INSTALLATION COMMISSIONING AND PERFORMANCE OF PHASE REFERENCE LINE FOR LCLS-II S.D. Murthy L. Doolittle LBNL Berkeley CA 94720 USA

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INSTALLATION, COMMISSIONING AND PERFORMANCE OF PHASE

REFERENCE LINE FOR LCLS-II∗

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

C. Xu, B. Hong, A. Benwell, J. Chen, SLAC, Stanford, CA 94309, USA

Abstract

Any cavity controller for a distributed system needs a

Phase Reference Line (PRL) signal from which to deﬁne

phases of a cavity ﬁeld measurement. The LCLS-II PRL

system at SLAC provides bidirectional (forward and reverse)

phase references at 1300 MHz to each rack of the LLRF

system. The PRL controller embedded with the Master Os-

cillator (MO) locks the average phase of the two directions

to the MO itself. Phase-averaging tracking loop is applied in

ﬁrmware which supports the feature of cancelling the phase

drift caused by changes in PRL cable length. FPGA logic

moves the phase of digital LO to get zero average phase of

the two PRL signals. This same LO is used for processing

cavity pickup signals, thus establishing a stable reference

phase for critical cavity RF measurements. At low frequen-

cies, open-loop PRL noise relative to the LO distribution

includes a strong environment and

1/𝑓

components, but the

closed-loop noise approaches the noise ﬂoor of the DSP. The

implication is that the close-in phase noise of the cavities

will be dominated by the chassis DAQ noise. The ﬁnal out-

of-loop phase noise relevant to machine operations is that

of the cavity ﬁeld relative to the beam.

INTRODUCTION

To achieve their physics-driven goals, all accelerator RF

sub-systems need a stable phase reference. The primary job

of the LLRF system is to hold the amplitude and phase of

each cavity’s voltage constant, where cavity phase is deﬁned

relative to the phase reference signal. The LLRF hardware

receives two frequencies, LO (1320 ) and PRL (1300 MHz).

Both of these signals are generated by the MO subsystem.

The speciﬁcations for cavity ﬁeld stability in LCLS-II are

stringent by historical accelerator standards: 0.01

◦

in phase

and

0.01%

in amplitude [1]. Careful analysis and design

minimizes cavity phase noise and drift.

A phase averaging reference line scheme has been used

in the ﬁeld before at both SLAC [2] and Fermilab [3], to

compensate for thermal drift in the cable used to distribute

phase information. The PRL design adopted for LCLS-II

starts from the same concept but uses digital techniques to

perform the averaging. When averaging is done using analog

components, expert ﬁeld setup is needed because of sensitiv-

ity to amplitude imbalance. The LCLS-II system at SLAC

provides bidirectional (forward and reverse) references to

each rack of the LLRF system at 1300 MHz. By digitizing

∗

This work was supported by the LCLS-II Project and the U.S. Department

of Energy, Contract DE-AC02-76SF00515

†sdmurthy@lbl.gov

the two signal separately, more diagnostics are possible, and

the equivalent adjustments performed by software.

For every oﬀset frequency from DC up to the closed-loop

cavity bandwidth, the LLRF system locks the cavity phase

using combination of LO and PRL sources. At low frequen-

cies (below a few kHz), the PRL contribution dominates, so

the information it provides (after averaging) needs to have

minimum phase drifts [5].

DESIGN

Phase Reference System

A Voltage Controlled Oscillator (VCO) drives the remote

end (far from the MO) of the physical PRL cable to generate

a forward phase reference signal at 1300 MHz. In the MO

rack, that forward signal is actively reﬂected to generate the

reverse wave on the physical PRL cable. A phase lock circuit

pins the average of those two signals to the phase of the MO

itself, creating the reﬂectometer technique [4]. The block

diagram of the phase averaging portion of the PRL is shown

in Fig. 1. At any arbitrary point on the phase reference line,

the directional coupler couples out the forward and reverse

signals. The distance from the coupler to the MO determines

the amount of phase diﬀerence. The forward and reverse

signals are sent to the LLRF chassis to perform digital phase

averaging.

Figure 1: LCLS-II phase averaging technique.

Installation

The overall LCLS-II linac with 35 1.3 GHz cryomodules,

spread out over 600 meters, is divided into segments, from

L0 near the gun (a single cryomodule) to the long (20 cry-

omodules) L3 for the ﬁnal on-crest energy boost. LH refers

to the 3.9 GHz energy linearizer (two cryomodules). The

PRL is divided up into three segments: L0+L1+LH, L2, and

L3. The MO and most PRL support electronics are located

between LH and L2. The phase-averaging feature described

arXiv:2210.05441v2 [physics.acc-ph] 15 Oct 2022

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INSTALLATION,COMMISSIONINGANDPERFORMANCEOFPHASEREFERENCELINEFORLCLS-II∗S.D.Murthy†,L.Doolittle,LBNL,Berkeley,CA94720,USAC.Xu,B.Hong,A.Benwell,J.Chen,SLAC,Stanford,CA94309,USAAbstractAnycavitycontrollerforadistributedsystemneedsaPhaseReferenceLine(PRL)signalfromwhichtodefinephasesofacavityfieldmeasur...

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INSTALLATION COMMISSIONING AND PERFORMANCE OF PHASE REFERENCE LINE FOR LCLS-II S.D. Murthy L. Doolittle LBNL Berkeley CA 94720 USA

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