http://ieeexplore.ieee.org/iel6/8233/25598/01142248.pdf

Microwave measurements

In preparation of the SPS as an LHC injector its impedance was significantly reduced in 1999 2000. A new SPS impedance reduction campaign is planned now for the High Luminosity (HL)-LHC project, which requires bunch intensities twice as high as the nominal one. One of the known intensity limitations is a longitudinal multibunch instability with a threshold 3 times below this operational intensity. The instability is presently cured using the 4th harmonic RF system and controlled emittance blowup, but reaching the HL-LHC parameters cannot be assured without improving the machine impedance. Recently the impedance sources responsible for this instability were identified and implementation of their shielding and damping is foreseen during the next long shutdown (2019 2020) in synergy with two other important upgrades: amorphous carbon coating of (part of) the vacuum chamber against the e-cloud effect and rearrangement of the 200 MHz RF system. In this paper the strategy of impedance reduction is presented together with beam intensity achievable after its realisation. The potential effect of other proposals on remaining limitations is also considered. KNOWN SPS INTENSITY LIMITATIONS The nominal bunches with intensity of 1.15 × 10 p/b spaced at 25 ns have been used since 2015 for physics in the LHC. In future, beams with intensity up to 2.4 × 10 p/b will be required by the HL-LHC project and the LIU (LHC Injector Upgrade) project [1] is responsible for preparation of this beam. The SPS ring is the last accelerator in the LHC injector chain. Various upgrades were launched by the LIU project to remove known intensity limitations in the SPS which include e-cloud effect, beam loading and longitudinal instabilities. The upgrade of other SPS devices is also planned so that these high intensity beams are safely injected, extracted and transferred to LHC [1]. Presently the bunch intensity of 4 batches with 72 bunches spaced at 25 ns is limited to ∼ 1.4 × 10 p/b since relative beam losses are increasing with bunch intensities, reaching 20% for 1.7 × 10 p/b injected. Possible reasons are e-cloud and beam-loading effects. At the moment, acceleration of the LHC beam with emittance of 0.4 eVs (defined by injector) with an intensity of 1.3×10 11 p/b requires higher RF power per 200 MHz cavity than the available 700 kW. The situation is similar on the SPS flat top, where short bunches are needed for clean injection into the bucket provided by the LHC 400 MHz RF system. The maximum bunch length allowed for extraction is established to be 1.9 ns with an average value of Figure 1: Bunch length (4σ Gaussian fit) measured for 4 batches with 72 bunches at injection (upper trace) and on the SPS flat top (lower trace) in double RF system. Average bunch length 1.65 ns and intensity 1.35 × 10 p/b. 1.7 ns, see example in Fig. 1. In the present SPS RF configuration (two 4-section and two 5-section cavities) the available RF voltage falls quickly with increased beam intensity, however the RF upgrade (6 cavities with a total of 20 sections, two additional RF power plants of 1.6 MW and four existing ones upgraded to 1.05 MW) should increase the voltage available for HL-LHC intensity to 10 MV [2], see Fig. 2. Nevertheless, due to longitudinal instability leading to emittance increase during the ramp as well as due to induced voltage (potential well distortion, PWD), the voltage required on the SPS flat top is also increasing with intensity; an estimation from analytical scaling (red line) gives 12 MV. So far this instability is cured by the 800 MHz RF system operating in the Bunch Shortening (BS) mode and controlled emittance blow-up, but to achieve beam parameters required by the HL-LHC, the SPS impedance reduction had to be included in the LIU baseline.

/pdf/removing-known-sps-intensity-limitations-for-high-luminosity-3do3joye4q.pdf

Removing Known SPS Intensity Limitations for High Luminosity LHC Goals

The Super Proton Synchrotron (SPS) at CERN is the injector of the Large Hadron Collider (LHC), the world’s largest particle collider. The High-Luminosity LHC (HL-LHC) project is a major step forward in the improvement of the LHC performances and it requires a doubling of the nominal bunch intensity of the current LHC beam. In the SPS, multi-bunch instabilities and particle losses limit the beam intensity that can be accelerated to 450 GeV/c and transferred to the LHC. Without mitigation measures, the bunch intensity threshold for longitudinal instabilities is three times below the nominal intensity of the LHC beam. Moreover, the present limited RF power is not sufficient to accelerate beams with intensities well above nominal without substantial particle losses and a reduction of the RF voltage available for the beam at the flat top energy. The SPS will undergo significant upgrades but they may not be sufficient to ensure the stability of the HL-LHC beam. The objectives of this doctoral research are to study the longitudinal intensity limitations of the LHC proton beam in the SPS and to find possible mitigation measures to ensure the beam stability and quality at HL-LHC intensity. Beam measurements and particle simulations are used in conjunction with analytical estimations to study the multi-bunch instabilities during the cycle in the SPS. This work attempts to identify the main sources of instabilities and beam quality degradation. Possible scenarios of mitigation measures are investigated to explore the future beam parameters achievable after upgrades. The effects on beam stability of the foreseen RF upgrade, the double RF operation and the reduction of various longitudinal beam-coupling impedances are analysed in detail. The scenario of a lower-harmonic RF system in the SPS, for particle losses reduction, is also studied.

Possible Mitigations of Longitudinal Intensity Limitations for HL-LHC Beam in the CERN SPS

The High Luminosity (HL)-LHC project at CERN aims at a luminosity increase by a factor ten and one of the necessary ingredients is doubling the bunch intensity to 2.4× 1011 ppb for beamswith 25 ns bunch spacing. Many improvements are already foreseen in the frame of the LHC Injector Upgrade (LIU) project, but probably this intensity would still not be reachable in the SPS due to longitudinal instabilities. Recently a lot of effort went into finding the impedance sources of the instabilities. Particle simulations based on the latest SPS impedance model are now able to reproduce the measured instability thresholds and were used to determine the most critical impedance sources by removing them one by one from the model. It was found that impedance of vacuum flanges and of the already damped 630 MHz HOM of the main RF system gave for 72 bunches the comparable intensity thresholds. Possible intensity gains are defined for realistic impedance modifications and for various beam configurations (number of bunches, longitudinal emittances) and RF programs (single and double RF). The results of this study are used as a guideline for planning of a new campaign of the SPS impedance reduction.

Effect of the Various Impedances on Longitudinal Beam Stability in the CERN SPS

The longitudinal single bunch instability observed in the SPS leads to uncontrolled emittance blow-up and limits the quality of high intensity beams required for the High Luminosity LHC and AWAKE projects at CERN. The present SPS impedance model developed from a thorough survey of machine elements was used in macro-particle simulations (with the code BLonD) of the bunch behavior through the acceleration cycle. Comparison of simulations with measurements of the synchrotron frequency shift, performed on the SPS flat bottom to probe the impedance, show a reasonable agreement. During extensive experimental studies various beam and machine parameters (bunch intensity, longitudinal emittance, RF voltage, with single and double RF systems) were scanned in order to further benchmark the SPS impedance model with measurements and to better understand the mechanism behind the instability. It was found that the dependence of instability threshold on longitudinal emittance and beam energy has an unexpected non-monotonic behavior, leading to islands of (in)stability. The results of this study are presented and can be used to define possible parameter settings for the future CERN projects.

/pdf/single-bunch-longitudinal-instability-in-the-cern-sps-3dxb628czc.pdf

Single Bunch Longitudinal Instability in the CERN SPS

Longitudinal multi-bunch instability in the CERN SPS with a very low intensity threshold is a serious limitation for the future doubling of bunch intensity required by the HLLHC project. A complete and accurate impedance model is essential to understand the nature of this instability and to plan possible cures. This contribution describes the current longitudinal impedance model of the SPS. Electromagnetic simulations and bench measurements were used to build the model. The contribution from each element is described and compared to the total machine impedance. Together with relevant beam measurements and simulations, the analysis of the different sources of impedance is used to identify the source of the longitudinal instability limiting the SPS performance so that the responsible elements can be acted upon.

/pdf/an-extended-sps-longitudinal-impedance-model-4qxo41vj1s.pdf

An Extended SPS Longitudinal Impedance Model

In the framework of the UA9 experiment, the Cherenkov detector is useful to measure the amount of particles deflected by a bent crystal, proving the crystal collimation principle. The tank used to host this device is taken as a case study for an in-depth analysis of different damping strategies for electromagnetic modes which otherwise would give rise to important beam-coupling impedance contributions. Such strategies involve the use of ferrite, damping resistors and a mode-coupler, a solution which intercepts the modes inside the cavity but damps the related power outside the vacuum tank (potentially avoiding heating). Such solutions are discussed through experimental measurements and the relative quality factor is taken as a figure of merit.

http://jacow.org/ipac2015/papers/mopje037.pdf

Study and Comparison of Mode Damping Strategies for the UA9 Cherenkov Detector Tank

A ferrite loaded accelerating cavity with a frequency sweep of 18 to 40 MHz is studied for a possible upgrade of the CERN accelerator complex. The resonance frequency of a ferrite loaded cavity shifts by applying an external magnetic bias field to the ferrite material by means of changing the relative permeability. We present the electromagnetic design of such a cavity with a special emphasis on the modeling of the nonlinear, anisotropic and dispersive characteristics of the ferrite’s relative permeability above magnetic saturation. For experimental crosscheck, a ferrite loaded resonant test setup was built which provides results for the material performance in a magnetic bias field. A comparison of numerical simulations and experimental measurements is shown and calculations are benchmarked by measurement data. Based on this study a preliminary design of a ferrite loaded accelerating cavity is described.

Preliminary Design of a Perpendicular Biased Ferrite Loaded Accelerating Cavity

In the frame of the LHC Injector Upgrade program involving the existing 80 MHz cavities in the CERN PS accelerator, an orthogonally biased ferrite tuner is foreseen to complement the current motor-driven piston tuner. This ferrite tuner shall provide the possibility of a fast frequency shift of about 200 kHz on the fundamental mode, to allow a fast switching between proton and ion frequencies. In order to avoid water cooling and related issues, the challenge was to bring magnetic losses in the tuner to a minimum such that a forced air cooling scheme will be sufficient. The tuner was first designed with simulation tools, a prototype was built and low-power RF testing was performed on the tuner-cavity combination to evaluate tuning range, bandwidth, and stability. These tests were carried out on a single-cell copper RF cavity mock-up with a resonance frequency of 88 MHz, where the ferrite tuner is connected via a tuning loop and the perpendicular magnetic bias for the ferrite tuner is provided by a DC bias supply. Simulations and test data will be presented.

Ferrite-tuner Development for 80 MHz Single-Cell RF-Cavity Using Orthogonally Biased Garnets

We present several measurement methods for evaluation of magnetic properties of magnetically biased and nonbiased ferrite samples in a coaxial test fixture. One important aspect is the crosscheck of results obtained by using different and independent measurement and evaluation methods. Since a rather high DC bias current has to be applied, a dedicated network was designed that allows the passage of up to 50 A DC without degradation of the RF performance. With a combination of calibration methods and a compensating topology with two identical sample holders, a good performance was achieved. In this context, magnetic material parameters for about 10 different types of ferrite were obtained. The orthogonal magnetic bias was added by placing the entire test fixture into a large toroidal coil. Thus, the bias field can be supplied independently from, and in addition to the classical parallel bias. An optimal combination between the two biasing fields was found, resulting in a reduction of magnetic losses up to 50 % on certain ferrites. We show that the mixed magnetization, normally used for garnets only, is beneficial also for other types of ferrites.

https://cds.cern.ch/record/2141867/files/wepmn070.pdf

Measurement Techniques and Application of Combined Parallel/Orthogonal Magnetic Bias on a Ferrite Tuned Resonator in Low Frequency Range (3-10 MHz)

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