ABSTRACT 1 CEBAF UPGRADE CAVITY/CRYOMODULE

Transcription

1 Energy Content (Normalized) SC Cavity Resonance Control System for the 12 GeV Upgrade Cavity: Requirements and Performance T. Plawski, T. Allison, R. Bachimanchi, D. Hardy, C. Hovater, Thomas Jefferson National Accelerator Facility, Newport News, VA 23606, USA ABSTRACT The CEBAF upgrade cryomodules are designed modulo eight superconducting cavities. Each cavity has an individual tuning system. The resonance tuning function is accomplished by mechanical tension of the cavity using two mechanical driving devices, a stepper motor and piezo-tuner, a combination of slow and fast control [2]. The stepper-motor driven coarse tuner has a range of +/- 200 khz and resolution of 10 Hz. The piezoelectric part allows the cavity to be continuously tuned to within 1 Hz of resonance and has a range of approximately 2 khz. The detuning signal is measured and processed by a digital Low Level RF (LLRF) system and is fed to the resonance control algorithm. The PZT system will be packaged with eight amplifiers in one 19 chassis. The stepper motor controls will be packaged in a similar fashion. In this note we present the tuner specifications of the newly designed system. 1.1 SC cavity 1 CEBAF UPGRADE CAVITY/CRYOMODULE Recent optimization in the SRF cavity design for the CEBAF Upgrade, particularly the elimination of the stiffening rings from the cells, yields a factor of eight increase in pressure sensitivity of cavity frequency from previous 50 Hz/torr to 420 Hz/torr. Because of this the Lorentz-force detuning coefficient K L increased from 1 to 2 Hz/(MV/m) 2, yielding a tune shift of 800 Hz at 20 MV/m(17 times the resonance bandwidth at the design Q ext, see Figure 1. Microphonics background measured in the Horizontal Test Bed (HTB) is 4 Hz RMS [4]. Table 1 presents parameters of the described cavity CEBAF 6 GeV CEBAF Upgrade , Detuning (Hz) Figure 1: Resonance curves for 5 and 7-cells cavities 0.0 1

2 Cavity Detuning (Hz) Frequency Q 0 Loaded Q Cavity Bandwidth Lorentz Force Detuning K Maximum Gradient Cavity Mechanical Sensitivity Microphonics He Pressure Sensitivity Table 1. CEBAF Upgrade cavity parameters 1497 MHz 10^10 3.2*10^7 46 Hz 2 for HBT, 4.3 for VTA 19.5 MV/m 300 Hz/um 6 ~ 24 Hz 420 Hz/Torr Another aspect of microphonics is the vibration induced by the stepper motor. Figure 2 shows results of the test when the tuner was driven by half-steps. Typically a frequency detuning of 60 Hz was observed at 8 Hz of mechanical vibration Time (sec) Figure 2. Microphonics induced by stepper tuner 2

3 GeV Upgrade Cryomodule Electrical Parameters and Specifications Piezo-actuator /amplifier In the Table 2 we present requirements specify mostly by the designer of the cryomodule in comparison to measured performance of the already build devices. Similar Table 3 will be presented in the next chapter describing stepper controller. amplifier output Table 2 PiezoTuner System formal requirements present performance of the prototype Operational voltage range V [V] [V] Current 0-10 [ma] 0-50 [ma] Input capacity of the piezoactuator 21 [uf] limited by current only Modes unipolar unipolar drive connection negative output grounded negative output grounded Full signal bandwidth 1 Hz 5 Hz Output impedance specified by designer < 1 [Ohm] Output noise (static) specified by designer <20 [mv] amplifier input formal requirements present performance of the prototype Control analog signal analog signal Input voltage range 0-10 [V] 0-10 [V] Input impedance specified by designer > 10 kohm Input connector BNC BNC amplifier features formal requirements present performance of the prototype Gain fixed [15] fixed or adjustable Temperature drift specified by designer 100 ppm/c Long time stability specified by designer 100 ppm/h (no cumulative) Interlock specified by designer adjustable power over Voltage monitor BNC, 1/100 of out value 1/100 of out value, BNC, locally only 3

4 1.2.2 Stepper motor tuner/ Motion control and drive Table 3 Stepper Motor System formal requirements present performance of the prototype Stepper motor type bipolar/2 phases bipolar/2 phases Max. current per phase 1.2 [A] 0.2 to 3.0 Amps per phase current in 0.2 Amp increments Max. voltage 24 [V] 24 [V] Full steps per revolution Micro steps per full step Max. speed 500 Hz/minute 1 limited down to 300 RPM Control EPICS EPICS/direct fiber-optic Upper limit switch NC, +24 VDC NC, +24 VDC Lower limit switch NC, +24 VDC NC, +24 VDC 2 MECHANICAL TUNER OBJECTIVES Initial tuning of the cavity for operation at 1497 MHz Compensate slow drift of frequency due to He pressure fluctuation Control of stepper motor induced microphonics Provides secondary Lorentz Force detuning compensation during cavity turn on where Self Exciting Loop (SEL) is a first choice [3]. Detune cavity for bypass operations 2.1 Stepper tuner controller & driver The stepper tuner will be controlled by an FPGA based, 8-axis motion controller with an embedded EPICS IOC which uses a PC A single zone system consists of one FPGA board, IOC-PC014 and eight commercial stepper driver modules MD2S-L [5]. See Table 3 for detailed performance description. 1 HTB test is necessary to determine required speed of stepper motor as well as full step/detuning angle ratio. 4

5 Cavity Detuning (Hz) Figure 3: Stepper controller block diagram Microstepping motor driver The MD2S-L is a universal programmable microstepping stepper motor driver capable of driving size 17 to 42 stepper motors with 31 microstepping resolutions from 2 to 256 microsteps per step. Motor currents are selectable from 0.2 to 3.6 A per winding in 0.05 A increments. The MD2S-L is powered by a single supply voltage from +16 to +50VDC. 2.2 PZT driver current The resonance control algorithm will control a high voltage amplifier via a 12 bit DAC. Since we set the detuning loop bandwidth limit at f=1 Hz, considering the maximum drive signal V dr =+/- 75V and the capacity of piezo-element is approximately C=21 F, the amplifier peak current should not exceed i 2 C f Vdr 10 ma. In fact for the worst case scenario the compensation of the 60 Hz cavity detuning at 8 Hz caused by stepper tuner motion, (see Figure 2) the actuator will require a control current of 4.75 ma. See the following formula: ma 30 ma Mechanical Tuner Amplitude 60 Hz Frequency 8 Hz ma ma Microphonic Frequency (Hz) Figure 4. PZT current needed to control detuning 5

6 2000 Hz is a piezo tuner range. 60Hz i d 2 C 8Hz V dr ma 2000Hz Figure 4 shows the PZT amplifier current requirements vs. microphonics detuning and frequency. The red star indicates the PZT response needed to compensate for the microphonics induced by the stepper motor PZT controller diagram Figure 5. PZT driver diagram Figure 5 presents a block diagram of piezo driver with the APEX power operational amplifier [6]. Basic features of the amplifier itself are following: - Low quiescent current - Over 350 V/μS slew rate - Single Supply: 20V to 200V - Split Supplies: +/ 10V to +/ 100V - output current 75mA cont. ; 100mA Pk - up to23 W dissipation capability - over 200 khz bandwidth The PZT chassis contains eight amplifiers each capable of volts output. The output voltage is linearly controlled with a 0-10 volt analog input. A voltage divider with a follower was used for Vout/100 monitoring. An analog multiplier was used to obtain a continuous calculation of power out to the PZT. A comparator was used to obtain a TTL power over limit interlock output. 6

7 S21 dbm PZT drive test result Figure 6 shows FFT data for 10 Hz drive signal, value of SNR is at least 45 db Hz PZT drive - FFT Hz Figure 6: Fast Fourier transform of piezo drive signal Figure 7 shows transfer functions for small (5 V pp ), medium (36 V pp ) and full scale (150 V pp ) outputs. As we can see the unit has 10 Hz full step band width which far exceeds the PZT amplifier requirement. 10 PZT amplifier+piezo transfer function Amp vout 36vpp Amp out 5vpp Amp out 150vpp Freq(Hz) Figure 7: PZT transfer function 7

8 3 SUMMARY Both parts of the cavity resonance control system, stepper controller and PZT driver, were designed and successfully tested at Jefferson Lab. In all cases their performance exceeded the requirements. The stepper motor chassis was tested on the bench and with the Renascence cryomodule, confirming all expectations. The PZT amplifier bench test verified the current drive capability, sufficient operational bandwidth and low output noise level. The most challenging task during the design process was to determine the 12 GeV upgrade cryomodule control requirements. In some cases (where cost permitted) the RF Controls team decided to expand the performance of the devices beyond specified requirements to avoid compromising the cryomodule performance in the future. REFERENCES [1] [2] [3] [4] [5] [6] C. Hovater, J. Delayen, L. Merminga, T. Powers, C. Reece, RF control requirements for the CEBAF energy upgrade cavities, LINAC 2000, August 2000, Monterey, CA E.F. Daly, Overview of existing tuner system, ERL Workshop, MAR 2005, Newport News, VA C. Hovater, T. L. Allison, J. R. Delayen, J. Musson, and T. E. Plawski, A digital self excited loop for accelerating cavity field control, Proceedings of 2007 IEEE Particle Accelerator Conference, Albuquerque, NM, June 2007 C.E. Reece et al., Optimization of the SRF Cavity Design for the CEBAF 12 GeV Upgrade, 2007 SRF Workshop, Peking University, Beijing, China 2007 US Digital ( Apex Microtechnology ( 8