Low-level rf control of Spallation Neutron Source: System and characterization

Abstract

The low-level rf control system currently commissioned throughout the Spallation Neutron Source (SNS) LINAC evolved from three design iterations over 1 yr intensive research and development. Its digital hardware implementation is efficient, and has succeeded in achieving a minimum latency of less than 150 ns which is the key for accomplishing an all-digital feedback control for the full bandwidth. The control bandwidth is analyzed in frequency domain and characterized by testing its transient response. The hardware implementation also includes the provision of a time-shared input channel for a superior phase differential measurement between the cavity field and the reference. A companion cosimulation system for the digital hardware was developed to ensure a reliable long-term supportability. A large effort has also been made in the operation software development for the practical issues such as the process automations, cavity filling, beam loading compensation, and the cavity mechanical resonance suppression.

Longitudinal beam parameter tolerances of NSLS-II

“…Fortunately this is within the capability of the present technology [6]. We will be following the development in the field of RF feedback and control and design our system with consideration of physics requirements, cost and available technologies.…”

Low level rf system for the European Spallation Source’s Bilbao linac

“…The current LLRF system can be favorably compared with those used in linacs with similar applications [3,4] and takes us to a next step where the aim will be a focus on the efficient control of the effects of rf phase and amplitude noise on the beam emittance, a territory for which charts have only recently been sketched [25]. Finally, some considerations of the present development, such as expanding the tuning system to control the tuners of each RFQ section independently, is now under study.…”

“…In the implementation of the SNS LLRF, a large effort has been made to keep the latency below 150 ns so that the required bandwidth can be achieved by an all-digital LLRF system. Also, mechanical modes of the superconducting cavities have been successfully damped out using adaptive feedforward (AFF) compensation resulting in a significant decrease of the amplitude and phase errors [3]. In the case of J-PARC, a digital LLRF system utilizing feedback and feed-forward control has been made and tested with amplitude and phase stabilities of 0.3% and 0.2 under beam loading [4].…”

High gain proportional rf control stability at TESLA cavities

“…Electrical voltage amplitudes couple like the square root of the coupling factor . Both the coupling at the power coupler and the pickup results with (3) in the second factor in (5). For a derivation of the last factor see [1].…”

“…The other FMs are unwanted and have a significant influence on the proportional control loop stability at high gains [1,5]. Digitalization acts like a filter and therefore influences the loop stability.…”