摘要
本论文首先介绍了国际上高能直线加速器的发展状况和一些先进加速器的概念,着重介绍了基于介质的加速结构。高加速梯度是未来直线加速器的方向之一。介质一金属膜片混合加载行波加速结构可使金属表面最大峰值电场与轴向加速电场之比E_s/E_a,,降为1左右(而盘荷波导为E_s/E_a＞2),有望提高加速梯度,相比较介质加载圆波导加速结构具有更高的Q值和分路阻抗.即该种新加速结构同时兼顾介质加载的圆波导加速结构和盘荷波导的优点。
本论文使用MAFIA软件,MATLAB,微波工作室和PAMELA软件,对X波段介质一金属膜片混合加载加速器进行了物理设计.假定功率源的功率为20MW,其最大加速梯度可达45 MV/m,长度1.47米,结构紧凑。通过调节加速腔的尺寸和聚束腔的相速度,得到其捕获效率为40%,能量为32MeV,能散为0.5%,加速管出口处束流的流强为100mA。
选择合适的陶瓷是研制介质.金属膜片混合加载加速结构的首要任务.其中介电常数的大小和介质损耗是首先要考虑的问题。本论文用微扰法研究了最新进口的陶瓷(真空性能比较好)的介电常数的色散特性。实验表明:该陶瓷的介电常数的大小在9.37GHz附近变化不大,介电常数ε_r=6.29,损耗正切tanδ_ε=0.000195,且进口陶瓷一致性好。为了研究进口陶瓷加载的混合盘荷波导结构在焊接前后性能的变化,我们对高温退火前后的陶瓷进行了X射线荧光光谱测试、XPS测试、电阻值测量和介电常数的测量。测试结果表明,陶瓷退火后性能基本稳定。
由于进口陶瓷硬度比较大,且比较脆,用普通的机床难以加工。为此我们探索了多种加工方法,摸索出一种较为合适的加工工艺,提高了加工精度。利用MAFIA软件模拟计算优化了加速结构的射频参数和结构尺寸,并进行了公差分析,为模型腔的研制提供了设计尺寸和公差要求。经过模拟计算和实验研究相结合的方法,研制了低损耗陶瓷加载的模型腔.研制后模型腔的工作频率精确到9.37GHz附近,Q值的测量值达到1148.6。
在行波直线加速器中,耦合器是一个非常重要的器件。开始,我们用微波丁作室时域求解器进行了大量的模拟计算,对国产陶瓷加载的X波段介质-金属膜片混合加载加速结构的耦合结构进行了设计,经过大量的实验研究,得到满足条件的耦合结构。但上述计算方法非常耗时且精度不够高,所以需要反复加工、实验。
经过大量的探索,获得了一种新的基于kyhl方法的思想,采用微波工作室本征模求解器和频域求解器快速、精确计算耦合器的计算方法。这种计算方法也适用于其他行波加速结构耦合器的设计。另外,通过大量计算,发现新结构与盘荷波导结构不同的是,使耦合腔失谐的短路活塞的端部位置不是位于耦合腔中间,而是位于靠近电子枪方向偏移中心位置3mm左右。
依照前面的计算结果对耦合器进行了实验研究,得到驻波比在9.37GHz时小于1.02,在20MHz的范围内驻波比小于1.10,实验结果与模拟计算的结果吻合得较好,进一步验证了上述计算方法的可靠性。另一方面,我们从本研究中摸索出了陶瓷加载盘荷波导加速结构焊接的基本工艺.本文还用拉珠法对焊接后的新加速结构的场分布和相位进行了初步研究。
对称的耦合结构相比单边耦合结构,可以抑制寄生的偶极模,对束流的干扰减小到最小,增加束流崩溃的阈值。本文对介质加载圆波导加速结构尤其耦合器进行了理论研究,设计了该种结构的同轴双耦合结构,为进一步工作提供了理论依据。
关于同轴负载所用Kanthal合金材料的高频磁导率的测量,公开发表的文章所采用的方法不能直接用于这种材料磁导率的测量。本文基于同轴谐振腔法,发展了这种材料的高频磁导率和电导率新的测量方法,得到工作频率下的μ_r=2.58,其喷涂工艺下的电导率σ=8978.27mhos/m,,并根据测量结果设计了同轴负载,为大功率同轴负载的研制提供了参考。本文提出的金属高频磁导率的测量方法适合不同波段的频率。
本文的研究为进一步研制X波段介质-金属膜片混合加载加速器奠定了理论和实验基础。
The development of the linac in the world and some advanced accelerator concepts are described in the thesis.The accelerating structure based on dielectric is emphasized here.The hybrid dielectric-iris-loaded structure may have lower ratio of peak surface electric field at the iris to axial accelerating electric field than the one of the iris-loaded waveguide structure by optimizing the geometric parameters,while r/Q of the new structure being comparable to iris-loaded accelerating structure.
With the help of MAFIA code,MATLAB code,Microwave Studio and PAMELA code,a small and high accelerating gradient x-band hybrid dielectric and iris loaded travelling wave electron linac was theoretically studied.The length of accelerator tube is 1.47m.The maximum accelerating gradient is 45MV/m.The beam energy is 32MeV.The capture efficiency is about 40%,the energy spread is about 0.5%and the beam emittance is about 5.7πmm·mrad under the conditions of the beam current being 100mA and the electron gun voltage being 50KV.
The choice of appropriate dielectric is crucial for the design of the new accelerating structure.The dielectric constant(εr) and dielectric loss(ranδe) of the composition Forsterite(Mg,Silicate) made in USA are measured using cavity perturbation method.The results show that:the values of permittivity vary slowly when the frequencies are near to 9.37GHz,the dielectric constant is 6.29 and the dielectric loss is 0.000195,the consistency of the dielectric is good.The configuration is tested by XFS,XPS and permittivity of the dielectric is measured using cavity perturbation method after annealing.The results show that the performance of the dielectric is steady on the whole.
Because of high hardness and brittleness of the dielectric,a lot of machining technology and methods are attempted.Through large numbers of experiments, some model cavities have been developed successfully at the frequency of 9.37GHz and the Q value of 1148.6 is obtained after brazing in a hydrogen furnace.
The RF coupler is an important component for an accelerator structure.Originally, using the dielectric made in China whose dielectric constant is 5.81,the coupler was simulated with the help of the Microwave Studio(MWS) T-Domain Solver.It took one much more time to simulate.The experimental results show that the actual value of the coupler aperture is very different from the theoretical value.
A novel calculation method based on the Kyhl method is obtained with F-domain solver of MWS for the design of the coupler.The calculation method can save much more time under the same precision than other methods.In addition,it is interesting that large numbers of calculations for the new structure show that the position of the plunger should be apart from the coupler cavity centre(3mm forward the electric gun) for detuning.To iris-loaded waveguide structure,the position is in the centre of the cavity.
According to above calculated results,the coupler was developed.The experimental results show that the SWR＜1.02 at the operation frequency(f=9.37GHz) and SWR＜1.10 within the bandwidth of 20 MHz.The experiment results are accorded with the calculating results after few processes.
On the other hand,the brazing technics of the dielectric loaded and iris-loaded waveguide structure are attempted firstly.The electric field in the axis and the phase shift of the accelerating mode are also investigated.
In order to eliminate parasitic dipole modes and to minimize beam perturbations that could lead to beam breakup effects in the accelerator,the dielectric loaded circle waveguide accelerating structure(especially the coupler) is investigated theoretically. The coaxial coupling approach employs a double-input design to symmetrize the coupling regions.
At last we developed a new method using coaxial resonant cavity to determine the high frequency permeability of the Kanthal alloy available for collinear load.Many measurement methods reported are not fit for the alloy.The relative permeability of 2.58 is obtained by this method and the conductivity of sprayed material ofσ= 8978.27mhos/m is got.Based on the results of the permeability and conductivity,the computer simulation for the design of the collinear load was made by Microwave Studio.
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