Journal of the Korean Physical Society, Vol. 63, No. 10, November 2013, pp. 1901∼1906

Design and Test of a Resonance Control System for Suppressing the PumpVibration Effects for the PEFP 13-MHz RF Cavity

Ying-Min Li,∗ Sung-Su Cha, Ji-Ho Jang, Hyeok-Jung Kwon,Young-Gi Song, Han-Sung Kim, Kyung-Tae Seol and Yong-Sub Cho

University of Science and Technology, Daejeon 305-350, Korea andProton Engineering Frontier Project, Korea Atomic Energy Research Institute, Daejeon 305-353, Korea

Tu-Anh Trinh

Korea Institute of Geoscience and Mineral Resources, Daejeon 305-350, Korea

(Received 26 October 2012, in final form 19 August 2013)

The Proton Engineering Frontier Project developed a 13-MHz pulsed, RF cavity for heavy-ionimplanter applications. Typically, slow changes in the room temperature and the mechanical vibra-tions of the vacuum device may be primary sources of disturbances, and the accelerating cavity ofthe implanter may not be able to operate at the resonance frequency owing to disturbance effects.We need a voltage-controlled oscillator phased-locked loop circuit to make a control system thatcould suppress the disturbance effects; thus, the accelerating gradient of the cavity always reacheda peak level for a given input power and coupling. An analog-circuit-based RF-frequency-trackingsystem was developed. Next, we obtained the optimal control parameters for the key control com-ponents. Finally, we measured the system performance between an open loop and a closed loop.The key point of the system design is to control the driving frequency that is used to operate theRF source by keeping the phase at around 0 degrees with respect to the resonance peak of thecavity. The experimental results showed that the fluctuations of the control loop error signal weresuppressed by about a factor of 10. The presented feedback loop is implemented as a standardproportional controller. The loop p-gain is 120 k.

PACS numbers: 07.05.Dz, 84.30.Qi, 84.40.DcKeywords: Frequency control, Accelerating cavity, Ion implanterDOI: 10.3938/jkps.63.1901

I. INTRODUCTION

For applications to materials and life science, the Pro-ton Engineering Frontier Project (PEFP) developed a13-MHz pulsed radio-frequency (pulsed-RF) cavity forheavy-ion implanter applications. Its geometrical struc-ture is shown in Fig. 1(a). The ion beam is acceleratedto typically around 120-keV for the helium ion by us-ing the harmonic time-dependent electrical fields alongthe drift tubes. The cavity has the amplitude-frequencyand the phase-frequency features shown in Fig. 1(b) [1].The nominal RF pulse duration is 1 millisecond, and itsnominal RF repetition rate is 1 Hz.

In the working environment of the cavity, a few un-expected disturbances always exist. The environmentaldisturbances are primarily from two sources: (1) the vac-uum devices’ mechanical vibrations (2) and the 13-MHzlinac’s room-temperature drift, and these results in res-onance frequency shifts for the cavity. When the im-

∗E-mail: liyingmin1979@kaeri.re.kr

planter is working, 3 pumps are always working. A dryscroll pump drives two turbo-molecular pumps (TMP)to work the system at a high vacuum level. Becausethe bellows connecting the dry scroll pump with the ionsource’s TMP are very long, the pump is very far fromthe cavity. Thus, in our system, we only care about thecavity’s TMP vibration disturbance, i.e., a 750-Hz dis-turbance. The pump’s quick vibrations resulted in fast±0.5-kHz resonance frequency variations of the RF cav-ity during the commissioning work for the implanter [1].We want to completely remove the effect for the reso-nance frequency of the cavity.

Because of these kinds of frequency perturbations, theaccelerating gradients of the cavity always fluctuate be-low the peak value for the given forwarded power andinput coupling coefficients. Our system is designed tosense the resonance frequency shift of the cavity and toadjust the output frequency of the RF source. Whenthe input signal’s frequency shifted from the resonancepeak of the RF cavity, the phase difference causes theRF source to change its output frequency, and the max-

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Fig. 1. (Color online) A 13-MHz RF cavity: (a) physicalstructure and (b) transmission features.

imized phase difference range is ±45◦. In the real ex-periment, we only care about the worst situation, whichis a frequency shift that does not go beyond the 3-dBhalf bandwidth of the cavity, i.e., less than 6.5-kHz [2,3]. The presented feedback loop is implemented as astandard proportional controller.

For the PEFP 13-MHz pulsed-RF implanter, we choseto control the RF source because it is cheaper and moreefficient. In this paper, Section II describes the systemdesign and a block diagram, the phase tracking of theresonance peaks, the structure of the experimental setup,and the component tests. In Section III, we present op-eration notes about the cavity test with this feedbacksystem and complete results, and we compare the per-formance for the given control loop by measuring thecontrol loop error signal between the open-loop case andthe closed-loop case. We also choose the cavity RF andcontrol parameters in an optimized way for the experi-ment. Section IV summarizes the achievements we ob-tained from the experiment.

II. EXPERIMENTAL SETUP

Figure 2(a) shows a block diagram of the fundamentalstructure of the control system, which includes a feed-back loop. The real experimental setup is shown in

Fig. 2. (Color online) Diagram of the control system: (a)structure of the RF control system for a 13-MHz heavy-ionimplanter, and (b) simplified control loop of the RF controlsystem represented with a transfer function.

Fig. 3. (Color online) RF control system’s experimentalsetup.

Fig. 3. Figure 4 shows the 13-MHz cavity’s measureddata: (a) the amplitude-frequency and (b) the phase-frequency S21 measuring curve. The optimal RF param-eter settings are shown in Table 1. The control systemconsists of 7 essential parts: (1) a RF source (250-kHzto 1-GHz RF signal generator, which consists of a pro-portional compensator, a frequency modulation (FM)function, a voltage-controlled oscillator (VCO) and aninternal trigger (pulse modulation), (2) a power ampli-fier (2500-W water-cooled), (3) a 13-MHz electric-field-accelerating copper-coated drum-shaped cavity (13-kHzbandwidth, 12.77-MHz resonance frequency), (4) a di-rect current (DC)-controlled phase shifter (0 - 7 V volt-age variable), (5) a 900-mV voltage bias circuit, (6) a

Design and Test of a Resonance Control System for Suppressing the Pump· · · – Ying-Min Li et al. -1903-

Table 1. RF test parameters in the resonation state of a 13-MHz cavity (δ = 0, room temperature 20 ◦C).

Item - Value Unit Remark Error

S11 -

- BW 13 kHz 18-dBm power, under-coupling for the input ±100 Hz

- Center 12.77 MHz Measured in 20 ◦C room environment ±1 kHz

- QL 1.045 k - Calculated with a 3-dB bandwidth ±100

- Loss –25.871 dB Return loss ±3 dB

S21 -

- Loss –46.18 dB Insertion loss ±3 dB

RF test-

settings

- PA average power 1200 Watt - -

- Source amplitude 7 dBm - -

- Source frequency 12.77 MHz - -

- PM pulse width 1 ms - -

- PM repetition rate 1 Hz - -

- FM deviations (LOOP proportional gain) 120 k - Linear range 0.1%

- Bias voltages 900 mV For protection of RF source -

- Phase shifter control voltages 3.1 V Phase difference kept to ±90◦ ±1 V

- Phase comparator response time 40 ns Phase output, any 15◦ change, 10% - 90% -

- Phase comparator dynamic range 143 ◦ Less than 10% deviation in instantaneous slope -

- Phase center point 900 mV AD8302, ϕa = ϕb ± 90◦ -

- Reference phase difference ±90 ◦ Between two signal paths -

- Slope –10 mV/◦ Opposite sign at –180◦ to 0◦ -

-Maximized

110 mV1. Depends on the loop configuration of total

±5 mVPick-up attenuations. 2. On-resonance. 3. Phase shifter

Signal cancels the natural phase difference between loops

phase comparator with –10 mV/degree phase sensitiv-ity (5-V power supply (PS)), (7) and an ‘EDLabs model330’ DC constant voltage source PS. We use it to sup-ply voltages to the phase shifter control and the biasingDC power supplies. Its control precision is around 0.1 V.This system has two important signals, the feedback sig-nal and a reference signal. The former was detected fromthe cavity, and the latter was directly obtained from theRF source. The two signals go into the phase compara-tor. The presented feedback loop is implemented as astandard proportional controller.

The typical disturbances added to the control loopwere 750-Hz single-frequency mechanical vibrationscaused by the Leybold TurboVac361 TMP. This TMPwas tightly attached to the cavity wall. If the cavity isworking in an off-resonance state due to various exter-nal disturbances, the detuning δ of the cavity becomesnon-zero; thus, the feedback signal has an extra phaseshift (∆Φcavity−detuning), which is induced by this dis-turbance. In general, between the reference and thefeedback signals, there is a phase difference owing to thephase shift effect of the detuning cavity. The RF source’sfrequency is controlled using this loop error signal, which

serves as an FM envelope to modulate the center fre-quency (i.e., carrier frequency) in the RF source [4–12].We call this input signal a “control loop error signal”,which comes from the phase comparator. We also testedthe FM modulation function of the RF source. We ex-amined the variation of the output frequency by using aspectrum analyzer. The RF source is an Agilent E4438C1GHz signal generator.

The phase comparator generates the corresponding er-ror signal based on two input signals, the FM outputsignal and the signal picked-up directly from the cav-ity, as mentioned above. Thus, the RF source can out-put a varying frequency that follows the variation in thephase difference between the two signals. For the 3-dBbandwidth (i.e., half-power point) of the cavity (see Fig.4(b)), we need to cover the range of a ±45◦ phase shift;thus, the system’s (lumped-circuit model) quiescent op-eration point Qsys (Q-point) should be located at 0 de-grees at 0 mV. The phase comparator has a phase centerpoint (PCP), which is approximately 900 mV, which cor-responds to a 90-degree phase difference between the twoinput RF signals. However, the phase comparator’s qui-escent operation point must work at mid range in the

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Fig. 4. The 13-MHz cavity features and resonation curves: (a) log amplitude-frequency curve and (b) phase-frequency curve.

range of 0 to 180 degrees (i.e., Qphase−comp = 90 degreesat 900 mV, just the PCP). If so, the device can work in avery good linear range. If this meaningful linear measur-ing range is to be covered and the measurement error isto be avoided, the reference (or ‘natural’ or ‘ordinary’)phase difference (between the feedback signal and thereference) should be set to 90◦ owing to inconsistent Q-points. We have to use a 900-mV bias circuit to reducethe 900-mV DC level to 0 mV for the loop error signals,i.e., to shift comparator’s Qphase−comp point (PCP, 90◦at 900 mV, range: 0 - 180◦) to the system’s Qsys point(0◦ at 0 mV, range: ±45◦). Thus, the system’s quiescentoperation point is 0 degrees for a 0 - V error signal.

If ∆Φcavity−detuning is equal to zero, the cavity has anon-resonance operation. Then, the cavity will not bringany extra phase shift to the control loop. Even underthese circumstances, the feedback loop signal still has adistinct phase with respect to the reference signal dueto the lengths of the cables and/or the devices installedin the two signal paths being different. We call this anordinary/natural phase shift. A voltage variable phaseshifter, which is used to cancel the natural difference,i.e., to compensate for the loop phase variations from amismatch in the cable lengths is in the feedback loop.We only use the range of a 0-V to 5-V control voltagewhen we adjust the loop phase shift ∆ϕ.

The phase shifter only changes the phase of the feed-back loop signal. Thus, the total phase difference Φtotal

between the two signals consists of both the phase-shifter-induced and the cavity-induced phase differences:Φtotal = Φshifter + Φcavity when the cavity is workingoff-resonance. The cavity will have a phase shift Φcavity

due to disturbances and this phase difference Φtotal be-tween the two signals will become non-zero. In an analogphase comparator, the phase difference generates a cor-responding voltage signal and is a linear function. InFig. 2(b), for the closed-loop operations, in the loop, theVCO has a linear transfer function of 120 kHz/V, the

cavity has a function of 7 degrees/kHz (half bandwidthof 6.5 kHz for cavity), and the comparator has a constanttransfer function of 0.01 V/degree. Thus, the total loopgain K = KV CO × Kcavity × Kcomp = 120 kHz/V × 7degree/kHz × 0.01 V/degree = 8.4. Assuming H(s) = 1for simplicity, one finds

yD =D

1 + G=

11 + 8.4

· D ≈ 110

· D . (1)

This closely matches the experimental data in this paper.Next, we tested the feedback loop circuit without a

cavity. The test included the shifter, the comparator,and the RF source. In this low-power test, we varied thephase in either path by adjusting that voltage variablephase shifter. We then checked the output frequencywith the same spectrum analyzer. We also changed theFM deviations (1 MHz, 500 kHz, and 100 kHz) in thetest. The output frequency can be simply regarded as aconvolution between the phase comparator device’s spe-cific feature curve and the phase shifter device’s specificfeature curve. From this experiment, if we choose therange of 2 - 5 V for the control voltage of the phaseshifter, thus the output frequency will have good linear-ity.

III. RESULT OF THE RF CONTROL LOOP

The setup for the high-power RF test for the wholecontrol loop is shown in Fig. 3. We have to measurethe control loop’s error signal. The RF source’s pulsesignal is amplified to around 60.7918 dBm by using ahigh-power amplifier. The high-power RF signal is for-warded into the 13-MHz cavity through a 20-cm loopcoupler. We picked up the cavity’s power signal througha 1-cm-long spear-shaped field probe. A crystal detectorwas used to measure the cavity’s power. It converts the

Design and Test of a Resonance Control System for Suppressing the Pump· · · – Ying-Min Li et al. -1905-

Fig. 5. (Color online) The 13-MHz cavity features andresonation curves: (a) log amplitude-frequency curve and (b)phase-frequency curve.

RF power of the pickup signal into a DC voltage at itsoutput load; we then utilized an oscilloscope to examinethe amplitude. The video output of the detector has asquare-law relation with the picked-up signal.

The following two initial conditions must be satisfiedin the high-power circuit test: (1) the pick-up signal mustbe the maximized value at the initial settings and (2) theloop phase difference must be 0◦ at the initial settings;i.e., the loop error signal should be 0 mV at the initialtime. The high-power test operating procedures can beexplained as below:

(1) SCAN CAVITY’S FREQUENCY

First, we use a network analyzer to measure the reso-nance frequency of the cavity. This is not necessary, butis just used to save time.

(2) LOCATE CENTER FREQUENCY

We then set this first value as the center frequency ofthe RF source. Then, we adjust the center frequency ofthe RF source so as to obtain the maximum amplitudeof that pick-up signal. When the cavity signal becomesmaximized, we can then confirm that this new value isthe resonance frequency of the cavity. We manually lockthis frequency in the RF source and turn on the FMmodulation switch.

(3) CLOSED LOOP

Next, we have to change the loop’s phase shift to ∆ϕ =0◦ to make the control loop’s error signal approach a zerovalue for as long as possible (The reason, why should wedo this, is that we have to keep the loop stability mar-gin as much as larger if possible). Figure 5 shows thedetected pick-up signal vs. the loop’s phase-shift controlvoltage. If we adjust to a different loop phase shift, the

Fig. 6. (Color online) Detected amplitude of the controlloop error signal during a one-hour period.

pick-up signal decreases due to a detuning of the cavity,and the goal is to find a proper value to compensate forthe cable-introduced phase shift (ordianry/natural phaseshift). And then we found that a 3.1-V DC control volt-age was the best value for this loop cable compensation,and the red arrow in Fig. 5 indicates this choice.

Figure 6 shows the amplitudes of the control loop er-ror signals for open and closed loops. In open-loop RFcontrol, the average value of the error signal is about–22.7459 mV. In closed-loop RF control, the error sig-nal was used to adjust the output frequency; the errorsignal was then controlled within a narrower range com-pared to the open-loop case. The signal statistics consistof 1-hour open-loop and closed-loop RF tests. The σ(standard deviation) of the open-loop case is 7.32 mV.For the closed-loop, σ is around 0.70 mV for the identicalforwarded power and input coupling. In the acceleratorsystem, a 750-Hz single-frequency disturbance results inremarkable fluctuations in the loop error signal, and theexperimental results prove that the fluctuations in thecontrol loop error signal can be reduced. When we com-pare the closed-loop control with the open-loop control,we can obtain a suppression factor of approximately 9.6%for the closed-loop control. The optimal loop p-gain is120 k.

IV. CONCLUSION

We employed a phased-locked loop (PLL) to stabilizethe operation point of the RF implanter cavity at its res-onance peak. The key point of the system design is tocontrol the frequency driven by the RF source by keep-ing the phase at around 0 degrees with respect to theresonance peak of the cavity. A 750-Hz single-frequencydisturbance exists in the implanter system. The distur-bance results in remarkable fluctuations in the loop errorsignal. With only a simple proportional compensation

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scheme, fluctuations in the cavity operation point due tovibrations can be easily suppressed by about 10 timesunder ordinary laboratory conditions. The optimal loopp-gain is 120 k.

The test results showed that the phase of the frequencydriven by the RF source can be quickly stabilized ataround 0 degrees with respect to the resonance peak ofthe cavity with a closed-loop operation. Also, the con-trol system can efficiently stabilize the cavity field at itsmaximum value.

ACKNOWLEDGMENTS

The work has been supported through the Proton En-gineering Frontier Project by the Ministry of Education,Science and Technology of the Republic of Korea.

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