In linear accelerators (linacs), the trajectory of the beam, which is comprised of accelerated particles, is stabilised and bent to follow a circular path or ring. This meticulously controlled path is referred to as the “orbit”.
The reference orbit to which the beam is to be steered may be either a “design orbit” or a “golden orbit”. A design orbit is an ideal orbit, flat except for where intentional bumps are applied. Bumps are used in colliders to separate the beams at the non-colliding interaction regions. In electron accelerators, the bumps are used for the generation of x-rays (as a result of bending the beam). A “golden orbit” is a previously acquired orbit recreated for experimentation purposes.
A linac’s global feedback loop is comprised of many components. First, a beam position monitoring (BPM) system, consisting of several hundreds of strategic locations along the ring, measures the position of the beam in the flat plane. Secondly, a global BPM data distribution network sends the measurements to a central orbit controller where a digital controller computes the corrections to be applied to the beam. These corrections are sent back to the orbit correction magnets (OCM) spread along the ring. Finally, corrector magnets are electronically driven to steer, stabilised, bend, and focus the beam.
The global feedback system uses as input position measurements from over one hundred BPMs, per plane (X-Y), per ring. The use of multiple BPMs and correctors enables a centralized algorithm to determine the applied corrections. The overall performance of the feedback system depends on the type of the digital controller and the rate of the loop. Target rates can vary from as low as one 1 Hz (slow orbit feedback) up to a few kHz (fast orbit feedback).
The following figure shows a typical global orbit feedback system.
Beam Position Monitor (BPM)
BPM modules are electronic modules that perform fast analog processing of beam pickup signals. They consist of wideband analog modules and are capable of non-interceptive beam position measurements.
BPM modules are found in linacs, microtrons, and transfer lines. They can measure the position of three types of beams: single bunch, macro-pulse, and continuous wave (CW).
- Can work in the S-band, L-band, and X-band. L-band and S-band beams can be processed, provided the bunch groups are short (<3 ns).
- Bunches at any repetition rate up to 500 MHz can be measured.
- A 5-MHz repetition of individual bunches can be distinguished from one another.
- Macro-pulses or single bunches up to few MHz repetition rate are measured individually.
- CW beams can be measured continuously.
- Beam position motions up to 5 MHz can be observed.
The front-ends’ X and Y outputs are typical analog ±1 or 2V signals. The precision of each position reading is approximately 10 μm. The analog X and Y coordinates from the BPM modules are continuously digitized with high speed analog-digital converters (ADCs). The digitized signal is referred to as the “beam position monitor data” (or simply “BPM data”).
Nutaq’s MI125 FPGA mezzanine card (FMC) provides 32 ADCs and can be used to digitize the X-Y data from up to 16 BPMs. Multiple MI125 cards can be combined in a uTCA chassis.
BPM data acquisition on the orbit controller is either interrupt or timer-driven, and data on all the channels is acquired simultaneously. Nutaq's RTDEx and Mestor interfaces, combined with the MI125's external clock input capability, provide a host-driven synchronous external triggering system for synchronised acquisitions across all nodes.
Orbit Controller (OC)
The orbit controller serves as the system's control center. It has the following responsibilities:
- Orbit display and BPM/corrector disabling functionality
- BPM data acquisition
- Response matrix measurement and building the constraint vectors
- Performing inversion and solving the system to calculate the orbit corrector values
- Calculating the correction using Singular Value Decomposition (SVD) calculation
- Running the orbit PDI control algorithm
- Computing and distributing the set points to the Horizontal and Vertical Corrector Magnets (HCM/VCM), to over one hundred corrector dipoles per plane per ring.
Nutaq's uTCA platform includes an SAMC-514 Intel QuadCore i7 embedded CPU to perform system’s management functionalities such as orbit display, enabling/disabling BPM/corrector, and logging data to a SATA hard disc.
Nutaq's uTCA platform uses the Nutaq Perseus 601X Virtex-6 FPGA carrier card for performing the real-time computations part of the control algorithm. Rahmati et al. (FPGA Based Singular Value Decomposition for Image Processing Applications, IEEE, 2008) shows that an FPGA implementation of the Jacobi-SVD algorithm is possible using a reasonable amount of FPGA logic resources. They observed a 3:1 computation time reduction on 20×20 and 30×30 SVDs when compared to optimal GPU implementations.
Orbit Correction Magnets (OCM)
Three types of configurations in Linacs use magnets: corrector, aligner, and sweeper.
Particles travel in a straight line. Dipole magnets bend the path of the particle beam to make it follow a ring shape. There can be several thousand dipoles along a multi-kilometer ring. Only the first magnet configuration (corrector) is part of a fast orbit feedback loop and will be discussed here.
Electromagnets are commonly used to generate the controllable magnetic fields. Possessing a current of several thousand amperes, they produce powerful magnetic fields which the dipole magnets use to enable the beam to handle tighter turns. The more energy a particle has, the greater the magnetic field needed to bend its path. High-speed digital to analog converters (ADCs) are used to control the correction magnetics' power supply. The electromagnets use a superconducting coil that enables a high current to flow without losing any energy to electrical resistance.
The first fully digitally controlled magnet power supplies were commissioned at Paul Scherrer Institute (PSI) in 1999.
Today, most correction magnets use a self-optimizing power supply control system to enable more complex control techniques at higher sampling rates.
Other type of magnets are also used:
- Insertion magnets (Quadrupole magnets) – Acting like lenses to focus a beam, they gather the particles closer together. Three quadrupoles are used to create a system called an inner triplet. Inner triplets tighten the beam, making it narrow by 10 times or more, depending on the type of accelerator, down to few micrometres across.
- Lattice magnets – Thousands of "lattice magnets" bend and tighten the particles’ trajectory. They keep the beam stable and precisely aligned. Beam motion is reduced to stay within a fraction of a micrometer in both the horizontal and the vertical plane.
Feedback rates can vary from as low as one 1 Hz (slow orbit feedback) up to a few kHz (fast orbit feedback). Corrections applied through slow orbit feedback suffer from poor synchronization, which causes undesirable orbit perturbations. A gain in beam stability and reproducibility of the experiments are observed when rate of the feedback loop is increased. With slow orbit feedback systems, the maximum corrector change has to be limited in each iteration. In the faster feedback systems, the corrector’s set points are simultaneously applied around the ring and it is necessary to limit the changes on an iteration less often. The increasing sensitivities of the experiments create the need for faster orbit feedback system.
Roundtrip latency, from BPM to OCM, is also a key factor when designing or selecting hardware platforms to implement a complete orbit feedback system. Most orbit controllers are designed to operate with a latency of approximately 100 μs. Most recent studies on this matter suggest a hardware architecture that can operate under very low latency constraints, in the range of approximately 20 μs.
The following Nutaq components implement the low-latency framework:
- The MI125 takes no longer than 9 clock cycles (90 ns @ 100MHz) to digitize the X-Y analog output of the BPM module.
- Aurora BSDK/MBDK cores provide ready-to-use implementations of the Xilinx Aurora communication protocol for the Perseus601x AMC carrier board. The cores can serialize and transmit 128 bits of data from one FPGA to another (including user FIFOs) in only 45 cycles (450 ns @ 100MHz).
- The second FPGA, equipped with Nutaq's QSFP(+)/SFP(+) transceiver modules, provides connectivity to 6 multi-gigabit transceivers (MGT) for rapid serial transmission over fiber optic links.
Combined, these components are the perfect framework for implementing the most “low latency demanding” orbit digital control algorithms.