ANALOG CAVITY EMULATORS TO SUPPORT LLRF DEVELOPMENT

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ANALOG CAVITY EMULATORS TO SUPPORT LLRF DEVELOPMENT∗

S.D. Murthy†, L. Doolittle, LBNL, Berkeley, CA 94720, USA

A. Benwell, SLAC, Stanford, CA 94309, USA

Abstract

The goal of a LLRF system is to control an actual RF cav-

ity with beam. While digital simulations have a place, having

an analog circuit to stand in for the cavity can be tremen-

dously helpful in validating hardware+ﬁrmware+software

under development. A wide range of cavity emulators have

been developed in collaboration with SLAC, and LBNL.

Cavity emulators are typically based on quartz crystals and

frequency conversion hardware. The choice of crystal fre-

quency and coupling mechanism depends in part on the

bandwidth and coupling of the cavity it’s intended to emu-

late. Examples of bandwidth range from 800 Hz (SLAC) as

a stand-in for a SRF cavity, to 31 kHz (LBNL) for a room-

temperature accumulator-ring cavity. An external LO is

used to tune the emulated cavity frequency. The coupling

properties are also of interest if the scope includes emulat-

ing reverse power waveforms. LLRF system checks such

as closed-loop bandwidth, and determining cavity detuning

can be performed interactively and as part of a Continuous

Integration (CI) process. This paper describes the design,

implementation, and performance of the cavity emulators.

INTRODUCTION

At most of the accelerator facilities, the real RF cavities

will still be under construction when the LLRF system is

being developed. Even when the cavities become available,

setting up the whole high-power system (e.g., cryomodule)

just to validate the LLRF system under development is diﬃ-

cult to justify. People also worry about damaging the cavities

with an unproven controller. Digital simulations are a great

tool at the beginning of the development process to under-

stand and test the basic functionality of the ﬁrmware [1].

In light sources, requirements ﬂow from the quality of the

X-ray beams to the electron beam quality and then to re-

quirements on the stability of the accelerating cavity ﬁelds.

Examples for the stability requirement range from

0.01◦

phase and

0.01%

in amplitude for SLAC’s Linear Coherent

Light Source Linac (LCLS-II) system [2] and

0.1◦

in phase

and

0.1%

in amplitude for LBNL’s Advanced Light Source

Upgrade (ALS-U) system. Careful engineering of the entire

LLRF system is required to attain this stability requirement.

To support the development process, cavity emulators

have been developed at various accelerator labs over the

years to validate the LLRF system before controlling a real

RF cavity. A collaboration between SLAC and LBNL has

resulted in the design of cavity emulators for SRF cavities

with 800 Hz bandwidth for LCLS-II and room temperature

accumulator-ring cavities with 31 kHz bandwidth.

∗

This work was supported by the LCLS-II and ALS-U Projects and the

U.S. Department of Energy, Contract DE-AC02-76SF00515

†sdmurthy@lbl.gov

CAVITY EMULATOR DESIGN

An analog cavity emulator consists of two components: a

crystal resonator, and frequency conversion chains as shown

in Fig. 1. They can either be constructed on Printed Circuit

Boards (PCBs), or with componentized parts, or a combina-

tion of both.

Figure 1: Cavity emulator components.

Resonator hardware

The resonator consists of a commercial quartz crystal,

often with additional passive components. A quartz crystal

is normally modeled as a series RLC circuit with a parallel

capacitor. The former represents the mechanical vibrations

of the crystal, and the latter represents the electrodes attached

to the crystal [3]. A crystal has two resonance modes, series

and parallel. The frequency at which the series inductance

and series capacitance combination exhibit a phase angle

of zero is deﬁned as series resonance frequency (

𝑓𝑠

). A

parallel resonance (

𝑓𝑝

) is created when series inductance

and capacitance interact with parallel capacitance.

All crystal parameters, such as Equivalent Series Resis-

tance (ESR), Shunt Capacitance (

𝐶0

), Motion Inductance

(

𝐿𝑚

), Motion Capacitance (

𝐶𝑚

), and Quality factor (

𝑄𝐿

)

need to be characterized using the measured transfer func-

tion (

𝑆21

) to achieve the desired bandwidth and minimize

broadband coupling [4].

𝐸𝑆𝑅 =2·𝑍0· (1/|𝑆21 | − 1)(1)

arXiv:2210.05430v2 [physics.acc-ph] 15 Oct 2022

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摘要：

ANALOGCAVITYEMULATORSTOSUPPORTLLRFDEVELOPMENT∗S.D.Murthy†,L.Doolittle,LBNL,Berkeley,CA94720,USAA.Benwell,SLAC,Stanford,CA94309,USAAbstractThegoalofaLLRFsystemistocontrolanactualRFcav-itywithbeam.Whiledigitalsimulationshaveaplace,havingananalogcircuittostandinforthecavitycanbetremen-douslyhelpfulinva...

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ANALOG CAVITY EMULATORS TO SUPPORT LLRF DEVELOPMENT

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