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SRF Cavity Emulator for PIP-II LLRF Lab and Field Testing

Ahmed Syed, Brian Chase, Philip Varghese, Sana Begum,

RF Department,

Fermilab

Batavia, IL, USA

Abstract— There are many stages in the LLRF and RF

system development process for any new accelerator that

can take advantage of hardware emulation of the high-

power RF system and RF cavities. LLRF development,

bench testing, control system development and testing of

installed systems must happen well before SRF cavities are

available for test. The PIP-II Linac has three frequencies of

SRF cavities, 162.5 MHz, 325 MHz and 650 MHz and a

simple analog emulator design has been chosen that can

meet the cavity bandwidth requirements, provide tuning

errors to emulate Lorentz force detuning and microphonics

for all cavity types. This emulator design utilizes a quartz

crystal with a bandwidth of 65Hz at an IF of ~ 4 MHz,

providing a Q of ~ 1.3 x 10^7 at 650MHz. This paper will

discuss the design and test results of this emulator.

I.INTRODUCTION

Low-level radio frequency (LLRF) systems are

needed in accelerators to control the rf fields in the rf cavities

that are used for beam acceleration. Generally, it is required to

build and test these LLRF systems before commissioning the

beam. Functional testing is frequently required for the study of

various LLRF control algorithms during beam commissioning.

However, the prerequisites for LLRF system testing, such as

super-conducting surroundings and the appropriate rf sources,

are typically pricey and occasionally unavailable.

State-space models of RF systems that are software-

based are prevalent and have useful applications in the design

phase of an LLRF system, but it is challenging to operate the

full rf system in real time [5]. This issue led to the creation of

cavity emulators to allow for LLRF system to be developed

without the need to connect to a real superconducting rf cavity.

Real-time behavior under LLRF control system operation can

be achieved using this emulator. However, the research is

largely concentrated on the cavity's fundamental mode [6].

Moreover, the performance of the LLRF system is limited by

disturbances like microphonics and detuning which are in scope

for this design.[1]

II.LLRF SYSTEM

As shown in Fig. 1, rf devices, such as rf sources (such as

klystron) and rf cavities, are essential in order to test the LLRF

system. Real-time emulators must be available as a stand-in for

actual cavities and RF sources in the development phase since

these devices are not always readily available. The RF system,

however, is prone to disturbances. Beam loading, high voltage

power supply ripples, Lorentz force detuning (LFD), and

unknown disturbances such microphonics, master oscillator

phase noise, and clock jitters are examples of typical sources [3-

4]. The LLRF system's performance is constrained by these

disturbances, consequently, useful information can be obtained

through the incorporation of disturbances into the cavity

emulator. According to the above discussion, this paper

discusses the design and implementation of a LLRF analog

cavity emulator based that integrates both RF cavity models and

RF disturbance models.

III.CAVITY EMULATOR DESIGN

Design of cavity emulator involves optimization of crystal

filter circuit in terms of achieving impedance matching and

frequency tuning. Crystal filter circuit design is developed in

ADS simulation. Once the crystal filter is optimized and

developed frequency upconversion and downconversion is

developed for IF – RF conversion.

Based on the input signals (RF drive) the cavity emulator

generates the following output signals:

• Cavity Forward

• Cavity Reverse

• Cavity Probe

The basic idea of the emulator is to use a simple crystal filter at

an IF 4.1938470 MHz and narrow bandwidth of ~77Hz. To

achieve the highest Q possible, crystal filter like quartz is used,

hence the Q factor of superconducting RF cavities that are used

in accelerator field can reach as high as 1011. For the crystal

board of 4.19 MHz, Q= 8.38 x 104. Best effort is made to pick

the crystal with least possible bandwidth as that would affect

quality factor of overall response.

Fig.1 Detailed Schematic of Analog Cavity Emulator

Design of the crystal board

The crystal board forms the crux of the cavity emulator circuit

and the output characteristics of the signal like- bandwidth and

return loss from this board determines the final output

characteristics. The crystal board is design involves DC

blocking capacitors and coupling capacitors as shown in Fig.2.

Firstly, RLC equivalent circuit of the crystal is determined by

measuring the crystals series and parallel resonance frequency

with a network analyzer. Upon obtaining the equivalent -Cs, Ls,

Rs and Cp we incorporate this in the circuit, and this forms the

quartz crystal equivalent.These obtained values of quartz filter

equivalent circuit are used in the ADS model.

Use of transformer for negative capacitance

RF transformer T4-1 inverts signals by producing a 180-degree

phase shift. It is in series with a equivalent capacitance equal

to parallel capacitance of the crystal. This combination of RF

transformer and series capacitor effectively cancels out the

effect of parallel resonance peak which is undesired. This is

done to improve the signal floor of the crystal side band and

compensates for parasitic capacitance.

Fig. 4 below shows ADS simulation without RF transformer

shows the parallel resonance peak fp is dominant and the series

resonance peak fp has a lower S21 magnitude at the center

frequency.

It also shows poor return loss at center frequency and doesn’t

depict the behavior of an RF superconducting cavity quite well.

Fig. 2 ADS Circuit Schematic- without transformer

Fig. 3 ADS simulation plots - without RF transformer.

In order to eliminate the parallel resonance peak fp of the circuit

we introduce RF transformer T4-1 which has center tapped

primary winding. Signal at the input is inverted by 180° and is

in series with capacitor C13 with almost same capacitance as

parallel equivalent capacitance C18 of crystal filter. The overall

response of the transformer and capacitor C13 effectively

cancels out the effect of parallel equivalent capacitance of

quartz crystal which is also known as case capacitance or

parasitic capacitance.

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XXX-X-XXXX-XXXX-X/XX/$XX.00©20XXIEEESRFCavityEmulatorforPIP-IILLRFLabandFieldTestingAhmedSyed,BrianChase,PhilipVarghese,SanaBegum,RFDepartment,FermilabBatavia,IL,USAAbstract—TherearemanystagesintheLLRFandRFsystemdevelopmentprocessforanynewacceleratorthatcantakeadvantageofhardwareemulationofthehigh-p...

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