THE LCLS-II GUN BUNCHER LLRF CONTROLLER UPGRADE C. Bakalis L. Doolittle D. Filippetto G. Huang S. Paiagua C. Serrano1 A. Benwell D. Chabot J. Dusatko2

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THE LCLS-II GUN & BUNCHER LLRF CONTROLLER UPGRADE

C. Bakalis∗, L. Doolittle, D. Filippetto, G. Huang, S. Paiagua, C. Serrano1

A. Benwell, D. Chabot, J. Dusatko2

1Lawrence Berkeley National Laboratory, Berkeley, CA 94720, USA

2SLAC National Accelerator Laboratory, Menlo Park, CA 94025, USA

Abstract

LCLS-II is currently in its commissioning phase at SLAC.

It is an X-ray FEL driven by a CW superconducting LINAC.

The beam injector plays a crucial role in the overall perfor-

mance of the accelerator, and is critical to the ﬁnal elec-

tron beam performance parameters. The LCLS-II injector

comprises of a

185.7 MHz

VHF copper gun cavity, and a

1.3 GHz

two-cell L-band copper buncher cavity. The FPGA-

based controller employs feedback and Self-Excited Loop

logic in order to regulate the cavity ﬁelds. It also features

several other functionalities, such as live detune computa-

tion, active frequency tracking, and waveform recording.

The LLRF system drives the cavities via two

60 kW

SSAs

through two power couplers, and thus stabilizes the ﬁelds

inside the plant. This paper provides an outline of the gen-

eral functionalities of the system, alongside a description

of its hardware, ﬁrmware and software architecture, before

ﬁnalizing with the current status of the project and its future

goals.

INTRODUCTION

LCLS-II is a Free-Electron Laser (FEL), operating

4 GeV

via a Continuous-Wave (CW) superconducting

LINAC, designed to provide pulses of electrons of

300 μA

at up to

∼1 MHz

. The soft X-rays delivered by the beam

and its undulators are expected to fall within a spectral range

200 −5000 eV

, thus allowing the newly-built machine to

host a broad set of groundbreaking experiments [1]. The

electron gun deployed at LCLS-II is a descendant of the

Advanced Photo-injector EXperiment (APEX), which was a

Very-High-Frequency gun (VHF-Gun), capable of CW op-

eration and optimized for the performance required by MHz-

rate X-ray FELs [2]. Alongside the LCLS-II injector system,

the High Repetition-rate Electron Scattering (HiRES) experi-

ment at LBNL shares the same gun and buncher concepts [3]

- this sets a fundamental requirement for the controller under

review in this paper, which is to be able to serve both of

these experiments without major modiﬁcations in any of its

hardware, ﬁrmware, or software components.

The injector of LCLS-II uses APEX-style RF cavities, in-

cluding a

185.7 MHz

VHF copper gun cavity and a

1.3 GHz

two-cell L-band copper buncher cavity, with a loaded Q in

the order of

15000

[4]. HiRES on the other hand uses a

similar scheme, with the critical frequencies of all aforemen-

tioned setups summarized in Table 1:

∗Main/Corresponding Author: cbakalis@lbl.gov

Table 1: Several important frequencies of all controller con-

ﬁgurations

MHz LCLS-II Gun LCLS-II Buncher HiRES Gun

𝑓𝐿𝑂 1320 1320 1225.7

𝑓𝐴𝐷𝐶 94.286 94.286 102.14

𝑓𝐷 𝐴𝐶 188.57 188.57 204.29

𝑓𝑅𝐹 185.7 1300 185.7

𝑓𝑢 𝑝

𝐼 𝐹 34.285 145 18.571

𝑓𝑑𝑛

𝐼 𝐹 20.714 20 74.288

𝑓𝑃𝑅𝐿 1300/7 1300 N/A

CONTROLLER SYSTEM

To ease the development process, all components of

the LCLS-II injector controller share the same hardware,

ﬁrmware and software base as the SRF part of the system.

The chassis for the gun control is diﬀerent than the one used

for the buncher. The basic function of the LLRF system is to

stabilize the RF ﬁeld in the cavities with feedback, and allow

the operator to monitor the RF signals by via a waveform

recording and display scheme. In the LCLS-II case, a high

precision phase reference is distributed globally by a hard

coaxial cable. The phase of the PRL is digitally averaged to

provide a drift-compensated phase reference. The system

needs to fulﬁll stringent requirements for the stability of the

cavity ﬁelds. The gun cavity needs to be within a

0.01%

ﬁeld amplitude error, and a

0.015◦

phase error. The buncher

cavity needs to be within a

0.3%

ﬁeld amplitude error, and

0.05◦

phase error [4]. What follows is a brief description

of each system component.

Hardware

The foundation of the hardware is based on a chassis,

comprised of devices operating both in the analog and in

the digital domain. The chassis can be seen in Figure 1.

The LO of the system is driven to one of the chassis inputs,

and is then distributed to various parts of the chassis. It

is used in analog downmixers, utilized in all of the chassis

inputs, which include cavity signals or the PRL. The analog

mixers provide downconverted signals to the digitizer board,

which also accepts the LO as a clock reference. The digitizer

employs a clocking chip that uses the LO as an input, and

in turn distributes a reference relevant to the setup at hand,

depending on its conﬁguration, which is software-controlled.

It also deploys ADCs to sample the downmixed RF signals,

and DACs that receive a digital stream before converting

them to an IF voltage. Naturally, the analog part of the

arXiv:2210.04005v2 [physics.acc-ph] 8 Nov 2022

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THELCLS-IIGUN&BUNCHERLLRFCONTROLLERUPGRADEC.Bakalis∗,L.Doolittle,D.Filippetto,G.Huang,S.Paiagua,C.Serrano1A.Benwell,D.Chabot,J.Dusatko21LawrenceBerkeleyNationalLaboratory,Berkeley,CA94720,USA2SLACNationalAcceleratorLaboratory,MenloPark,CA94025,USAAbstractLCLS-IIiscurrentlyinitscommissioningphaseatSL...

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THE LCLS-II GUN BUNCHER LLRF CONTROLLER UPGRADE C. Bakalis L. Doolittle D. Filippetto G. Huang S. Paiagua C. Serrano1 A. Benwell D. Chabot J. Dusatko2

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