PhD Position Study of Passive and Active Harmonic Cavity Systems for SOLEIL II
Located on the Paris-Saclay plateau, about 20 kilometers from the capital, the SOLEIL synchrotron is one of France's leading research facilities. Since it began operating in 2008, it has served the national and international scientific communities. Research conducted at SOLEIL covers a wide range of scientific and industrial fields — including physics, biology, chemistry, materials science, environmental science, Earth sciences, and cultural and natural heritage — all connected to current societal challenges. Experiments carried out on the beamlines rely on the use of synchrotron light emitted by electrons accelerated to nearly the speed of light within a storage ring. SOLEIL is jointly supervised by the CNRS and the CEA and offers its staff a dynamic, innovative, multidisciplinary, and international working environment.
At the forefront of technology, SOLEIL II is an ambitious project designed to provide new opportunities for scientific and industrial research. SOLEIL II consists of a major upgrade of the existing infrastructure and aims to significantly improve the performance of electron accelerators and beamlines. It is designed to address major current and future societal challenges, particularly in areas such as advanced materials research, energy and sustainable development, health and well-being, and the environment.
Construction work on the SOLEIL II project began in 2024, marking the start of a phase of development and technological innovation. In parallel, the existing facility will remain operational until the fall of 2028. SOLEIL II is scheduled to start operations in 2030, with a gradual ramp-up phase continuing until 2035.
1. Context and Motivation
The performance of fourth-generation synchrotron light sources based on storage rings can be limited by collective effects driven by the very high beam densities achieved in these machines, such as intra-beam scattering (IBS) and Touschek scattering. To mitigate these effects, harmonic cavities are routinely employed to stretch the beam longitudinally, thereby reducing the electron density and delivering high brightness and a beam lifetime compatible with the applicable radiological constraints.
As part of the upgrade of the French synchrotron radiation source SOLEIL to SOLEIL II, warm passive harmonic cavities will be installed. These cavities will include a coupling port enabling a switch to active operation if required. Prior to final installation on SOLEIL II, the first-of-series fundamental and harmonic cavities will be installed and commissioned on SOLEIL, providing a unique opportunity for experimental validation under real operating conditions.
2. Scientific Challenges
2.1 Beam Stability with the IQ (In-Phase/Quadrature) Feedback System
The proper operation of the double RF system, comprising active fundamental cavities and passive harmonic cavities, critically depends on a feedback loop on the fundamental cavities to broaden their bandwidth, thereby avoiding the dipole-quadrupole beam instability that limits bunch lengthening. The Low-Level RF (LLRF) I/Q feedback developed for SOLEIL II is expected to fulfill this requirement, although several uncertainties remain:
This feedback has not yet been modelled in the longitudinal beam dynamics simulations (mbtrack2).
Simulations show that an excessively high proportional gain for a similar feedback type (amplitude/phase) can, under certain conditions, trigger the very instability it is intended to suppress, potentially severely limiting the effectiveness of the harmonic cavities.
A phase feedback for damping oscillations on mode 0, analogous to the one in operation at MAX IV, should be integrable into the SOLEIL II LLRF system. These different feedback schemes must be modelled in mbtrack2 to evaluate their respective performances.
2.2 Operational Constraints of Passive Cavities
The use of passive harmonic cavities introduces specific operational constraints compared to an active system:
The average beam current must be sufficiently high to induce the required voltage in the passive cavity.
The bunch filling pattern must be uniformly distributed; otherwise, the increased sensitivity to the transient beam-loading effect causes variations in the phase and length of bunches along the train.
Active operation of the harmonic cavity would restore greater operational flexibility, particularly at low current and with exotic filling patterns, when the generator voltage greatly exceeds the beam loading. However, experience from third-generation synchrotron sources shows that at high average current, active harmonic systems are highly susceptible to beam instabilities, requiring complex RF feedback loops to stabilise the double active RF system.
3. Objectives and Work Plan
This PhD project pursues two objectives. The first is to verify that the chosen solutions will meet the bunch-lengthening performance targets for SOLEIL II. The second is to investigate bunch-lengthening schemes beyond the passive harmonic cavity solution, which could serve as a long-term alternative providing longer bunches or increased operational flexibility.
Objective 1 — Validation of the I/Q LLRF System for SOLEIL II
Modelling of the I/Q feedback in longitudinal beam dynamics simulations (mbtrack2).
Quantification of the risk of self-triggered dipole-quadrupole instability as a function of gain.
Modelling of the mode-0 phase feedback in beam dynamics simulations and evaluation of its performance as a complement to the I/Q feedback (optimization of the respective parameters to ensure compatibility and effectiveness of both feedback types).
Participation in the installation and commissioning tests of SOLEIL II cavities on SOLEIL.
Experimental validation of the LLRF simulation models on SOLEIL.
Objective 2 — Active Operation of the Harmonic Cavity
Study of beam instabilities in active operation of the harmonic RF system.
Study of the RF feedback loops required to stabilize the double active RF system.
Characterization of the operational flexibility gained in active mode at low currents and for exotic filling patterns (32 bunches, 16 bunches, single-bunch).
4. Perspectives
Transient Beam-Loading Compensation
It has been demonstrated numerically that compensation of the induced voltage can contribute to reducing the transient beam-loading effect. The actuator may be either an active fundamental or harmonic cavity, or a broadband kicker cavity. Such compensation would be of interest to achieve full bunch lengthening from the harmonic system even when gaps are present in the filling pattern (for ion clearing or specific operating modes).
Multi-Harmonic Systems
In the longer term, the use of a combination of passive and active cavities at different harmonics to further lengthen the beam will be of interest. Multi-harmonic systems have not yet been studied from the perspective of beam stability, however, it is likely that the reduction of longitudinal focusing by the much flatter RF potential will impose stringent constraints for such systems to operate reliably.
5. Supervision and Environment
The PhD student will be integrated within the Accelerator Physics group of Synchrotron SOLEIL, in close collaboration with the RF group. They will have access to the SOLEIL machine for experimental validation and will contribute to the ongoing developments for SOLEIL II.
The position is funded by a 3-year doctoral contract.