Chris Adolphsen

Influence of fabrication errors on wake function suppression in NC X-band accelerating structures for linear colliders

Wake function suppression is effected by ensuring that the mode frequencies of an X-band normal conducting (NC) accelerating structure of multiple cells are detuned and moderately damped by waveguide manifolds attached to the outer wall of the accelerator. We report on the dilution in the wake function suppression that occurs due to errors resulting from the fabrication process. After diffusion bonding 206 cells a non-uniform expansion in the cell geometry forces a substantial shift in the frequencies of select cells. We remap all circuit parameters to these shifted cell frequencies to predict the wake function. Experiments performed on the SLC at the SLAC National Accelerator Laboratory indicate that the wake function is well predicted by the circuit model.

STATUS OF A LINAC RF UNIT DEMONSTRATION FOR THE NLC/GLC X-BAND LINEAR COLLIDER*

Designs for a future TeV scale electron-positron Xband linear collider (NLC/GLC) require main linac units which produce and deliver 450 MW of rf power at 11.424 GHz to eight 60 cm accelerator structures. The design of this rf unit includes a SLED-II pulse compression system with a gain of approximately three at a compression ratio of four, followed by an over-moded transmission and distribution system. We have designed, constructed, and operated such a system as part of the 8-Pack project at SLAC. Four 50 MW X-band klystrons, running off a common 400 kV solid-state modulator, drive a dualmoded SLED-II pulse compression system. The compressed power is delivered to structures in the NLCTA beamline. Four 60 cm accelerator structures are currently installed and powered, with four additional structures and associated high power components available for installation late in 2004. We describe the layout of our system and the various high-power components which comprise it. We also present preliminary data on the processing and initial high-power operation of this system.

RF thermal and new cold part design studies on a TTF-III input coupler for Project-X

An RF power coupler is one of the key components in a superconducting (SC) linac. It provides RF power to the SC cavity and interconnects different temperature layers (1.8 K, 4.2 K, 70 K and 300 K). The TTF-III coupler is one of the most promising candidates for the High Energy (HE) linac of Project X, but it cannot meet the average power requirements because of the relatively high temperature rise on the warm inner conductor, so some design modifications will be required. In this paper, we describe our simulation studies on the copper coating thickness on the warm inner conductor with RRR values of 10 and 100. Our purpose is to rebalance the dynamic and static loads, and finally lower the temperature rise along the warm inner conductor. In addition, to get stronger coupling, better power handling and less multipacting probability, one new cold part design was proposed using a 60 mm coaxial line; the corresponding multipacting simulation studies have also been investigated.

Modified TTF3 Couplers for LCLS-II

The LCLS-II 4 GeV SCRF electron linac will use 280 TESLA-style cavities with TTF-3 power couplers that are modified for CW operation with input powers up to about 7 kW. The coupler modifications include shortening the antenna to achieve higher Qext and thickening the copper plating on the warm section inner conductor to lower the peak temperature. Another change is the use of a waveguide transition box that is machined out of a solid piece of aluminium, significantly reducing its cost and improving its fit to the warm coupler window section. This paper describes the changes, plating and surface issues, simulations and measurements of the coupler operation (heat loads and temperatures) and RF processing considerations. INTRODUCTION The LCLS-II project has adopted the TTF-3 coupler design (see Fig. 1) to power the 1.3 GHz TESLA-style cavities in its 4 GeV SCRF linac [1]. These couplers were designed for pulsed operation with several hundred kilowatts of input power at a ~ 1% duty factor, so the peak fields for 7 kW operation at LCLS-II will be much smaller. The low duty required the static heat load to be kept small, which was done by using only a thin (1030 m) layer of copper plating on the inner stainless steel surfaces. For LCLS-II CW operation, however, the inner conductor in the ‘warm’ section (between the windows) would overheat. Also the Qext range of the coupler is too low for the small LCLS-II beam currents (< 300 A). Thus modifications were made as described below. MODIFICATIONS Thicker Copper Plating Simulations show that with 7 kW fully reflected input power (worst case), the peak temperature of the warm inner conductor decreases from about 700 K to 400 K if its plating thickness is increased from 30 m to 150 m. This reduces the temperature below the 450 K level at which the couplers are baked so the vacuum levels should be manageable. LCLS-II adopted this thickness, which increases the 45 K total (static + dynamic) cryogenic load by 14 %. As a test, several ILC coupler warm sections were modified by removing their 30 m plating and replating to 150 m. The photo in Fig. 1 shows the achieved plating uniformity in the bellows region, which meets requirements. So far these couplers have worked well and vendors are producing new ones with this plating thickness. Shorter Antenna The TTF-3 couplers can move over a 15 mm range, which changes Qext by about 19 % per mm. LCLS-II will run with Qext = 4.1×10 (about 10 times higher than XFEL), which is above the nominal TTF-3 Qext range. To shift up the range, the flared antenna tip will be shortened by 8.5 mm, keeping the same flare angle and 3 mm radius edges. For the first two cryomodules, ILC cold sections have been modified by milling down the existing antenna tips using a fixture that prevents the couplers from being damaged. Aluminium WG Box and Flex Rings To lower cost and improve performance, the copper waveguide box that attaches to the warm window will be replaced by one machined from a single block of aluminium, without RF matching posts (see Fig. 2). Also the capacitor ring that allows HV holdoff will be replaced by a copper flex ring to provide a better RF seal between the waveguide and the coupler body. ___________________________________________ *Work supported by the Department of Energy, Office of Science, Office of Basic Energy Science, under Contract No. DE-AC0276SF00515 #star@slac.stanford.edu Figure 1: (top) TTF-3 coupler, (bottom) sectioned inner bellows after plating to 150 m thickness. Figure 2: Pair of aluminium waveguide boxes being used for coupler RF processing. THPB077 Proceedings of SRF2015, Whistler, BC, Canada ISBN 978-3-95450-178-6 1306 C op yr ig ht

EXPERIMENTAL STUDY OF SURFACE RF MAGNETIC FIELD ENHANCEMENT CAUSED BY CLOSELY SPACED PROTRUSIONS

The RF magnetic field enhancement between two closely spaced protrusions on a metallic surface has been studied theoretically. It is found that a large enhancement occurs when the field is perpendicular to the gap between the protrusions. This mechanism could help explain the melting that has been observed on cavity surfaces subjected to pulsed heating that would nominally be well below the melting temperature of the surface material. To test this possibility, an experiment was carried out in which a pair of copper “pins” was attached to the base plate of an X-band cavity normally used to study pulsed heating. Melting was observed between the pins when the predicted peak temperature was near or exceeded the copper melting temperature. INTRODUCTION We have theoretically investigated the effect of a pair of identical, very small diameter, parallel, cylindrical, metallic, surface protrusions on an applied RF magnetic field in which the flux lines thread between them. The pins concentrate the flux lines, enhancing the magnetic field on the inner surfaces [1]. This is to be distinguished from the magnetic enhancement from general surface features, for example, those associated with sharp edges in normal conducting structures [2] and surface bumps and pits in superconducting cavities [3-6], where the radius of curvature of the surface is the main factor in determining the enhancement, as opposed to the gap opening between above-surface structures. To verify our theoretical findings and explore the surface damage due to high pulsed heating temperatures (close to the melting point), we fabricated 4 pairs of short, cylindrical, copper pins and brazed them to a sample plate of a TE01 cavity used for pulsed-heating studies [7]. The electric field on the surface of the plate is nominally zero so only magnetic field effects are observed. The pins have a height of 2.5 mm and radius of 0.5 mm. They were all located at the radius of maximum pulsed heating on the sample plate. Different gap distances were used for the pairs, two that should produce a magnetic field enhancement of about 3.3 and two with about 2.3. This is illustrated in Fig. 1, where the solid line is the result of a simulation for the pin size, and the squares indicate the enhancement deduced from the measured pin spacing. The surface field around the pin pair is shown in the color maps of Fig. 2. The magnetic field is enhanced almost uniformly from the bottom of the pin to the start of its rounded tip. The electric field, which nominally increases linearly above the surface, is enhanced in small areas at the rounded tips. The ratio of maximum electric field to the maximum magnetic field is about 190 Ω with a magnetic field enhancement of 3.2. Figure 1: Peak surface magnetic field enhancement between the pin pairs with different gap distances.

Simulating a new low Q 6-cell X-band klystron output cavity

X-band klystron research and development has been underway at SLAC for over two decades culminating in the current workhorse X-band RF source, the XL-4. The XL-4 traveling wave output structure consists of 4-cells and will be extended to 6-cells to reduce breakdown and further improve upon the klystrons robustness. Simulations show the gradient can be reduced by roughly 25% and that the cavity is stable. The new output cavity will be cold tested and implemented on an existing XL-4 in the near future.

Stability review of SLAC's L-band sheet beam klystron

A review of L-band sheet beam development at SLAC National Accelerator Laboratorys Klystron Department. The measured current density profile of a 40:1 elliptical beam is presented. A review of instabilities discovered while simulating the klystron are presented.

Performance Limiting Effects in X‐Band Accelerators

Acceleration gradient is a critical parameter for the design of future TeV‐scale linear colliders. The major obstacle to higher gradient in room‐temperature accelerators is rf breakdown, which is still a very mysterious phenomenon that depends on the geometry and material of the accelerator as well as the input power and operating frequency. Pulsed heating has been associated with breakdown for many years however there have been no experiments that clearly separate field and heating effects on the breakdown rate. Recently, such experiments have been performed at SLAC with both standing‐wave and travelling‐wave structures. These experiments have demonstrated that pulsed heating is limiting the gradient. Also, a dual‐moded cavity has been designed to better distinguish the electric field, magnetic field and pulsed heating effects on breakdown.