超导高频腔的频率调谐与ATLAS能量升级射频系统
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"FREQUENCY TUNING AND RF SYSTEMS FOR THE ATLAS ENERGY UPGRADE CAVITIES - 国际著名实验室经验总结，主要涉及超导高频腔的频率调谐技术及其在ATLAS加速器能量升级项目中的应用。"
本文讨论了在ATLAS加速器能量升级项目中所设计和构建的新式低温超导射频（SRF）四分之一波铌腔的频率调谐方法以及相关的射频系统开发。这个项目的技术是下一代超导重离子加速器的基础。文章深入探讨了如何将腔体的固有频率调整到与加速器主振荡器频率匹配，并详述了用于驱动腔体并在运行时保持相位锁定的射频系统的开发过程。
1. **频率调谐**：
频率调谐是超导射频腔的核心环节，目的是确保腔体的工作频率与加速器的需求精确匹配。在ATLAS的升级项目中，七个109MHz、β=0.15的超导四分之一波腔被用来增加现有超导直线加速器的电压15MV。腔体的构造允许其频率进行精细调节，以适应不同的加速需求。
2. **超导射频腔**：
超导射频腔利用超导材料的特性实现极低的损耗，从而提高粒子加速的效率。β值是腔体几何参数，反映了粒子在腔体内运动与电磁波传播速度的关系。在本案例中，β=0.15表示粒子速度大约是光速的15%。
3. **新式低温超导模块**：
新的低温超导模块包含七个超导腔，这是对现有加速器的一个重大提升。低温环境是保持超导材料零电阻状态的关键，它能保证腔体高效工作，降低能量损失。
4. **射频系统**：
开发的射频系统不仅要驱动腔体工作，还要在腔体运行时保持其相位锁定。这要求系统能够实时监测并控制腔体频率，确保粒子束的稳定加速。此外，射频系统的设计需要考虑到系统的稳定性、可靠性和对复杂操作环境的适应性。
5. **技术开发与应用**：
该项目的技术创新不仅服务于ATLAS，也为未来超导加速器的设计提供了借鉴。通过这个项目，科研人员积累了丰富的经验，进一步推动了超导技术在粒子物理实验和加速器领域的应用。
6. **腔体调谐方法**：
调谐方法可能包括改变腔体的几何尺寸、使用可调谐电容或改变腔体内部的磁场分布等。这些方法的实施需要精密的机械调整和精确的控制系统。
7. **相位锁定**：
相位锁定是保证加速器性能的关键，通过反馈控制系统来维持腔体与主振荡器之间的相位关系，确保粒子在每次通过腔体时都能得到正确的能量提升。
"FREQUENCY TUNING AND RF SYSTEMS FOR THE ATLAS ENERGY UPGRADE CAVITIES"这一主题涵盖了超导技术在粒子加速器中的应用，特别是针对腔体频率调谐和射频系统设计的先进方法，这对于理解高能物理实验和加速器技术的发展具有重要意义。
FREQUENCY TUNING AND RF SYSTEMS FOR THE ATLAS ENERGY
UPGRADE SC CAVITIES*
G. P. Zinkann, J. D. Fuerst, S.M. Gerbick, M.P. Kelly, M. Kedzie, S. Macdonald, P.N. Ostroumov,
R.C. Pardo, S. Sharamentov; K.W. Shepard, Techsource, Inc.
Z. Conway, Presently at Cornell University
Abstract
A new cryomodule with seven low-beta
superconducting radio frequency (SRF) quarter wave
niobium cavities has been designed and constructed as an
energy upgrade project for the ATLAS accelerator at
Argonne National Laboratory [1]. The technology
developed for this project is the basis for the next
generation superconducting heavy ion accelerators. This
paper will discuss the methods employed to tune the
cavities eigenfrequency to match the accelerator master
oscillator frequency and the development of the RF
systems used to both drive the cavity and keep the cavity
phase locked during operation.
INTRODUCTION
The ATLAS Energy Upgrade Project includes a new
cryomodule containing seven 109 MHz, β=0.15c quarter-
wave superconducting cavities to provide an additional 15
MV voltage to the existing superconducting linac. The
cavities are constructed so that there are no demountable
joints. Due to the fact that all of the manufactured
sections of the cavity are electron beam welded into the
final cavity configuration, the dimensions of the parts
prior to welding must be properly sized in order to
achieve the final eigenfrequency demanded by the
accelerator’s Master Oscillator system. Also, in order to
phase lock to the accelerator’s Master Oscillator, two
other systems are employed; a slow tuner system [2] is
used to compensate for frequency shifts due to pressure
changes, and a fast tuner system [3] is used to compensate
for frequency shifts due to microphonics.
CAVITY TUNING DURING
CONSTRUCTION
The individual cavity parts are formed and then
electron beam (e-beam) welded into four sections. These
four sections are illustrated in Figure 1. They are the
housing, the toroid, the center conductor and the dome.
All four sections are manufactured with their length
longer than needed. They will all be trimmed to length in
increments so that in the end, the proper frequency will be
achieved. The first step is to trim the dome and the toroid
once, using Electrical Discharge Machining (EDM). This
trim cut is to establish a square edge that will facilitate
alignment to the housing and center conductor.
Toroid
Housing
Center conductor
Dome
Figure 1: Four main cavity sections.
Several parameters must be known to determine what
length the housing and center conductor will be trimmed
to. For example, when the individual pieces are e-beam
welded together, shrinkage of the niobium occurs that
results in a frequency shift. This frequency shift must be
accounted for in the sizing of the cavity parts. In
addition, when the parts are electro-polished [4] the
removal of the niobium results in a frequency shift which
also must be accounted for. Table 1 lists the desired
frequency at 4.5K and other parameters that must be
known to achieve the final sizing results.
Table 1: Development Parameters
Master Oscillator Frequency
(for B =.15c)
109125.0 kHz
Slow Tuner Half-range +20 kHz
1-shot hydraulic tuning +/-20 kHz
ΔFreq / ΔElectro-Polish (uniform)
282.2 kHz/mm
ΔFreq / ΔEP(Dome & Toroid)
1078.9 kHz/mm
ΔFreq / ΔPressure (Helium Jacket)
-9 kHz/atm
ΔFreq / ΔAIR
(20C, 740 Torr, 40% Humid)
-34 kHz
ΔFreq / ΔT(293K - 4K)
-156 kHz
ΔFreq / ΔIndium wire .010" thick
-26 kHz
ΔFreq / ΔLength(center conductor)
-132 kHz/mm
ΔFreq / ΔLength(distance to dome)
30 kHz/mm
The four sections are assembled in a fixture which
keeps them alligned and clamped together. Induim wire is
compressed between each clamped joint of this assembly.
______________
*
This work was supported by the U.S. Department of Energy, Office of
Nuclear Physics
, under Contract No. DE-AC02-06CH11357.
#zinkann@anl.gov
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