Digital RF control for spallation neutron source accumulator ring

The proposed upgrade plan for the rf control of the Spallation Neutron Source (SNS) accumulator ring requires that the new digital field control module (FCM) have the capability of adaptive feed forward (AFF) compensation for the heavy beam loading in the cavities. The feed forward compensation capability should support both the conventional software driven AFF algorithm and a new beam current driven AFF algorithm. A user-friendly Epics GUI for all the FCM functionalities is also a part of the requirement. The first prototype for test has been built. It is a narrowband system based on the conventional I/Q sampling scheme. Both the hardware and software are currently under test in the lab, and the field test in a near future has been planned.

INTRODUCTION

The Spallation Neutron Source (SNS) accumulator ring has a circumference of 248 m and revolution frequency of 1.058 MHz. The injected beam energy is 1 GeV The beam current ramps up to maximum 50 A at the end of the full 1060 turn stacking [1], [2], [6]. There are three fundamental and one 2^nd harmonic cavity powered by four tetrode RF power amplifiers. Based on the original design specifications and operation experience, some specific requirement for the rf control has been updated and defined [3], [5], and they are listed in Table 1. Table 1: Requirements for Ring RF Control

Parameter/Functionality	Specification
Control scheme	AFF + feedback combined
Control bandwidth	100 kHz
Field Regulation	(fundamental only)
Phase	+1–2.5 deg
Amplitude	±5%
Bean loading handling	50 A
Gap signal dynamic range	40 dB min.
RF vector input	Gap_V, Grid_V, Beam_I
RF vector output	LLRF Drive
Conventional AFF	supported
Beam current AFF	supported
Cavity dynamic tuning	supported
Tetrode Grid boost	supported
Grid-Gap phase monitor	yes
Regulation error monitor	yes
RF protection response	less than