1. SUPERCONDUCTING RF CAVITIES
The past two decades have seen the advent of superconducting RF cavities in most of the accelerators recently built or under construction, resulting from the dramatic breakthroughs in the accelerating field (from 5 MV/m to 30 MV/m) and cryogenic consumption at roughly constant fabrication cost and unsurpassed operation efficiency. After LEP200 operational success, the usage of niobium based SRF technology became widespread and almost unavoidable for circular and linear accelerator projects using electron, proton and heavy ion beams in pulsed or CW mode or operation. It also opened up new operation modes included beam recirculation and beam energy recovery, and new applications like SRF electron guns and transverse deflection. Furthermore, some advances are still the result of recent R&D demonstrating that higher performances are to be expected in the medium term future.
- High Q0 / high gradient SRF structures, requesting special furnaces, and other demanding infrastructure.
- Surface preparation of SRF cavities requesting innovative sophisticated surface modification methods like Nitrogen doping or infusion.
- New fabrication techniques using large grain bulk niobium or coated cavities with higher Tc material.
- SRF electron sources of high demand for future CW FEL operation.
- Optimized cryostat design for particle-free assembly and robotics, and cost reduction.
2. RADIO-FREQUENCY POWER SOURCES
The electrical power consumption of future accelerators will be driven to a large part by their RF systems. A significant part of the initial investment and running cost of the large scale machines will be determined by the purchasing cost and the efficiency of their RF sources. Increasing the efficiency of existing RF systems to higher levels means several millions euros saved per year on the electricity bill. The upcoming large scale accelerators are expected to require RF power in the range of 10 to 100 MW (for comparison, the Large Hadron Collider (LHC) has a total RF drive of 5 MW). This is particularly true for electrons colliders, circular (e.g. FCC-ee or CEPC) or linear (e.g. ILC or CLIC), High power hadron Linacs (e.g. PIP2) and Accelerator Driven Systems (e.g. MYRRHA).
- Modulators: today’s modulator are already operating with very high efficiency (85-92%) almost independent of their output power (kW-MW), Voltage (1-100kV), and pulse length. For short pulses (< 500 μs), the modulator rise time becomes an important factor in the system efficiency and this is where further developments, such as the Stacked Multi-Level (SML) design, are expected to make a significant difference.
- Klystrons: Current State of the art Klystrons can deliver a maximum efficiency of approximatively 65%. The limiting factor is the electron bunch profile as it approaches the output cavity of klystrons, as well as the velocity of the lowest electron leaving the output gap. With the advance of modern beam dynamics tools, number of novel electron bunching mechanisms such as the Core Oscillation Method (COM), the Bunching, Alignment and Collecting (BAC) method and the Core Stabilisation Method (CSM), have shown an improve on the efficiency through numerical investigations.
- Solid State Amplifiers (SSA): SSAs promise cost efficient RF power generation and the advantages of modular systems. This imply an effective combination of the single units (of 1 kW) to reach high power values (> 100kW). A promising solution is to use combiner cavities that combine all the output of single units in one stage. The difficulty still to match hundreds of input antennas and minimise the reflected power due to manufacturing tolerances of the electronics or to failed units.
Cryogenics is a base technology for the numerous worldwide research facilities that utilize large superconducting (SC) magnets or SC particle accelerators, be it with superconductive radio-frequency cavities or high-field magnets. The rarity and potential shortage of helium gas, as the most used cooling liquid for the large facilities, call for critical advances in reducing the overall helium and raising efficiency, as well as alternative cryo-cooling technologies. Industry, especially European firms, and research laboratories are leading these developments. European PED certification is not fully adapted to the peculiar risk and technical solutions of the Helium cryostats used in accelerators.
- Higher efficiency cryo-plants through improved performance sub-system components such as heat exchangers, turbines, instrumentation and process control optimisation.
- Cryo-coolers adopting development technologies employed from space industry applications and possibly new cryogenic fluid mixtures.
- Safety specifically for accelerator equipment, by developing dynamic models for improving mitigation of cryogenic incidents and new European standards to certify cryostat design and fabrication.
4. BEAM INSTRUMENTATION
Beam instrumentation is the accelerator artificial intelligence: it keeps particle beams alive and bring them to their required performance level. As an analogy, beams stored during one day on powerful circular accelerators travel the same distance as the orbit of Pluto (about 30 billion kilometers), while keeping their sub-millimeter orbit and size characteristics. Beam instrumentation includes beam diagnostics, i.e. instruments that sense the electro/ optical signals of charged particles and monitor their evolution front-end systems, i.e. ultra-fast electronics systems that process these signals and dynamically generate corrective actions control systems, i.e. specialized software suites that command and regulate the operation of the many technical individual constituents and systems cooperating collectively to stabilize the accelerator operation, from the beam source to beam delivery.
- Non-invasive diagnostics based on RF or optical signals, particularly for high intensity beams.
- Longitudinal diagnostics for ultra-short bunches based in RF structures, electro-optics or Terahertz detectors.
- Digital conversion of electric or light signals with high resolution and large bandwidth.
- Ultra-fast electronics based on parallel developments in computer industry and high-speed communications
- Innovative feedback algorithms in control systems improved with AI.
5. PARTICLE SOURCES
Ion sources determine the research potential of the accelerator or post-accelerator facilities. The essential parameters are the ion species, the ion current intensity and the polarization. Proton accelerators (e.g. LHC, SNS, ESS, etc…) are using high intensity H+ or H- ion sources, while heavy ions accelerator are based on the widest range of stable ions from helium to heavy stable ions (e.g. lead or uranium). Radioactive ions beams are using secondary sources including a target. Fusion reactors are using MeV-energy intense ion sources (e.g. D-) for neutral injection as a plasma heating mechanism. For spatial (e.g. ion thruster) and industry (e.g. ion beam lithography) application, compactness and reliability are crucial properties of the ion sources. There is also a strong interest in material science for ion traps producing highly charged ions at rest. For electron sources, the new generation of CW FELs calls for high current high repetition rate (10 kHz-1 MHz) RF photo-injectors, a possible new application of superconducting RF cavities. Pushing the intensity of positron sources is mandatory for colliders, together with polarization, and for very low energy positron beams used in anti-matter research and material science. Creating low-emittance energetic muon beams would open the way for a new generation of lepton colliders.
- Electron Cyclotron Resonance (ECR) ion source breakthroughs in intensity can be expected by increasing the microwave RF frequency (up to 45 GHz) and using superconducting coils (up to 11 T) to match the resonance condition. On the other hand, the standard ECR sources (0.8 T, 2.45 GHz) can increase their power efficiency, ease of operation and compactness by using permanent magnets for spatial and industry applications.
- Electron Beam ion sources (EBIS) are more efficient for highly charge heavy ion sources. A large range of studies are in progress to improve their reliability and ease of operation.
- The creation of electron-positron pairs in matter is the basic mechanism for the positron sources; hence thermal effects on the solid target limit the beam intensity. New schemes are proposed using intense photon radiation from laser collision or undulators, preferably superconducting, or atomic crystals, followed by photon conversion on thin targets. Some schemes are amenable to producing polarized positron beams, others are considered to produce muon pairs.
6. MAGNETS AND VACUUM SYSTEMS
Warm magnet technology is indeed basic for guiding particle beams along accelerators, with a widespread group of laboratories and industries capable of producing and testing conventional magnets in large series. However, new promising beam transport techniques have recently been discovered or implemented successfully (e.g. ultimatebrilliance synchrotron-radiation storage rings (DLSR), fixed-field alternating gradient accelerators (FFAG), beam channels for plasma acceleration) that extend the design and manufacturability needs for warm magnets, be they resistive magnets or permanent magnets, beyond their current state of the art. In many cases, small aperture vacuum chambers with high pumping speed and low desorption are required to implement these techniques.
- Manufacturing of combined function magnets with complex magnetic polar pieces.
- Permanent magnets introducing high permeability material like Neodymium or Praseodymium.
- Coating of small aperture vacuum chambers using the Non-Evaporable-Getter (NEG) technology and which reduces electron secondary-emission yield.
- Surface treatment of vacuum chamber to reduce secondary electron emission yield.
7. HIGH FIELD SUPERCONDUCTING MAGNETS
Large and powerful high-field superconducting magnets are routinely used in science, research and technological development (RTD) and in medical diagnostics, using Magnetic Resonance Imaging (MRI), the latter representing the biggest current market for superconductivity. In addition, superconductors bring potentially large energy savings in power applications, with demonstrations of power cables, transformers, motors or current limiters already have been made. There is a strong drive on new superconductor developments to increase the power of hadron colliders and MRI for scientific research, and for tokamaks for the next demonstrators by raising the magnetic field strength in large volumes using new innovative superconducting materials, such as Nb3Sn or High Temperature Superconductors (HTS), and developing cable and coil innovative cooling technologies.
- Development and use of ultimate performance Nb3Sn conductors, the most mature option so far, to overcome cost and coil fabrication issues.
- Development and use of HTS conductors still needing R&D on material science to electromechanical engineering.
- Innovative conductor and cold mass cooling methods to increase the operating temperature margin.
- Reinforcement of the conductor mechanical strength to take on much higher internal magnetic forces.
8. NORMAL CONDUCTING RF STUCTURES
Radio Frequency acceleration technology (normal conducting) was introduced 90 years ago (e.g. drift tube linacs) and is still the standard reference for acceleration, used in the majority of particle accelerators worldwide. There are irreplaceable in the front-end injection systems of proton and ion linear accelerators (e.g, Radio-Frequency-Quadrupole). Normalconducting RF cavities are characterised by a large variety of designs (single cell, multi-cell, TE-mode, TM-mode, RFQ, etc.), operating frequencies (from the kHz to the multi-GHz range), operating modes (CW or pulsed, tunable or fixed frequency, coupled or stand-alone, etc.), and of construction technologies (Cu-plated or full copper, bolted, welded or brazed). The availability of simple designs using conventional fabrication techniques makes normal-conducting RF accessible to small universities and laboratories without the need for specialised infrastructure. Conversely, sophisticated designs reaching challenging parameters and/or large-scale productions require specific technological infrastructure for the manufacturing and for the processing of the cavities.
- The main ongoing developments for accelerating RF cavities are related to increasing the accelerating gradient to reduce the length the accelerator, and to increase the power efficiency. This latter challenge leads to increasing the operating frequency reducing at the same time the cavity dimensions, thus imposing additional challenges on the manufacturing in terms of precision machining and surface quality.
- Given the pressing need of proton or ion high intensity injectors, the fabrication of RFQs in industry requires sophisticated machining, thermal treatment and firing of ultra-pure copper, with some commonalities with the fabrication of high power RF input couplers and CW RF guns. Consolidating these techniques over time and regions would benefit to the lead time and cost of their fabrication.
THE ROLE OF THE KEY TECHNOLOGICAL AREAS FOR FUTURE MAJOR PROJECTS
This table illustrates the criticality of KTAs for the major Research Infrastructures of the global landscape, regarding the following criteria:
- Being widely needed for the future projects.
- Presenting a high development potential allowing to meet the needs of future challenging machines.
- Presenting a high development potential allowing to reduce the construction and/or operation costs of future machines.
THE ROLE OF AMICI TECHNOLOGICAL FACILITIES FOR KTA’S DEVELOPMENT
This table illustrates the matching of Key Technology Areas with the Technological Facilities that constitute the current AMICI Technology Infrastructure: for the checked entries, scientists at Technological Facilities recognize running cutting-edge technical developments aimed at breakthroughs in performance, cost or reliability of the given KTA.