Inside the Large Hadron Collider at CERN
Guest Columnist Richard Soden, product manager at Agilent Technologies, describes the creation of high-energy particle beams and the control of such beams inside CERN’s Large Hadron Collider (LHC), the Australian Synchrotron and HIT.
Photos: A tour of CERN’s Large Hadron Collider
High-energy particle accelerators are helping researchers investigate big questions about the nature of matter and the origins of the universe. Typical experiments involve carefully controlled collisions between either intersecting particle beams or a particle beam and an atomic-scale target.
Creating collisions at nanometer scale with picoseconds of duration requires extreme precision in spatial and temporal control.
At facilities such as the European Organization for Nuclear Research, more commonly known as CERN, and the Australian Synchrotron, high-performance digitizers are helping researchers achieve the precision and control needed to perform more and better experiments in less time.
At the Heidelberg Ion-Beam Therapy (HIT) Centre in Germany, precisely controlled beam energy is used in radiotherapy treatment of cancerous tumors. In applications such as these, digitizers must provide fast measurement throughput, very short “dead time” between measurements and excellent measurement fidelity.
Generating high-energy particle beams
There are two types of accelerators: linear accelerators (“linacs”) and synchrotrons. Linacs operate in a straight line and, due to size constraints, have limited power. Synchrotrons are circular and have multiple stages, enabling them to produce much higher power levels than linacs.
To illustrate the structure of a synchrotron, Figure 1 shows a simplified diagram of the LHC. Listed in order of operation, here is a summary of the acronyms in the figure:
• Linac 2: Linear accelerator for protons
• PSB: Proton Synchrotron Booster
• PS: Proton Synchrotron
• SPS: Super Proton Synchrotron
• LHC: Large Hadron Collider
The process starts in Linac 2, which generates 50-MeV protons that are fed into the PSB. It boosts the protons to 1.7 GeV before they are injected into the PS, which raises the energy to 26 GeV.
Next, the SPS raises the energy level to 450 GeV. Over a period of about 20 minutes the protons are injected into the LHC’s main ring.
In this final stage, protons accelerate to 7 TeV and circulate as two separate beams traveling in opposite directions. Collisions can be created at four intersections located around the LHC.
The term “beam” is a misnomer: Rather than a continuous stream, hadrons (protons or neutrons) are formed into “bunches” that have durations (or “widths”) measured in picoseconds.
Bunching is intrinsic to the physics of using RF fields to achieve high gradients of acceleration within the synchrotron. Bunches, which may contain several billion protons, travel at velocities approaching the speed of light. Depending on the accelerator’s circumference, transit time ranges from a few hundred nanoseconds to tens of microseconds.
A particle accelerator is a giant scientific instrument. It includes numerous subsystems, ranging from interconnected arrays of accelerators to tiny photo-detector diodes.
Clusters of traditional test equipment are used to measure, monitor and control the quality of particle beams. One key measure of beam quality is focus: Tightly focused beams help ensure high rates of particle collisions and interactions.
Within a typical accelerator, essential monitoring equipment includes detectors and digitisers. Detectors sense attributes such as the light intensity or energy level produced by individual bunches.
Digitisers convert a detector’s analog output into a digital representation that can be quantified, analyzed and used for beam control. In some cases a PC mounted in the instrument chassis performs initial data reduction and sends the preprocessed results across the local area network (LAN).
Monitoring particle beams in the LHC
The CERN Control Centre (CCC) manages the LHC and oversees operation of the laboratory’s eight accelerators, consolidating all control rooms under one roof (Figure 2). It also manages the lab’s technical and cryogenic infrastructures, which controls the cooling of 1,600 superconducting magnets spaced throughout the LHC.
Capturing light-speed signals
Within the LHC, each counter-circulating beam consists of about 2,808 bunches, each of which contains up to 100 billion protons. The bunches travel at 99.9999991% of the speed of light—and at that rate one transit of the 27-km LHC takes about 90microseconds.
The performance of the LHC depends on its injector chain. Along that chain, the Open Analogue Signal Information System (OASIS) acquires and displays analog signals in the acceleration domain. The signals come from every CERN accelerator and are sampled using various types of high-performance digitizers.
More than 2,000 individual signals are available to the operators and physicists who use OASIS. They are looking for a variety of indicators from the LHC and its injector chain: instabilities in proton bunches; currents in fast-pulsing magnets (“kickers”); phase relationships between kickers; and the state of RF signals in the accelerating cavities. These characteristics help determine if conditions are right for the experiments scheduled during any given shift.
With bunches traveling near the speed of light, fast measurements are critical and digitisers must have very short dead time between measurements.
Synchrotron light sources accelerate electrons to produce synchrotron radiation that spreads across the electromagnetic spectrum, spanning infrared, visible light and X-rays. The resulting photon beams are more than one million times brighter than the sun. This intense light is used to perform imaging experiments via spectroscopy or diffraction in materials science, biology and medicine at millimeter, micrometer, nanometer and sub-nanometer scales.
To ensure a high-quality beam, the Australian Synchrotron uses a technique called fill-pattern monitoring (FPM) to measure real-time intensity distribution of electron bunches in the storage ring (Figure 3).
Knowledge of the electron fill-pattern profile is important for experiments that require precise spatial and temporal control. When performing such experiments, a source with a known temporal-intensity profile makes it possible to analyse the effects of the radiation source on the results.
In FPM, the standard measurement approach uses a pickup-type monitor. One example is a capacitive strip that measures the voltage induced by bunches as they pass through the accelerator’s vacuum chamber. An alternative approach is to detect and measure the optical synchrotron radiation generated by each bunch. This requires an ultra-fast optical diode and a high-performance digitiser to perform direct measurements of emitted radiation intensity.
Testing the alternate approach
The Australian Synchrotron team worked with Agilent Technologies to develop a diode/digitiser detector and has demonstrated its ability to provide bunch-by-bunch resolution.
To validate the diode/digitiser detector, the synchrotron team performed a variety of tests and compared the results to those of the pickup-based approach.
In the tests, the diode/digitiser-based FPM was used to measure the fill pattern of the electron beam circulating in the storage ring. This capability can be used for dynamic control, which lends itself to “beam top-up” modes.
These modes enable, when needed, computer-controlled injection of additional electrons into the storage ring to compensate for losses, or to create custom fill patterns that meet the requirements of specific experiments.
Two control-related tests were performed. One simulated controlled loss of the beam, which tested FPM sensitivity. The other gauged FPM resolution by measuring single-bunch injections. In both cases the FPM outperformed the strip recorder. As an example from beam-loss testing, the FPM provided more precise characterisation of the fill pattern (Figure 4) and better sensitivity as beam power decreased.
During single-bunch injection testing, a single RF bucket was filled using an injection mode at 0.05 mA per shot.
A comparison of the injection into the storage ring and the post-injection energy confirmed precise injection into a single RF bucket. This level of sensitivity enables finer control over the selection of injection currents used in the dynamic top-up protocol.
Treating cancer with particle beams
Precise control of particle beam power and size has opened the door to medical applications. One example is radiotherapy, which is used in cancer treatment. The goal is to direct a beam within the volume of a cancerous tumor and destroy the DNA inside tumor-cell nuclei.
Some systems use carbon ions because they deliver greater linear energy transfer (LET) per unit length than protons. This translates into greater kinetic energy at greater depth within the body, thereby improving treatment efficacy. An example of this approach is the recently commissioned Heidelberg Ion-beam Therapy Centre (HIT) in Germany.
Controlling beam energy
The beam source is a linac that includes a 400 keV/u radio frequency quadrupole (RFQ) followed by an Interdigital-H-mode drift tube linac (IH-DTL) that produces an end energy of 7 MeV/u.
The linac is controlled in part with a measurement configuration that detects beam energy using the time-of-flight (TOF) method. Signals from four embedded phase probes are boosted with +60-dB preamplifiers before being fed into high-performance Agilent Acqiris 4 GSa/s digitisers.
Phase shifts between signals reflect the TOF difference between any two probes, and a calculation of cross-correlation between signals is used to manage phase relationships and thereby control beam intensity. This beam diagnostic is one of several techniques being used to ensure precise control of the radiotherapy beam.