High Gradient Accelerating Structure

The following sections are included:

• Preface
• Contents

The following sections are included:

• Introduction
• Juwen Wang's Early History
• Juwen Wang's Ph.D. Thesis
• Juwen Wang's Work since his Ph.D. in 1989
• Acknowledgment
• References

The design of high-gradient room-temperature RF linear accelerators requires that special attention to be paid on several phenomena which limit their performance: electron field emission, dark current, RF breakdown, pulse heating, and cavity damage.

In this talk, the motivation, experimental and theoretical studies, and adventures in past more than 30 years at SLAC are introduced and reviewed. As most critical issues, these basic studies have greatly benefited the successful development for 100 MV/m high gradient accelerator structures applied in multi TeV linear colliders. Some important design practices and results as well as future work are also presented.

Advances in the fundamental understanding of vacuum breakdown achieved during the program to develop high-gradient accelerating structures for a TeV-range linear collider project are described.

X-band accelerator structure development at KEK has been driven targeting the linear colliders in worldwide collaborations. It is based on the technologies developed with high-precision machining, precise assembly and bonding method to preserve the precision. With maximally utilizing the merits of such technologies, the long-range wakefield was suppressed in parallel to realize the high gradient. The latter needs more study and development to actually realize the stable operation at a gradient of 100 MV/m or higher in the view point of the present paper. The worldwide collaboration studies are extensively on-going and the understanding of the vacuum breakdown has been advancing. By describing the development at KEK toward the X-band wakefield suppressed high-gradient accelerator structure, this paper shows how such structures have been evolved and may serve to show a room for the future studies.

The conceptual design of a compact, photoinjector-based, facility for high precision measurements of wakefields is presented. This work is motivated by the need for a thorough understanding of beam induced wakefield effects for any future linear collider. We propose to use a high brightness photoinjector to generate (approximately) a 2 nC, 2 mm-mrad drive beam at 20 MeV to excite wakefields and a second photoinjector to generate a 5 MeV, variably delayed, trailing witness beam to probe both the longitudinal and transverse wakefields in the structure under test. Initial estimates show that we can detect a minimum measurable dipole transverse wake function of 0.1 V/pC/m/mm and a minimum measurable monopole longitudinal wake function of 2.5 V/pC/m. Simulations results for the high brightness photoinjector, calculations of the facility's wakefield measurement resolution, and the facility layout are presented.

Under the support of the U.S. DOE SciDAC program, SLAC has been developing a suite of 3D parallel finite-element codes aimed at high-accuracy, high-fidelity electromagnetic and beam physics simulations for the design and optimization of next-generation particle accelerators. Running on the latest supercomputers, these codes have made great strides in advancing the state of the art in applied math and computer science at the petascale that enable the integrated modeling of electromagnetics, self-consistent Particle-In-Cell (PIC) particle dynamics as well as thermal, mechanical, and multi-physics effects. This paper will present the latest development and application of ACE3P to a wide range of accelerator projects.

The symposium held at Tsinghua University, in honor of Dr. Juwen Wang, is a testament to his engagement with diverse areas of linear accelerators. My work has overlapped with his during the period of the Next Linear Collider/Japanese linear Collider (NLC/JLC) programe in particular. Here I report on the beam-exited long-range wakefield in these linacs, and also on subsequent developments in the context of the Compact Linear Collider programme (CLIC). This self-excited wakefield, if left unchecked, can at the very least cause a marked dilution in the beam emittance, and in the worst case can cause a catastrophic beam break up (BBU) instability rendering the particle beam unusable. To ameliorate this affect, there are two main strategies: one can either aim at heavily damping the wakefield (in practise requiring a Q value as low as ~10) or one can detune each of the accelerator's cells to ensure that each mode is excited at slightly different frequency, in a precise manner. The former approach entails placing damping waveguides and damping materials in relatively close proximity to the beam, and this is the approach adopted by the CLIC collaboration. For the NLC/JLC we collectively followed the latter approach, in which the dipole mode of each cell is detuned with an erf function profile along each accelerator structure. Eventually the modes, which form the wakefield, recohere and, to ensure the wakefield remains below a specified level a portion is coupled out through slots cut into each cell to an attached waveguide-like manifold. Typically medium Q values are aimed at, between 500 and 1000. This scheme entails suppressing the modes which comprise the wakefield–using damped and detuned structures (DDS). Sampling an attenuated portion of this manifold radiation also provides both a beam and structure diagnostic. A similar DDS design, but with more stringent wakefield suppression requirements, has been followed as an alternative to the CLIC damping scheme. Wakefield suppression for the NLC/JLC and CLIC is reported on, with a focus on the DDS approach.

When the beam is passing through an accelerator, it will generate higher order modes (HOM), which will affect to the beam quality especially in high energy accelerators such as International Linear accelerator Collider (ILC). In order to preserve the beam quality, HOM couplers are required to be installed to extract HOM power. Most of HOM couplers are 3D complex structures including small features. In addition, many physics process are involved in HOM coupler design such as RF heating and multipacting. Numerical modeling and simulation are essential for HOM coupler design and optimization for successful operation of high energy accelerators. SLAC developed 3D finite element parallel electromagnetics code suite ACE3P can be used to accelerator modeling with higher accuracy in fast turnaround time. In this paper, ACE3P application for HOM damping simulation and multipating analysis is presented for ILC 3.9 GHz crab cavity.

R&D of a C-band (5712 MHz) high gradient traveling-wave accelerating structure has been in progress at Shanghai Institute of Applied Physics (SINAP). The prototype and new C-band accelerating structures have already been fabricated. In this paper, some details of R&D are introduced.

A Global Effort — The effort to develop high gradient room temperature accelerator structures for the Next Linear Collider or the Global Linear Collider was a global effort involving many people from a number of laboratories around the world including SLAC, FNAL and LLNL in the US, KEK in Japan and CERN in Europe. Critical ideas came from many sources, so my attributions may not always be accurate, but indicate where I first heard the concept. Juwen Wang's role was very important in accepting, modifying where appropriate, and implementing ideas to produce a long series of structures finally leading to a successful structure.

The following section is included:

• Author Index