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The LCLS SC-Linac will employ superconducting (SC) cavities to accelerate electron beam at up to 1 MHz repetition rate.
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Most slide images are from Dan's, Sebastian's, and Janice's talks on 31 Aug 2021. Some are from Andy Benwell's talk 22 May 2020.
Also highly recommended is the Cryomodule Internal Photo Tourwhich details the components of a cryomodule and their functions.
Motivation
To make operation at such a high repetition rate economically viable, it is critical to minimize the amount of input RF power that is wasted as heat loss. Cavity performance is characterized by the quality factor, Q0, a ratio of energy stored insiide the cavity to power lost through cavity walls. Because the nitrogen-oped niobium cavities of the SC-Linac are designed to dissipate far less heat through the walls than the copper cavities of the original LCLS linac, they have a higher Q0 so can transfer more of the power generated by the RF to the electron beam.
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Motivation
If you want a very high repetition rate machine, you need superconducting RF (SRF) cavities to manage the heat from the RF. Because of Due to the superfluid nature of low temperature liquid helium, it conducts the heat to the surface of the liquid, away from the cavities. From Dan Gonnella's talk 31 Aug 2021, if the cavities were made of copper and you fed in 20 MV of CW RF, the dissipated power in the cavity walls would be 15 MW. With niobium at 2K, the dissipated power is 15W. The cryogenic costs increase the wall power to 15 kW., an three orders of magnitude lower than power lost through copper walls!
Features
Cavity Description
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Each cavity also has two higher-order-mode couplers (HOM couplers) which are small cylinders (a few cm in diameter and ~10cm long) attached to each end of the cavity. These cans are tuned to absorb frequencies above 1.3 GHz. They also contain antenna for signal monitoring, but these aren't connected in the housing.
Frequency Tuning
When the drive frequency matches the resonant frequency of an accelerating cavity, standing waves form inside the cavity storing electromagnetic energy which can be used to accelerate a particle beam. Due to thermal expansion, a cavity's volume will change with temperature, shifting the resonant frequency of the cavity. Cavities also experience Lorentz force detuning which is a distortion of the cavity walls due to the pressure from the electromagnetic fields of the RF.
Frequency tuners are used to change the shape of the cavity to get it back on resonance. There are two types of tuners: stepper motors that can make slow, coarse changes to the cavity frequency and piezo tuners that make fast, incremental adjustments to maintain the resonant frequency during normal running.
In section 1 of this paperis a brief description of Lorentz force detuning.
Magnets
Due to spatial constraints, there is one multifunction conductively-cooled magnet in each cryomodule. The four quadrants of the magnet are each three concentric coils wired as a quadrupole, x corrector, and y corrector.
Important Concepts
Amplitude vs Gradient
RF Amplitude is given in units of volts. A electron after passing through a 1MV potential field will have 1 MeV of energy (E=qV). In our system, the LLRF is given a desired RF amplitude in MV (ADES) which is the total energy imparted to the beam if the cavity is phased correctly. Gradient (in MV/m) is amplitude divided by the cavity length. The LLRF also takes as input the desired phase (PDES). After initially phasing the cavities with the beam and calibrating the amplitude, a PDES of 0 should be the phase that imparts the most energy to the beam (fully forward phased). If PDES=0, the energy imparted to the beam by the cavity should be ADES.
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