1.School of Physics, Beihang University, Beijing 100191, China 2.Beijing Advanced Innovation Center for Big Data-based Precision Medicine, Beihang University, Beijing 100191, China 3.Beijing Key Laboratory of Advanced Nuclear Energy Materials and Physics, Beihang University, Beijing 100191, China 4.National Research Tomsk Polytechnic University, Tomsk 634050, Russia 5.Institute of Heavy Ion Physics, Peking University, Beijing 100871, China
Abstract:Short-pulse length and high-power density, intense pulsed ion beam (IPIB) has been widely studied in material processing during past decades. Ablation effect plays a great role in the interaction between IPIB and material and may affect the energy deposition of IPIB, thus further influencing the beam application and diagnostics. Therefore, the investigation of ablation effect on energy deposition of IPIB in the irradiated material is of great significance for its applications and diagnostic techniques. In this work, experiments on the IPIB irradiation are carried out on the BIPPAB-450 accelerator at Beihang University. Its maximum accelerating voltage is 450 kV, peak current density is 150 A/cm2, energy density is 1.5–1.8 J/cm2 and pulse duration (FWHM) is 80 ns. Polymer materials which have low thermal conductivity, low decomposition temperature and thus yield to ablation under low beam density, such as polycarbonate (PC), polyvinyl chloride (PVC) and polymethyl methacrylate (PMMA), are chosen in the present research. The 304 stainless steel is used for calorimetric beam diagnostics and comparative analysis. Energy deposition in polymer material and 304 stainless steel are obtained by high infrared imaging diagnostics. It is revealed that the distributions of energy deposition in these two kinds of materials differ from each other obviously. The highest energy density deposited in the 304 stainless steel appears in the center of the irradiated area where focused is the beam with a higher energy density. However, the central energy density in polymer material turns out to be lower than the surrounding area, indicating that a large portion of the ion beam is prevented from reaching the target. Meanwhile, the simulation based on the finite element method is carried out for the thermal filed distribution and evolution under the IPIB irradiation. The simulation result indicates that the strong ablation can be generated on the target surface since the highest temperature caused by IPIB irradiation is much higher than its decomposition temperature. According to the results of experiments and simulation, the polymer material can start to be ablated at the initial stage of IPIB irradiation which will consume partial energy and the products of ablation may act as shielding to block the energy deposition in the same pulse. Keywords:intense pulsed ion beam/ ablation/ energy deposition/ shielding
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--> --> --> 1.引 言强脉冲离子束(intense pulsed ion beam, IPIB)源于20世纪60年代对惯性约束核聚变(inertial confinement fusion, ICF)点火技术的研究[1]. 当IPIB作用于材料表面时, 能够在材料表面微米尺度深度内形成极高的功率密度沉积, 导致材料表面温度剧烈地上升和下降, 伴随产生快速熔化、汽化、重凝. 在这个过程中, 材料表面的硬度和韧性等参数能够得到显著的提高[2,3]. IPIB的这种特性使其在材料处理及表面改性方面获得应用并展现出良好的发展前景[4,5]. 当IPIB能量密度较高时, 靶表面在剧烈的辐照热效应下会产生烧蚀等离子体并向外扩散[6,7]. 利用IPIB产生的烧蚀等离子体可以以较高的沉积速率进行薄膜制备[8,9]. 在一定反应气体条件下, 利用IPIB产生的烧蚀等离子体可以制备纳米粉末[10,11]. 在束流辐照过程中, 烧蚀产物可能与束流发生相互作用从而对束流的能量沉积产生影响. 尤其是束流中的离子可能被烧蚀产物阻止, 使其能量耗散在烧蚀产物中而不能充分沉积在靶上, 即烧蚀产物可以对离子在靶上的能量沉积产生屏蔽. 这对较高能量密度下束流的诊断和在烧蚀条件下束流辐照效应的分析, 都会产生影响. 以往的研究对IPIB烧蚀效应的探索主要集中在对烧蚀质量损失和烧蚀产物的研究上[12-14], 在这些研究中, 辐照材料主要为金属, 烧蚀效应相对较弱, 烧蚀产物密度较低, 对束流的屏蔽效应并不明显. 而在采用IPIB对高分子材料进行的改性研究中, 由于靶材导热率和分解温度均较低, 在较弱的束流辐照下靶材表面即可能产生烧蚀, 并和束流相互作用对其能量沉积产生影响. 故在有较为稠密烧蚀产物的情况下, 研究辐照过程中烧蚀产物和束的相互作用对于认识辐照参数、理解辐照机制具有重要的意义. 本文选用具有较低的热导率以及较低的分解温度的高分子材料聚碳酸酯(polycarbonate, PC)、聚氯乙烯(Polyvinyl chloride, PVC)、聚甲基丙烯酸甲酯(polymethyl methacrylate, PMMA)作为靶材, 选用304不锈钢作为对比材料研究了较为稠密的烧蚀产物对IPIB在靶上能量沉积的影响. 为了对于烧蚀的程度进行预测, 采用蒙特卡罗和有限元方法对辐照产生的温度场分布进行了计算, 并结合实验数据进行了分析. 2.实 验IPIB辐照实验在北京航空航天大学物理学院BIPPAB-450强脉冲粒子加速器上进行. 该加速器加速电压最高达450 kV, 最大束流密度150 A/cm2, 束流横截面能量密度达1.8 J/cm2, 脉冲长度(半高宽)约80 ns. IPIB通过磁绝缘二极管产生, 离子成分为70%的H+和30%的Cn+, 为了提高束流密度, 采用15 cm的圆锥型铜束流器辅助束流聚焦. 烧蚀实验选用尺寸分别为150 mm × 150 mm × 0.125 mm, 150 mm × 150 mm × 0.15 mm, 150 mm × 150 mm × 0.25 mm和150 mm × 150 mm × 0.1 mm的PC, PVC, PMMA和304不锈钢作为靶材. 靶材垂直于束流的传输方向放置于聚束铜罩的出口. IPIB在靶材上形成的热斑通过机械臂控制的FLUKE Ti25红外相机在脉冲发射后0.1 s内获得, 并由此计算束流沉积于靶上的横截面能量密度分布[15].
4.结果与讨论离子束二极管采用几何聚焦, 同时采用锥形聚束器对束流横向分布进行约束以提高束流密度[17]. 图1为IPIB辐照前后304不锈钢和高分子材料背面温度分布图. 束流在不锈钢上产生的热斑近似圆形, 而且束流辐照区域和未辐照区域存在较为清晰的边界, 如图1(b)所示. 根据不锈钢上的热斑, 束流中心区域能量密度最高, 但对于高分子材料, 如图1(c)—图1(e)所示, 束斑中心区域存在较为明显的低温区域, 即在束流能量较高的位置, 沉积的能量密度反而较低. 图 1 IPIB辐照前后靶背面温度分布图 (a) 辐照前304不锈钢; (b) 辐照后304不锈钢; (c) 辐照后PC; (d) 辐照后PVC; (e) 辐照后PMMA Figure1. Distribution of temperature on rear face before and after IPIB irradiation: (a) 304 stainless steel, before irradiation; (b) 304 stainless steel, after irradiation; (c) PC, after irradiation; (d) PVC, after irradiation; (e) PMMA, after irradiation.
图2为IPIB在304不锈钢和高分子材料上束流中心部位横截面能量密度分布. 根据温度变化的范围可知, IPIB在不锈钢和高分子材料上产生的能量沉积区域是近似的, 但是当能量密度超过一定阈值之后, 在高分子材料上, 沉积能量密度会产生明显的下降, 且随着束流能量密度的提高, 沉积能量的下降更为显著. 如图2(b)所示, 当能量密度大于0.28 J/cm2时, IPIB沉积在PC上的能量密度开始明显降低, 直到束流焦点位置, 能量沉积降低至0.16 J/cm2. 对于PVC和PMMC, 束流密度在不超过0.2 J/cm2时会引发类似效应, 如图2(c)—图2(d)所示. 图 2 沿x方向能量密度分布图 (a) 304不锈钢; (b) PC; (c) PVC; (d) PMMA Figure2. Distribution of energy density along x direction: (a) 304 stainless steel; (b) PC; (c) PVC; (d) PMMA.
为了更准确地分析出现这种现象的原因, 使用有限元方法对IPIB辐照材料产生的热场分布进行分析. 图3为IPIB辐照304不锈钢和PC产生的功率密度, 由于离子射程的差异, 能量密度为1 J/cm2的IPIB辐照与304不锈钢和PC表面产生的最大功率密度分别为1.7 × 1017和6.6 × 1016 W/m3. 以现有参数对PC进行热场模拟, 如图4(a)所示, 在能量密度为1 J/cm2的IPIB辐照下, 其表面数微米深度内在脉冲前期约100 ns时已达到热解温度(约580 K)[18-20], 这意味着在束流能量沉积的初期, 材料表面区域会发生剧烈的烧蚀, 表面数微米深度范围内都会由于烧蚀而脱离材料表面, 使得烧蚀产物足够稠密, 可以屏蔽束流中后续的离子, 从而对束流的能量沉积产生显著影响. 对于更高能量密度的IPIB, 材料发生烧蚀的时间更早, 程度也更强烈, 产生的对能量的屏蔽效应也更明显, 因此在能量密度更高的辐照中心区域沉积在材料上的能量反而更少, 如图2(b)—图2(d)所示. 由于PC的热导率很低, 束流辐照会产生急遽的温度上升, 且在表面达到最高温度之后, 温度降低的速率很小, 会有利于烧蚀的产生. 对于该能量密度的IPIB产生的温度场, 如图4(b)所示, 在304不锈钢上最高的温度约为1800 K, 由于该温度显著低于沸点, 可以认为没有剧烈的烧蚀产生, 不会发生能量屏蔽现象, 如图2(a)所示. 由于304不锈钢的热导率较高, 在达到温度峰值之后, 热场向靶内部扩展, 并使近表面区域温度降低的速率显著高于PC, 进一步降低了烧蚀发生的可能性. 图 3 能量密度为1 J/cm2的IPIB产生的功率密度 (a) 304不锈钢; (b) PC Figure3. IPIB power density distribution with cross-sectional energy density 1 J/cm2 in (a) 304 stainless steel; (b) PC.
图 4 1 J/cm2的IPIB作用下的热场变化 (a) 304不锈钢; (b) PC Figure4. Thermal field distribution after irradiation of IPIB with cross-sectional energy density of 1 J/cm2 in (a) 304 stainless steel; (b) PC
图 1 IPIB辐照前后靶背面温度分布图 (a) 辐照前304不锈钢; (b) 辐照后304不锈钢; (c) 辐照后PC; (d) 辐照后PVC; (e) 辐照后PMMA
图 2 沿x方向能量密度分布图 (a) 304不锈钢; (b) PC; (c) PVC; (d) PMMA
图 3 能量密度为1 J/cm2的IPIB产生的功率密度 (a) 304不锈钢; (b) PC
图 4 1 J/cm2的IPIB作用下的热场变化 (a) 304不锈钢; (b) PC