LLRF System Analysis for the Fermilab PIP-II Superconducting LINAC

TL;DR

The paper analyzes the PIP-II LLRF system, a superconducting linac using a SEL-based architecture derived from LCLS-II, to assess field and resonance control under high beam loading. It combines calibration, gradient sensing, and SSA modeling to determine feasible gain ranges and regulation performance, highlighting the impact of saturation on stability and the need for feedforward and latency reductions. Beam loading at 2 mA poses a substantial challenge relative to LCLS-II, motivating mode choices like SELAP and targeted firmware improvements. The findings establish the practical limits of current hardware and outline concrete improvements to achieve robust regulation for future PIP-II operation.

Abstract

PIP-II is a superconducting linac that is in the initial acceleration chain for the Fermilab accelerator complex. The RF system consists of a warm front-end with an RFQ and buncher cavities along with 25 superconducting cryo-modules comprised of cavities with five different acceleration \(β\). The LLRF system for the linac has to provide field and resonance control for a total of 125 RF cavities. Various components of the LLRF system have been tested with and without beam at the PIP-II test stands. The LLRF system design is derived from the LCLS-II project with its self-excited loop architecture used in the majority of the cryo-modules. The PIP-II beam loading at 2 mA is much higher than the LCLS-II linac. The control system architecture is analyzed and evaluated for the operational limits of feedback gains and their ability to meet the project regulation requirements for cavity field amplitude and phase regulation.

Figures (6)

• Figure 1: LLRF System Architecture - SEL Control
• Figure 2: LLRF Feedback Loop Components
• Figure 3: SSA Calibration
• Figure 4: LCLS-II Cavity Bode Analysis
• Figure 5: Feedback Regulation