DEVELOPMENT AND APPLICATION OF A FREQUENCY SCAN-BASEDAND A BEAM-BASED CALIBRATION METHOD FOR THE LLRF
SYSTEMS AT KEK STF
M. Omet #,A), A. Kuramoto A), H. Hayano B), T. Matsumoto B), S. Michizono B), K. Watanabe B)A)
The Graduate University for Advanced Studies (SOKENDAI)
Shonan Village, Hayama, Kanagawa, 240-0193
B)High Energy Accelerator Research Organization (KEK)
1-1 Oho, Tsukuba, Ibaraki, 305-0801

Abstract
In order to calibrate the ratio of the forward andreflected signal in the waveguide system of theSuperconducting RF Test Facility (STF) RF gun atechnique based on a frequency scan was developed. The
application allowed the reconstruction of the cavity
pick-up signal and therewith a stable feedback operationin absence of an actual pick-up antenna.
For the determination of the RF phase for on-crest
beam acceleration by superconducting 9-cell L band
cavities as well as for the calibration of the cavity
gradient a method using beam loading effects during anRF phase scan was developed. The optimal RF phase foron-crest beam acceleration was found with an accuracyof 3°. A stable feedback operation was established. The
cavity gradient was calibrated with an error due to thecalibration method of less than 5%.

INTRODUCTION
In preparation for the International Linear Collider(ILC) [1] STF is operated at the High Energy Accelerator
Research Organization (KEK) in Tsukuba. In the current
configuration for the quantum beam project [2] the linear
accelerator for electrons consists beside others of a 1.3
GHz normal conducting photo-cathode RF gun as well as
two superconducting (SC) 9-cell TESLA type L band
cavities, which are driven by one klystron in theDistributed RF Scheme (DRFS) [3]. Both systems areoperated using digital Low Level RF (LLRF) control
techniques [4, 5].
Due to the absence of a pickup antenna in the STF RF
gun cavity a calibration method for the forward and thereflected signal in the waveguide had to be developed inorder to reconstruct the cavity signal, which is therequirement for the feedback operation.
For the on-crest acceleration using the SC cavities andthe feedback operation of the DRFS a method for thedetermination of an appropriate RF phase as well as forsetting the loop phase to 0° had to be developed.
Furthermore the beam-based calibration of the cavity
gradient was intended.

#RF GUN CALIBRATION AND
FEEDBACK OPERATION
Since the actual cavity signal is a superposition of theactual forward and reflected signal [6], it can bereconstructed by𝑈cav = 𝑈for + 𝑈ref .
(1)
The calibration of 𝑈for and 𝑈ref corresponds tofinding the complex matrix elements of∗𝑏 ) (𝑈for ),∗𝑑 𝑈ref𝑈( for ) = (𝑎𝑈ref𝑐(2)∗∗and ref
where forare measured values. It wasshown that the cross talk is negligible small so that 𝑏and 𝑐 can be considered as zero. There with equation(2) can be rewritten as𝑈∗𝑈
10( for ) = () ( for∗ ) .
0 𝑑/𝑎 𝑈ref𝑈ref(3)

This means 𝑑/𝑎 has to be determined, which can bearchived by using the reflection coefficient
𝑑𝑈𝑈

∗

𝑑∗
Γ = 𝑈ref = 𝑎 𝑈ref.
∗ = 𝑎Γ
forfor(4)

Performing an RF frequency scan (typical range of±100 kHz) by changing the phase constantly over time
during the second flattop of the pulse, which corresponds
to a value different from zero for y3 of the phase asΓ∗shown in Figure 1, and plottingΓ ∗ versus
results in a resonance circle as shown in Figures 2 and 3(blue). The point of full reflection can be identified as𝑐Γ∞∗ = |𝑐| + 𝑟 |𝑐| ,(5)Figure 1: RF pulse shape properties for amplitude (top)and phase (bottom).

momet@post.kek.jp- 265 -

Figure 2: Imaginary part versus real part of Γ ∗ (blue)and Γ (red).

Figure 3: Imaginary part versus real part of Γ ∗ :Measured data (blue) and fit (green).
where c and r result from a fit of a circle to the measured
data (see Figure 3). The point Γ∞∗ corresponds to
1 0 in reality, which allows the normalization
𝑑=𝑎1Γ∗∞.

Figure 5: Zoomed in region of amplitude (top) and phase(bottom) at first flattop versus time of reconstructedcavity signal (red) and set point (light blue) during
feedback operation.
Beside this the calibration method allows to obtain thecoupling coefficientby1+Γ

= 1−Γ0 .
0(6)Insertion of this in equation (3) and in equation (1) yields
the reconstructed cavity signal.
Once the calibration factor 𝑑/𝑎 is determined and setto the system, 𝑈cav is calculated in real-time. Thereby astable feedback operation was established yielding alongtime stability of/=00
in amplitude
and= 00
in phase, respectively. Figures 4and 5 are showing amplitude and phase of the respectivepulse.

Figure 4: Amplitude (left) and phase (right) versus timeof the forward signal (blue), the reflected signal (green),
the reconstructed cavity signal (red), and the set point(light blue) of an RF pulse while feedback operation.

(7)

BEAM-BASED CALIBRATION OF
SUPERCONDUCING CAVITIESAs shown in Figure 6 the DRFS waveguide system
consists beside others of several waveguide phaseshifters (WPS) and waveguide reflectors (WR). Thereby
the combination of WR 1 and WPS 3 as well as WR 2and WPS 4 are used to set thevalues of cavity 1(MHI 12) and cavity 2 (MHI 13), respectively [7].

Figure 6: Schematic of the DRFS waveguide system:
Klystron, variable hybrid, circulators, waveguide phaseshifters (WPS 1 and WPS 2), waveguide reflectors (WR1 and WR 2), waveguide phase shifters (WPS 3 andWPS 4), and superconducting cavities (MHI 12 and MHI13).

- 266 -By scanning the RF phase using WPS 1 and WPS 2consecutively while observing the beam induced change
in amplitude
at the flattop of the pulses [8] as shownin Figure 7 for each cavity the RF phases for on-crest
beam acceleration were determined.

Figure 7: Amplitude versus time of an RF pulse
including a beam induced drop-off during the flat top.

In a low power measurement of the two paths between
the klystron and the two cavities in the DRFS waveguidesystem (see Figure 6) after a RF phase shift calibration itwas shown that the phase difference between the twocavities is cav1cav =. Since the cavities aremounted in / distance the measured value is in good
agreement with the expected and required value of 0 ,showing that the developed and applied method iseffective.
The beam induced voltage, which causes to a drop in
amplitude as shown in Figure 9 for five different step
motor positions of WPS 1, can be calculated by𝑟𝑉ind = 𝐼b ∙ 𝑡 ∙ ∙ π ∙ 𝑓0 .
(8)𝑄

Those correspond to the maximal negativeandare identified by a fitting procedure. By taking the charge
per bunch into account as well as averaging over ten
pulses per step motor position, the calibration method
was improved so that the fluctuation due to the change ofthe charge per bunch over time could be smoothed out.
Figure 8 shows a respective phase scan. Besides
determining the RF phases the loop phase for thefeedback loop was set to zero and a reference table wascreated, which allowed a stable feedback operation.
Figure 9: Zoomed in region of amplitude versus steps intime of flattop including beam for five different WPS 1step motor positions.
Comparing this value with the ADC values allows acalibration of those. Taking the length of the cavities into
account yielded cavity gradients of cav1 = 1/and cav = 1/ . The errors for these values
due to the calibration method areandcav1 =. In respect of the estimated errors, thesecav =cavity gradients are in good agreement with themeasured beam energy of 40 MeV.

SUMMARY

Figure 8: Beam induced amplitude/charge per bunch
versus RF phase shift for cavity 1 (top) and cavity 2(bottom). The drop at cavity 1 above 77° of RF phaseshift originates from effects of far off-crest beam
operation.

By developing and applying a calibration method forthe forward and reflected signal in the waveguide of theSTF RF gun, the cavity signal could be reconstructed,
which allowed a stable feedback operation even in
absence of an actual pick-up antenna with a longtime
stability of/= 00
in amplitude and= 00
in phase, respectively.
A beam-based calibration method for the RF phase foron-crest acceleration as well as for the cavity gradientswas developed and successfully applied. The determinedcavity gradients were/andcav1 = 1=1/withanerrorof=cav
cav1
anddue to the calibration method.
cav1 =Furthermore the stable feedback operation of the DRFS
system was achieved.

- 267 -REFERENCES[1] http://www.linearcollider.org[2] H. Hayano, “Status of Superconducting RF Test
Facility (STF)”, Presentation, The 9th Annual
Meeting of Particle Accelerator Society of Japan,
Toyonaka, 2012, THPS096.
[3] S. Fukuda, “Distributed RF Scheme (DRFS) –Newly Proposed HLRF Scheme for ILC”,Proceedings of Linear Accelerator Conference
LINAC2010, Tsukuba, 2010, MOP027.
[4] T. Miura et al., “Performance of the μTCA DigitalFeedback Board for DRFS Test at KEK-STF”,
Proceedings of IPAC2011, San Sebastián, 2011,MOPC155.
[5] S. Michizono et al., “Performance of LLRF System
at S1-Global in KEK”, Proceedings of IPAC2011,
San Sebastián, 2011, MOPC157.
[6] A. Brandt, “Development of a Finite State Machinefor the Automated Operation of the LLRF Control
at FLASH”, Doctoral Thesis, Universität Hamburg,Hamburg, 2007.
[7] S. Kazakov et al., “L-band Waveguide Elements for
SRF Application”, Proceeding of Particle
Accelerator Society Meeting, Tokai, 2009,
FPAC22.
[8] M. Liepe, „Regelung supraleitender Resonatoren
mit Strahlbelastung am TESLA-Test-Linearbeschleuniger“, Diploma Thesis, Universität
Hamburg, Hamburg, 1998.

- 268 -