Study of the Transverse Mode Coupling Instability in the CERN Large Hadron Collider

The High-Luminosity upgrade of the CERN LHC will increase the performance of the accelerator and the potential physics discoveries. The beam intensity will be multiplied by two to increase the collider luminosity. With such high intensities, collective effects and in particular beam coupling impedance are a possible performance limitation for the accelerator. After an introduction to accelerator physics and wake-fields, two collective effects codes will be detailed: PyHEADTAIL, a time-domain macro-particle code, and DELPHI, a Vlasov equation solver. Both are important to estimate coherent beam stability margins in the CERN accelerator complex, therefore a detailed comparison for different wakes and impedances, including the LHC model, will be presented. The current LHC stability limits will then be investigated with DELPHI simulations. In particular the Transverse Mode Coupling Instability, a fast instability occurring for high intensity beams with chromaticities close to zero, will be studied. The results will then be compared to measurements performed in the accelerator. Beam based measurements of several collimators will also be presented and compared to predictions from the impedance model. Combining these measurements and their comparison to simulations, we will estimate the uncertainty on the LHC impedance model. To cope with the increased beam intensity, the impedance of the High Luminosity upgrade of the LHC will be reduced. The impact of different upgrade scenarios will be studied from the Transverse Mode Coupling Instability perspective. The potential benefits of an impedance reduction will be demonstrated through measurements in the LHC. Measurements performed in the LHC on a low impedance prototype collimator, the key component to the impedance reduction, will also be presented. These simulations and measurements will confirm the increase in the mode coupling threshold towards a value three times higher than the nominal bunch intensity.