2012 4th International Conference on Computer Modeling and Simulation (ICCMS 2012)
IPCSIT vol.22 (2012) © (2012) IACSIT Press, Singapore
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1 Department of Electrical Engineering, Indian Institute of Technology Kanpur, India
2 Visiting Scientist, Fermi National Accelerator Laboratory, Batavia, Illinois USA 60510
In present research work, a possible design for a control system that can be used to eliminate radiation and attenuation noise that results in high power and frequency applications, such as in superconducting RF cavities [1] used in particle accelerators is described. Simulation and experimental testing of the design has been performed on a
LabVIEW-8.6 console and modelling of the controller has been done using real time data capturing. The solution employs modulating a second signal on the main carrier and measuring the phase difference between two successive points in the waveguide to calculate the detuning of the resonator, which is equal to the difference between the cavity’s natural frequency and the actual. Based on this information, the tuner automatically tries to compensate for the deviation observed. Finally the entire system is assembled on a movable cart and we test our system that achieves the desired tuning capabilities. The design, simulations, experimental testing and results of the setup have all been described.
High power Tuners, Instrumentation and Control systems, Super-conducting RF cavity tuning
Modern particle accelerators involving linear acceleration systems (LINACs) use super-cooled niobium cavities [2] to accelerate particles to about 0.9995c (where c= Propagation velocity of light in free space).
The cavity is super-cooled using liquid helium and at the desired temperature, it enters into a superconductive state. A high frequency electromagnetic wave is then generated in this waveguide using a suitable source and particles are accelerated using this wave. It is necessary to tune the cavity such that its natural frequency matches that of the carrier electromagnetic wave travelling through it [3] . This is needed to ensure no attenuation losses or noise generation occurs due to a non-resonating state of the cavity. Since the natural frequency of the cavity depends on its physical dimensions, it is required to minimally stretch or compress the cavity for optimal tuned conditions. Manufacturing technologies for cavities create small but significant dimension errors and other deformations are caused due to irregular heat generated by electromagnetic waves. Thus a fast, real time tuning mechanism is needed to maintain the efficiency of this system. Small deviations from resonance can lead to severe losses in energy being effectively available for particle acceleration.
The setup is based on the AD8333 I/Q Demodulator board from Analog Devices . The phase shift between the two waveforms is measured and used to calculate the detuning of the cavity. This is achieved by modulating a observation signal on the base carrier and use two signal probes; one at the source which provides the power to the RF superconducting cavity (forward signal) and the second at the end of the cavity (backward signal).
The signals are demodulated to baseband waveforms using the AD8333 I/Q board and fed into an Analog to Digital conversion (ADC) card. A LabVIEW 8.6
program (Fig.1) calculates the phase difference between the two waves. Once that phase difference and the difference in the magnitude of the signals is calculated, it is easy to calculate the detuning of the resonator, which is the deviation of the resonance frequency from the nominal one.
The board has two RF inputs and a Local oscillator input for synchronization. The local oscillator is fed a sinusoidal wave whose frequency is four times of that of the RF input. A divide by four circuits generates
1 Corresponding Author: Tel:+91-9795530426
Email(s)-pkagrawal89@gmail.com,agrawalp@fnal.gov,agrawalp@iitk.ac.in
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the internal 0° and 90° phases of the local oscillator that drives the pair of matched I/Q Demodulators. This module is used in a cart, which serves as the tuner for the real tests conducted in the laboratory.
The signals received by the system are investigated and the condition for the best possible distortion free output is acquired from the subsequent simulations. Noise levels and higher harmonics are seen by the
First Fourier Transforms [4] of the signals. The block diagram of the setup is shown in (Fig.2)
Fig. 1: The LabVIEW program on the computer analyzing the samples from the IQ Demodulator
Note: The Signal to Noise ratio is approximately 30dB. Clockwise from top a) The two signal waveforms from the front (red) and the back (white) of the cavity, The phase difference between the two waves is clearly evident b)
Amplitude versus time for one half cycle c) The First Fourier transforms of the signals with noise highlighted in green and red, sampling done at 50k samples per second
Analog to Digital conversion [5] : For the analog to digital conversion a National Instruments ADC (PXI
6259) is used .
However the sampling frequency is not very high and the issue of aliasing is evident. To avoid aliasing, the input to an ADC must be low-pass filtered to remove frequencies above half the sampling rate. This filter is called an anti-aliasing filter , and is essential for a practical ADC system that is applied to analog signals with higher frequency content.
All ADCs work by sampling the continuous waveform at discrete time intervals at intervals of the sampling time. Their output is therefore an incomplete picture of the behavior of the input. If the input is known to be changing slowly compared to the sampling rate, then we can average and predict the value of the signal to be in between the two measured values. However this is not applicable in the case of a signal that varies at a rate greater than the sampling rate.
If the input signal is changing rapidly than the sampling rate, then spurious signals called aliases are produced at the output. Since electromagnetic wave frequencies are in the order of 10 9 Hz , the problem of aliasing is particularly evident in our situation. An electronic anti-aliasing filter in LabVIEW achieves the desired results.
Using the methodology described above, a SNR (Signal to Noise Ratio) of approximately 30dB is easily achievable. But, since this is practically insufficient to achieve a stable and reliable system, there is a need to improve the design by further modelling and improving the control system.
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The probes used in the system are standard Hall Effect [6] probes that work on the principle of electric drift current being produced inside a conductor placed inside a magnetic field. Since all Hall Effect probes carry a certain voltage to sense the effect of the field present inside the cavity, the noise generated is experimentally correlated to the probe voltage level.
Fig. 2: Block Diagram of the LabVIEW program to analyze the output of the I/Q Demodulators
The probe voltage is safely reduced to 63mV and a SNR ratio of about 80dB is achieved for the first time
Care must be taken to reduce the voltage to such low levels. If the cavity is not operating at super-cooled temperatures, the above correlation will not remain true. This is due to the inevitable addition of thermal noise voltage v=kT (Where k is the Boltzmann’s constant and T is the temperature of the cavity ). At an operating temperature of about 300 K this noise figure is of the order of 26 mV and causes instability of the control system, being significant to the probe input power. The probe voltage could be safely reduced below 63mV if the cavity is at liquid helium temperatures; however detection of sinusoidal waveforms is particularly difficult due to evident distortion patterns at extremely low voltages.
For the RF inputs the AD8333 chip acts as an amplifier . The gain of this stage is approximately observed to be 9.50. Thus, the ranges of the signals coming from the cavity should be matched to the input ranges of the IQ demodulator. The maximum level for them is 280 mV at 50-ohm impedance. The optimum results are hence seen at much lower amplitude in the range, at about 100mV . The input level is changed to see if there are any changes in the output. It is seen that the output is the same each time, but certain higher frequencies were added, leading to noise. This is evident from the First Fourier transforms of the output signal, which starts showing peaks at higher frequencies. Hence a level of 100mV is chosen for the sinusoidal input to the AD8333. This voltage level achieves a SNR of about 80dB (Fig.3), which is sufficiently large for our application.
The offset at the outputs of the IQ demodulator must be compensated. The offset level varies on the input amplitude and changes with time. Hence it is sufficient to compensate it with the value of the output when there is no input and the power supply is on. This being done over a large time interval suffices to compensate the offsets to a tolerable level.
In a cryomodule , the cavity is composed of eight individual cells. Hence sixteen probes are used and the input signals for each cavity cell are acquired. Two eight ended output splitters are used to measure the signals and supply them to eight AD8333 boards. The NI-PXI is set to sample at a rate of 102.4 kS/s , which is reasonable for cavity tuners.
A Stepper motor is used for the tuning device mechanism. A stepper motor is a brushless, synchronous electric motor that can divide a full rotation into a large number of steps [7] . The motor's position can be
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controlled precisely without any precise feedback mechanism as long as the motor is carefully sized to the application. A stepper motor, which had which two windings and a multiple stack type rotor, is used. For the motion controller, a NI card, the PXI-7340 Motion Controller and a Danaher [8] P70530-SDN power amplifier are utilized. The Danaher is a stepper driver and is used to provide the required current to the motor. The motor axle is fixed to the external cavity enclosure and this serves the purpose of compression or elongation required in tuning. A LabVIEW 8.6 program serves to control the motor motion of the stepper motor (Fig.4).
Fig. 3: Improved setup with an 80 dB signal to noise ratio achieved from the AD 8333.
Various options are added in the user interface for the number of steps, step size, rotation speed
(steps/second) and the acceleration or deceleration (steps/sec 2 ). The program involves a two-axis control for up to two stepper motors, for optimum tuning in x-y coordinate plane. The setup does not involve tuning on the z-axis as the two-axis setup provides reasonably effective control for the niobium cavities described above.
Fig. 4: Block diagram of the motor motion control panel in LabVIEW
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The method to construct an efficient, space saving tuner for applications where the power involved with the operation and the levels of precision required are both very high is described. The entire mechanism can be assembled on a portable cart (Fig.5) for easy transfer and installation. The system can also be effectively employed in other high power and high frequency applications for effective results. Future work in this area could possibly include complete 3-D control in the x, y and z coordinate system, which could prove to be far more versatile and find its application in a variety of emerging technologies such as in automobiles and nuclear power systems.
Fig. 5: The assembled tuner cart
The author would like to thank his mentors Dr.Shekhar Mishra, Dr.Yuriy Pischalnikov, Mr.Roman
Pipilenko all at the Fermi National Accelerator Laboratory, USA, Mr.Sanjay Shukla at the Indian Institute of Technology Kanpur and Mr. Manoj Agrawal, Executive Director Research, Indian Railways for their invaluable support and guidance throughout the course of this work and the documentation of this paper.
Finally the publication of this work has been possible by the generous funds available to the author through the Dean Academic Affairs, Dean of Resource Planning and Generation, Dean Research and Development and Head of the Department of Electrical Engineering, also at the Indian Institute of Technology Kanpur.
[1] Safa H. et al, Progress and trends in SCRF Cavities for future accelerators, Proceedings of the EPAC 2000,
Vienna, Austria
[2] Hasan Padamsee: RF superconductivity: Science, Technology and Applications, 2009 Wiley-VCH Verlag
GmbH & Co. KGaA, Weinheim
[3] Harms, E et al., Status of 3.9GHz superconducting RF cavity technology at Fermilab, Particle Accelerator
Conference, 2007. PAC, IEEE
[4] AV Oppenheim, AS Wilsky: Signals and Systems, books.google.com
[5] DH Sheingold, Analog-digital conversion handbook, Analog Devices Inc., 1972
[6] CL Chien, the Hall Effect and its applications, 1979
Motor www.solarbotics.net/library/pdflib/pdf/motorbas.pdf
[8] Danaher Motor drivers, P7000 series specifications, http://www.danahermotion.com/
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