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Study of High-gradient Structures for the Linear Collider

直线对撞机高梯度结构研究

基本信息

批准号：
03044141
负责人：
TAKATA Koji
金额：
$ 5.76万
依托单位：
National Laboratory for High Energy Physics (KEK)
依托单位国家：
日本
项目类别：
Grant-in-Aid for international Scientific Research
财政年份：
1991
资助国家：
日本
起止时间：
1991 至 1992
项目状态：
已结题

来源：
https://kaken.nii.ac.jp/grant/KAKENHI-PROJECT-03044141/
关键词：
linear collider linac klystron high gradient acceleration high electric field dark current oxygen free copper precision machining リニアコライダ-加速管高電界加速無酸素銅ろう付電界放出電子

项目摘要

Experimental and theoretical studies have been carried out for accelerating structures of the main linac of linear colliders. The structure is of a traveling wave type driven with 11.4GHz X-band RF power. The accelerating gradient is desired to be as high as 100MV/m. In this fiscal year, we fabricated a proto-type structure of oxygen free copper with a very accurate and fine surface finish. It was then high power tested at KEK by using a 30MW klystron which had been developed at KEK. We were able to reach the high gradient after a 500 hour conditioning. We also made detailed measurements of dark currents due to surface emission under the very high RF electric field.In order to achieve a high gradient, a high power klystron is absolutely necessary. However, the fabrication technology of klystrons becomes extremely difficult as the operation frequency raises. Therefore high gradient experiments were carried out at the S-band(2.86GHz), at which frequency the high power klystrons are easil … More y available. In fact at this frequency we achieved over 80MV/m average gradients successfully and also tested beam acceleration too. For the TeV energy linear collider, however, the RF frequency should be an X-band(11.4GHz) one at the lowest, since otherwise the total ac power becomes impractically high. KEK has been developing X-band high power klystron for a long time, and recently we succeeded to get a first tube which can deliver 30MW RF power. This experiment is carried out by using this tube.CERN has a different idea for the TeV linear collider. Particularly they are considering a K-band(30GHz) frequency generated by a FEL. Its realization is still far in the future. They are, on the other hand, developing technologies to fabricate accelerating structure which is capable of such high gradients. They are therefore wanting to test the structure as soon as possible even at lower frequencies. Hence a joint experiment started at KEK.In this experiment CERN and KEK made structures of the same dimensions. The main specification is : number of cell 20, coupler cell 2, cell length 2.9mm, phase shift per cell 2pi/3, constant impedance structure, beam aperture 6mm, cell diameter 10.4mm, disk thickness 1mm. The Q value is 6500, attenuation is 1.54/m, impedance is 104MOMEGA/m. The maximum surface field over the beam aperture is 3.9 times the average gradient. The most critical point in the fabrication was to achieve a sufficient smoothness on the beam aperture. It is accomplished by carefully choosing diamond bits. We also paid attention to avoid metallic and organic dusts falling inside the structure. They are usually a serious cause to limit the maximum attainable gradient by inducing heavy discharges.The test was carried out at the klystron repetition rate 50Hz. The RF pulse length was typically 100ns. The KEK structure reached 70MV/m after 60million pulses, While the CERN one did 100MV/m after 9 million pulses. In the CERN case, we also used a SLED system to boost the RF power.Observed dark currents were analized by use of a modified Fowler-Nordheim formula. The important measure therein is the field enhancement factor beta. For ther both structures, we obtained similar values around 50 for beta.The absolute value of the dark current was found to be rather high for the KEK structure and further studies are necessitated to resolve this. Less

对直线对撞机主直线加速器的加速结构进行了实验和理论研究。该结构是一个行波型驱动11.4GHz的X波段射频功率。加速梯度期望高达100 MV/m。在本财政年度，我们制造了一个具有非常精确和精细表面光洁度的无氧铜原型结构。然后在KEK使用KEK开发的30 MW速调管进行高功率测试。经过500小时的处理后，我们能够达到高梯度。我们还详细测量了在极高射频电场下表面发射产生的暗电流。为了获得高梯度，高功率速调管是绝对必要的。然而，随着工作频率的提高，速调管的制造技术变得非常困难。因此，在S波段（2.86GHz）进行了高梯度实验，在此频率下，高功率速调管的工作频率为2.86GHz。 ...更多信息 y可用。事实上，在这个频率下，我们成功地实现了超过80 MV/m的平均梯度，并且还测试了射束加速。然而，对于TeV能量的直线对撞机，RF频率最低应该是X波段（11.4GHz），因为否则总的交流功率会变得不切实际地高。KEK公司长期致力于X波段高功率速调管的研制，最近成功研制出了第一台可输出30 MW射频功率的速调管。CERN对TeV直线对撞机有不同的想法。特别是他们正在考虑由FEL产生的K波段（30 GHz）频率。实现这一目标还遥遥无期。另一方面，他们正在开发制造能够实现如此高梯度的加速结构的技术。因此，他们希望尽快测试结构，即使在较低的频率。因此，KEK开始了一项联合实验。在这项实验中，CERN和KEK制造了相同尺寸的结构。主要规格为：单元数量20，耦合器单元2，单元长度2.9mm，每个单元的相移为2 π/3，恒定阻抗结构，波束孔径6 mm，单元直径10.4mm，盘厚度1 mm。Q值为6500，衰减为1.54/m，阻抗为104 MOMEGA/m。束孔上的最大表面场是平均梯度的3.9倍。制造中最关键的一点是在光束孔径上实现足够的平滑度。这是通过仔细选择金刚石钻头来实现的。我们还注意避免金属和有机粉尘落入结构内。它们通常是一个严重的原因，限制了最大可达到的梯度诱导重放电。RF脉冲长度通常为100 ns。KEK结构在6000万脉冲后达到70 MV/m，而CERN结构在900万脉冲后达到100 MV/m。在欧洲核子研究中心的实验中，我们也使用SLED系统来提升射频功率，并利用修正的Fowler-Nordheim公式来分析暗电流。其中重要的量度是场增强因子β。对于这两种结构，我们得到了类似的值约为50 β。暗电流的绝对值被发现是相当高的KEK结构和进一步的研究是必要的，以解决这个问题。少