Magnesium diboride superconducting RF resonant cavities for high energy particle acceleration
Magnesium diboride superconducting RF resonant cavities for high energy particle acceleration
复制标题
用于高能粒子加速的二硼化镁超导射频谐振腔
DOI：
10.1088/0953-2048/17/9/026
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发表时间：
2004
影响因子：
3.6
通讯作者：
T. Tajima
中科院分区：
文献类型：
--
作者：
E. Collings;M. Sumption;T. Tajima
支持任何特定超导涂层的论点必须以其基本的热力学性质为依据。超导转变温度T_c决定了表面电阻，从而决定了腔的Q。这必须保持足够高，以使系统能够以所需的场梯度和频率驱动，而不会导致过大的功率损失。在这一点上，MgB_2的39K T_c是有利的。其预期最大加速磁场EaccMAX为77 mV？m1，BCS表面电阻RsBCS(4K，500 MHz)为2.5N？如后面所讨论的，作为SRF腔的涂层，MgB2代表了一种有趣的可能性。此外，MgB_2的HC2比Nb的高，导致估计的俘获通量敏感性略低。最近在洛斯阿拉莫斯国家实验室(LANL)对一种MgB_2薄膜的测量表明，在4K时，其射频表面电阻低于Nb的表面电阻，这是对MgB_2吸引力的原则证据。我们的计算是基于500 MHz频率下4K的保守计算。然而，当T_c为39K时，涂有镁B_2涂层的腔体应该比低T_c的腔体更不容易发生热击穿。用于GHz频率和电压梯度的超导材料>；40 mV？m？1，最近引用的一个目标，将要求低Rs(高Tc)和高HSH值。在T_c为39K的情况下，如果薄膜制备良好且没有缺陷和颗粒，则MgB_2明显具有降低RsBCS的潜力。此外，当MgB_2的HCl相对较低时，过热临界场HSH高于Nb。目前，关于HCl和HSH在确定EACC限度中的确切作用还存在一些争议。然而，较高的氢氧化镁的HSH值确实暗示了增强EACC值的可能性。HCl和HSH的确切作用还有待进一步研究。存在可使腔状结构内部涂覆一层硼化镁薄膜的技术。
Arguments in support of any particular superconducting coating must be framed in terms of its fundamental thermodynamic properties. The superconducting transition temperature, Tc, determines the surface resistance, and thus the Q of the cavity. This must remain sufficiently high that the system can be driven at the required field gradients and frequencies without leading to excessive power loss. In this regard the 39?K Tc of MgB2 is advantageous. With an anticipated maximum accelerating field, EaccMAX, of 77?MV?m?1 and a BCS surface resistance, RsBCS (4?K, 500?MHz), of 2.5?n ? as discussed later, MgB2 represents an interesting possibility as a coating for SRF cavities. In addition, the higher Hc2 of MgB2 than Nb results in a slightly lower estimated trapped flux sensitivity. Recent measurements of an MgB2 film at the Los Alamos National Laboratory (LANL) have shown an RF surface resistance lower than that of Nb at 4?K, which is proof-of-principle evidence of the attractiveness of MgB2. Our calculations are based conservatively on 4?K operation at 500?MHz. However, with a Tc of 39?K, MgB2-coated cavities should be less susceptible to thermal breakdown than low-Tc ones. Superconducting materials for use at GHz frequencies at voltage gradients >40?MV?m?1, a recently cited target, will require both low Rs (high Tc) and high Hsh values. With a Tc of 39?K, MgB2 clearly has the potential to reduce RsBCS if the films are well prepared and free from defects and particles. Additionally, while the Hc1 for MgB2 is relatively low, the superheating critical field, Hsh, is higher than that of Nb. Currently, there is some debate about the exact roles of Hc1 and Hsh in the determination of Eacc limits. However, the higher values of Hsh for MgB2 do suggest the possibility of enhanced Eacc values. The exact roles of Hc1 and Hsh should be further investigated. Techniques exist that may enable cavity-like structures to be internally coated with a MgB2 film.