阳极杆箍缩二极管（Rod-pinch二极管）是一种电子束二极管，工作于MV级电压下，产生几十至上百kA的强流相对论电子束，通过与靶材料的轫致辐射作用形成高剂量率、小焦斑的脉冲X射线源，用于闪光照相等领域。本论文主要研究 Rod-pinch二极管强流脉冲电子束的箍缩特性。对Rod-pinch二极管的工作过程进行数值模拟，获得不同参数对电子束聚焦效果的影响规律，并且利用新建成的2MV IVA驱动源对Rod-pinch二极管进行初步的实验研究。利用PIC(Particle-in-Cell)程序模拟Rod-pinch二极管的工作过程和束流动力学行为，研究阳极等离子体的近似模拟方法，获得与三阶段模型相对应的模拟图像。在此基础上，研究电子束发射位置、离子流密度和阴极盘厚度等参数对箍缩特性的影响。得到模拟结果：只存在电子流的情况下，Rod-pinch二极管无法在MV级电压下实现电子束箍缩；阳极离子流对电子束聚焦效果的影响存在阈值，离子流达到阈值以上进一步增加，对二极管总电流和阳极杆尖端电流密度基本没有影响，降低阳极离子流有助于提高二极管X射线产生效率；阴极电子发射位置影响到电子束的箍缩特性，从后表面出射的电子束箍缩质量最好，下表面次之，前表面出射的电子束箍缩质量差；增大阴极厚度不能缩短电子束实现箍缩的时间，但是可以在一定程度上提高箍缩质量。根据数值模拟结果设计Rod-pinch二极管及测量装置，在新建成的2MV IVA驱动源上开展Rod-pinch二极管实验。二极管工作电压峰值1.95MV，电流峰值52.7kA，电压和电流波形在时间上基本吻合，二极管工作阻抗37Ω，形成的X射线源正面焦点直径0.95mm，正对针尖1m处X射线剂量2.75rads。实验结果表明，二极管及测量诊断装置符合设计要求。结合理论分析与数值模拟结果，对Rod-pinch二极管中阴阳极等离子体引起短路的物理机制进行了讨论，表明：在磁绝缘强箍缩阶段，阳极表面电子和阴极表面的离子被束流自身角向磁场约束在电极表面附近很薄的鞘层内，二极管中形成稳定的双向流，等离子体的运动不会导致阴阳极间隙的闭合；二极管工作电压和电流降低时，束流自身角向磁场不足以约束电极表面的等离子体，阴阳极等离子体相向运动，二极管间隙闭合。对比电阻分压器型负载与Rod-pinch二极管的实验结果，检验上述结论。
The rod-pinch diode is an electron beam diode, operating at the voltage of MV class, which can produce electron currents of several tens to hundreds of kA. A high dose rate, small spot size, pulsed X-ray source which is mainly used for radiography, can be formed with the bremsstrahlung effect of the electrons produced and the high-Z target. This paper is mainly focusing on the investigation of the electron pinching characteristics of the diode. The operating process is simulated numerically, from which the influence of the structural parameters on the electron pinching quality is obtained. Preliminary rod-pinch diode experiments are performed on the 2MV IVA driver.The PIC (Particle-in-Cell) code is used to simulate the beam’s kinetic in a rod-pinch diode. The approximate simulation methods of the anode plasma are investigated, with which the simulation pictures of the three regimes are obtained. The influence of electron emission sites, the ion current, and the cathode thickness on the electron pinching characteristics are investigated. The simulation results indicate that the electrons can’t pinch to the rod tip at the voltage of MV class without the consideration of ion currents. When the ion current density emitted from the rod surface reaches a certain value, the threshold, further increase of ion current will not influence the total current and the current density at the rod tip of the rod-pinch diode. Therefore, reducing the ion current is beneficial to increasing the X-ray production rate. The electron emission sites influence the electron pinching characteristics. The electrons emitted from the upstream surface have the best pinching quality, which those from the downstream surface pinch worst. The increase of the cathode thickness can’t reduce the time needed to obtain good pinching quality, but it can increase the pinching quality to some extent.The rod-pinch diode and relative diagnostic devices were designed according to results of numerical simulation results. Rod-pinch diode experiments were carried out using the newly-built 2MV IVA driver. The peak voltage was 1.95MV and the peak current was 52.7kA. The voltage and current waveforms matched well in the time interval. The operating impedance of the diode was calculated to be 37Ω, and the X-ray spot diameter was 0.95mm. The forward X-ray dose at 1m from the rod tip reached 2.75rads. It was shown that the rod-pinch diode and diagnostic systems could meet the design requirements for radiography. The short circuit mechanics of the motions of the anode and cathode plasmas are discussed with theoretical analysis and numerical simulations, which shows that in the magnetically limited phase, the electrons in the anode surface and the ions in the cathode surface are restricted by the beams’ self-magnetic field with the thin sheaths near the electrodes and a stable bipolar current is formed accross the anode-cathode gap. The motions of the plasmas can’t cause the short circuit phenomenon. When the operating voltage and current decrease, the self-magnetic field is not large enough to limit the plasmas’ drift in the electrode surfaces. The ions and electrons drift in the opposite directions and short circuit the anode-cathode gap. The above conclusions are verified with the experimental results of the resistive load and the rod-pinch diode.
