than 0.01 wt%/day [79]. We note that the melting point of LiH ceramic is 688°C, which is slightly higher than that of Al (660°C). This indicates that LiH ceramic can withstand stronger particle beam irradiation [80]. Furthermore, the density of LiH ceramic is only 0.77 g/cm³ [81], which is much less than the 2.7 g/cm³ of Al. The stopping power of ion beams in LiH ceramic is thus far below that in Al [82–84], resulting in lower energy loss. Compared with the Al+Be converter, LiH+Be is therefore more beneficial for neutron production.

To evaluate the performance of the LiH+Be converter, we further employ the Geant4 code to simulate neutron production. For comparison, neutron information for pure Be and Al+Be converters is also provided. The thickness of the side and front/back plates of the LiH and Al wrapping layers is d₀ = d₁ = 0.1 cm. For the LiH+Be, Al+Be, and pure Be converters, all Be materials are cubic with a thickness of 3 cm and side length of 10 cm. Figures 8(b)-8(f) show the neutron spectrum, yield, and angular distribution for the three converters. We see that the spectral structure for all three converters is almost identical, demonstrating that the wrapping layer does not induce spectral distortion.

When P is low (e.g., P ≤ 0.36 PW), the differences in E_n, Y_n, and dN_n/dθ among these three converters are significant. For the LiH+Be converter, E_n and Y_n are only slightly inferior to those of the pure Be converter and much higher than those for the Al+Be converter. With increasing P, the differences gradually decrease since the two ion species are accelerated to higher energies and their relative energy loss in the wrapping layer becomes smaller. This indicates that the LiH+Be converter can achieve a retention rate closer to that of the pure Be converter. With a laser intensity of 4 × 10²¹ W/cm², duration of 30 fs, and power of 1.45 PW, simulations demonstrate that a forward-peaked neutron pulse with yield of 1.95 × 10¹⁰, duration of 500 ps, and maximum energy of ∼70 MeV can be obtained with the LiH+Be converter.

Fig. 8 [FIGURE:8] (Color online). (a) Structure of the proposed novel converter with LiH ceramic wrapping Be material (LiH+Be). (b) Yield, (c) spectrum, (d)-(e) angular distributions of neutrons, and (f) evolution of neutron production rate P_n(t) for LiH ceramic wrapping Be (LiH+Be), Al wrapping Be (Al+Be), and pure Be (Be) converters from MC simulation.

VI. Summary

To summarize, we have investigated laser prepulse ablation, laser-driven ion acceleration, and neutron production from nuclear reactions during the interaction of a PW femtosecond laser pulse with a pitcher-catcher target using full-process simulations. We find that preplasma formation in front of the pitcher layer is beneficial for neutron production. The selection of converter material is of great importance, and a Be converter as the catcher layer offers superior performance in improving neutron source quality. Considering experimental operation requirements, we propose a novel converter with LiH ceramic wrapping beryllium material to maintain emissions within safety limits and achieve a high retention rate. In particular, with a laser intensity of 4 × 10²¹ W/cm², duration of 30 fs, and power of 1.45 PW, simulations demonstrate that a forward-peaked neutron pulse with yield of 1.95 × 10¹⁰, duration of 500 ps, and maximum energy of ∼70 MeV can be obtained with the novel converter. Our results should be helpful for present experiments on PW femtosecond laser-driven pulsed neutron sources and their related applications.

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