Technical Systems
3.9 LOW LEVEL RF CONTROLS .1 Overview
The Low-Level RF system (LLRF) controls the phase and amplitude of the RF cavities used to accelerate the beam, and is essential for stable and reliable beam operation. The LLRF includes feedback and feed-forward, exception handling and extensive built-in diagnostics with suitable speed and accuracy. Each of the ∼650 L-Band RF units in the main linacs, sources and bunch compressors have a LLRF controller, as do the damping ring RF stations.
LLRF also controls the crab cavities in the beam delivery and various RF diagnostic devices.
A primary challenge for the ILC LLRF is the large number of cavities driven by a single klystron. The LLRF controls the vector-sum of all cavities as well as controlling the individual cavities. Most of the needed requirements have been demonstrated in the LLRF systems in operation at the FLASH facility at DESY [123]. The DESY LLRF uses state-of-the-art technologies for digital control of the operational parameters. Similar systems are being implemented at FNAL and KEK.
3.9.2 Technical Description
The performance requirements for the LLRF are set by the gradient desired from the cavities and by the stability required for beam parameters such as energy and energy spread, both bunch to bunch and pulse to pulse. There are also stringent requirements on the bunch compressor RF to set the arrival time of the beams at the IP, and on the crab cavity RF to fix the beam position at the IP.
Three issues of particular importance for the ILC LLRF are:
1. Lorentz force detuning: The radiation pressure of the electromagnetic field during the RF pulse deforms the cavity and pulls it off resonance. The static detuning (∆f ) due to the Lorentz forces is proportional to the square of the accelerating field (Eacc) and is approximately 600 Hz [124] for operation at design gradient in the main linac (31.5 MV/m).
To maximize the RF power efficiency, and to reduce the electric fields at the cavity input coupler, it is essential to cancel the Lorentz force detuning by a fast frequency tuner (for example, piezoelectric actuators).
2. Microphonics: External mechanical vibrations can be transferred to the cavities via the supporting system within the cryostat. Modulation of the resonant frequency due to microphonics is estimated to be ∼10 Hz rms. This modulation is not correlated to the macro pulse and and therefore can only be corrected by the feedback system.
3. Beam loading: The beam loading by individual bunches is about 0.15However, slow bunch charge fluctuations within the bandwidth of the RF system cause cavity vector disturbances that need to be controlled on the order of 0.05% at each station as the bunch charge fluctuations are correlated through the accelerator chain. Bunch charge is measured in the DRs and processed by the LLRF to create a correction feedforward term before beam is injected into the linac.
The RF systems in the main linacs and RTML require tight field control on the order of up to 0.07% for amplitude errors and 0.35◦for the phase. Due to microphonics, the measurement of the vector sum must be calibrated to an accuracy on the order of 1% for amplitude and
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Summary of tolerances for phase and amplitude control. These tolerances limit the average luminosity loss to <2% and limit the increase in RMS center-of-mass energy spread to <10% of the nominal energy spread.
Location Phase (degree) Amplitude (%) limitation correlated uncorr. correlated uncorr.
Bunch Compressor 0.24 0.48 0.5 1.6 timing stability at IP (luminosity)
Main Linac 0.35 5.6 0.07 1.05 energy stability ≤0.1%
1.0◦ for phase. The phases of crab cavities in the beam delivery system must be stabilized to better than 0.015◦. Table 3.9-1 gives an overview of the regulation requirements of the Main Linac and RTML bunch compressor.
Besides field stabilization, the LLRF provides automatic beam-based system calibration and diagnostic signals to the accelerator control system. Exception handling is required to avoid unnecessary beam loss and to allow for maximum operable gradient.
Availability and maintainability are also critical considerations in the LLRF system de-sign. Although most of the LLRF system components are located in the service tunnel, the large number of units requires a high availability design. Possible failure modes must be understood, their operational impacts examined, and mitigation measures developed and implemented. Adequate redundancy such as a simple feed-forward technique in the complex feedback scheme should be an integral part of the system design. Built-in diagnostics for both hardware and software are required to support preventative maintenance and increase reliability.
3.9.3 Technical Issues
3.9.3.1 Hardware Architecture
The most basic function of any LLRF control is a feedback that measures the cavity field vector and attempts to hold it to a desired set-point. The vector difference between the measured field and the set-point is filtered and amplified, then used to modulate the klystron drive and thereby the incident power to the cavities. The forward and reflected power signals are also processed to measure the resonant frequencies of the ILC cavities, for automated adjustment by slow motor-controlled tuners and fast piezoelectric actuators. The architecture of a typical LLRF control system is shown in Figure 3.9-1. The signal from the master oscillator, brought through the RF distribution system, is used as the RF reference.
The LLRF has to combat numerous perturbations with various time patterns and fre-quencies. Some of these perturbations recur at the machine repetition rate (5 Hz for ILC), like Lorentz force detuning and beam loading. An adaptive feedforward system is used to compensate for the average repetitive errors. The set-points for cavity fields are also imple-mented in a table to accommodate the time-varying gradient and phase during the cavity filling.
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FIGURE 3.9-1. Typical configuration of an RF control system using digital feedback control.
3.9.3.2 Digital Technologies
The key technologies to be used are modern Analog to Digital Converters (ADCs), Digital to Analog Converters (DACs), as well as powerful Field Programmable Gate Arrays (FPGAs) and Digital Signal Processors (DSPs) for signal processing. Low latency can be realized, with time delays from ADC input to DAC output ranging from a few 100 ns to several µs depending on the chosen processor and the complexity of the algorithms. Gigabit links are used for the high speed data transfer between the large number of analog input and output channels and the digital processor as well as for communication between various signal processing units.
Typical parameters for the ADCs and DACs are sample rates of 65-125 MHz and 14-bit resolution. The signal processing uses FPGAs with several million gates, including many fast multipliers. More complex algorithms are implemented on slower floating point DSPs
A down-converter module translates the 1.3 GHz RF cavity probe signal to the Intermedi-ate Frequency (IF) where it can be digitized and processed further. The down converter can degrade overall performance if not properly designed. Problems with nonlinearities, thermal noise, phase noise and thermal stability must be addressed in order to maintain the integrity of the detected signal from the cavity. The up-converter module translates a digitally gen-erated IF signal back to the RF in a process similar to that of the down converter. The up converter has less stability issues since it is within the feedback loop.
A fast piezoelectric actuator and a slow motor-driven tuner control the resonant frequency of each individual cavity. The frequency error of the cavity is measured during and after the flattop. This error can be reduced by suitable excitation of the piezoelectric actuator (fast tuner), or it can be compensated via additional RF power. The motor-driven tuner is only used to correct for long-term drifts. The station LLRF system must interface to High Level RF, beam transfer control, machine protection, sector and global energy and phase
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regulation, and the control system. A control system IOC is built into the LLRF system to handle parameter and data collection.
3.9.3.3 Software Architecture
A major benefit of a digital RF feedback and feed-forward system is that it supports auto-mated operation with minimal operator intervention. This is accomplished by deploying a number of algorithms to maintain best field stability (i.e. lowest possible rms amplitude and phase errors), to allow for fast trip recovery, and to support sophisticated exception handling.
Beam-based feed-forward further improves the field stability. Figure 3.9-2 shows the basic functional diagram of the LLRF software system.
FIGURE 3.9-2. Basic functional diagram of the LLRF software system.
The software implementation of the RF control system must also support high availability.
The main requirements for the algorithms are low latency for feedback, modularity to simplify interfacing, and support of a high degree of automation. Important applications include exception handling, built-in diagnostics and beam-based feedback.
subsubsectionSoftware Implementation
The massive parallel processing in the FPGAs provides low latency for the feedback algorithm. Complex algorithms requiring floating point calculations such as adaptive feed-forward can be also implemented.
The setting of system parameters and piezoelectric tuner control are implemented on floating point DSP processors since the latency requirements are not as stringent. Auto-mated operation can also be implemented on a middle layer server CPU since the timing requirements are not as critical.
The distribution of the modular algorithms requires well-defined interfaces to ensure sim-plicity in performing trouble shooting, maintenance, and upgrades. Low latency links use in-house protocols while commercial protocols are available for links needing high bandwidth but not low latency.
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System redundancy is achieved with algorithms, which calculate the key results from multiple signal sources. It is, for example, possible to calculate the cavity field from forward and reflected power although the measurement error is larger. Any discrepancy between the independently derived signals flags potential errors in hardware or algorithms.
Data storage is provided locally on most processor boards and is distributed to the central servers between pulses for further signal processing. With almost 15,000 cavities to control, automation is essential to ensure simplicity of operation and high availability. To support automation, the front-end hardware and software must as a minimum include the following features: field vector measurement, loop phase and loop gain, loaded Q and cavity detuning, beam phase and beam induced voltage, calibration of cavity field and phase, vector-sum cal-ibration, calibration of forward and reflected wave, beam loading compensation (current and phase), klystron linearization, exception detection and handling, RMS field errors, warnings and alarms.
It is desirable to implement the algorithms as close a possible to the LLRF station con-troller to reduce network traffic. However, if the algorithms and applications are implemented in middle layer servers or as client applications, it can simplify the programming, facilitate later upgrades and improve maintainability.
3.9.4 Components
Table 3.9-2 gives a rough parts count for the components in the baseline LLRF system for a single RF unit in the main linac.
TABLE 3.9-2
Rough part count for the components in the baseline LLRF system for a single RF unit at the main linac.
Module Specification Quantity
Precision cable 1/2 Coax–low temp.coef. 94
Down converter 1300MHz to IF 95
ADC channel 14 bit, 65MHz or higher 95
FPGA & DSP State of the art 3 to 10 each
DACs 16 bit, 100 MHz or higher 6
There are a total of 14,540 cavity modules in the main linacs, where 560 klystrons (i.e.
560 RF units) provide the drive power for 26 cavities each. The e−source, e+source, RTMLs have 11, 39 and 36 RF units, respectively. The e− and e+ damping rings have 10 klystrons driving 36 superconducting cavity modules in total. Each of these cavity modules has three signals monitored by the LLRF, a cavity field probe, and a forward and reflected power signal.
Each signal is routed in temperature-stabilized coaxial cable.