The Advanced Performance of HIAF

HIAF is the first advanced heavy ion research facility combined with superconducting linear accelerator, synchrotron and storage ring, achieving an optimal combination of the high pulse current intensity of linear accelerator and the high energy characteristics of synchrotron. It adopted a series of advanced technologies, including the most advanced superconducting ECR ion source of 45 GHz, superconducting accelerator, the energy storage fast cycle pulse power, fast pulse diode magnet, high gradient, wide band and fast response magnetic alloy loading cavity high-frequency system, ultra-thin wall ultra-high vacuum chamber lined with ceramic, high-precision multi-pole combined superconducting coil technology, low beam loss bi-plane coating Corner Septum, etc.

1. Superconducting ECR ion source of 45 GHz

The world's first fourth-generation ECR ion source technology, realizing the international initiative and breakthrough of the core technology of 45 GHz ECR ion source with high charge state based on all Nb3Sn strand composite superconducting magnet. The high-current heavy ion accelerator represented by HIAF needs to provide a high-current and high-charge heavy ion beam (such as U35+) of the magnitude of 1 emA. The existing ion source technology cannot meet the demand and is extremely challenging. As an international problem, it is necessary to develop more advanced ion source technology. The newly designed 45 GHz ECR ion source will provide a new solution for the heavy ion injector with high current and high charge state. The main breakthroughs are as follows: firstly, the world's first Nb3Sn fully superconducting ECR ion source magnet will challenge the limits of existing Nb3Sn magnet technology and process, and successfully develop the composite magnet based on single strand Nb3Sn superconducting hexpole magnet and four superconducting solenoid for the first time in the world. The highest magnetic field on the magnet is about 12T. China is the first to achieve ms class fast response active desuperlocking protection, Bladder+Keys based superconducting magnet assembly structure, high precision (plus or minus 0.05mm) cold body assembly process based on full simulation guidance, Mirror based six-pole coil test process; Design and development of a more than 10 W class redundant cryostat system with large cooling capacity; secondly, for the first time in the world to solve the key technical and physical problems of high density and high electron energy ECR plasma with high efficiency transmission of 20 kW/45 GHz microwave and coupled heating; thirdly, innovatively solving an international problem of developing more than 10 MW/m² high power density thin-walled plasma arc cavity; fourthly, innovatively solving the international problem of developing a high-temperature metal evaporation furnace with long-term stable operation in the environment of high magnetic field (more than 6T); finally, to solve the key technical and physical problems of efficient extraction and high-quality transport of 30-50mA heavy ion beam with high charge state and efficient matching with RFQ accelerator.

2. Superconducting linear accelerator

The first fully superconducting heavy ion superconducting linear accelerator in China, using continuous wave and pulse dual operation mode, whose designing current intensity is 2emA, is currently the highest superconducting linear accelerator in the world. The main design features are as follows: firstly, the design energy of the heavy-ion RFQ accelerator is 0.8MeV/u, which improves the acceleration efficiency and beam quality of the whole accelerator, and reduces the probability of beam loss in the superconducting acceleration section; secondly, adopting 3D virtual integration design and assembly technology to realize the scene of “what you see is what you get” and improve the assembly, maintainability and detectability of the system; thirdly, the superconducting accelerator unit adopts bottom-up mode and Kelvin support structure to accurately control the cold shrinkage deformation and position deviation of the on-line components in low temperature environment; finally, the radio frequency system adopts standardized all-solid-state power amplifier and digital control technology to improve system scalability, maintainability and reliability.

3. Variable forward excitation full energy storage fast cycle pulse power source

The scheme of "variable-forward excitation full energy storage fast cycle pulse power supply" proposed for the first time in the world has solved the international problem of non-resonant extremely fast change rate of inductive load power supply. Owing to space charge effect and dynamic vacuum effect, ions need to be accelerated to high energy to avoid avalanche effect of ion loss. It requires the magnet power supply to output 4000A current pulse, current rise rate to 38000A/s, rise time to less than 100ms, tracking error to less than plus minus 1×10^-4; The output and adjustment range of the power supply is large, and the high precision should be maintained throughout the pulse cycle. At present, the resonant scheme is adopted to obtain the fast rate pulse current of large inductor magnet in the world, and the pulse current frequency can generally reach dozens of Hertz, such as the Spallation neutron source SNS in the United States and the J-PARC RCS intensifier in Japan. However, the resonant scheme can only generate sinusoidal current waveform at a fixed frequency, and the frequency cannot be adjusted, so it is only suitable for the accelerator operating in sinusoidal resonance mode. It is a technical problem for international synchrotrons to satisfy the extremely fast rate and non-resonant power supply of heavy ion accelerator. The variable excitation fast cycle total energy storage high power and high precision power supply scheme put forward by IMP, adopts innovative way of the energy storage to control energy in orderly flow between the magnets and inductance energy storage capacitor. Only a small part of energy consumed by magnet heating is supplied from the grid each time, and the ordered fluctuation of bus voltage is controlled by vector rectifier. The scheme not only reduces the power distribution to one third of the original power supply, but also solves the problem of power network impact well.

4. High voltage gradient, wide frequency band, fast response magnetic alloy loading cavity high frequency system

Magnetic alloy high frequency system is one of the core technologies of the new generation of fast cycle synchrotron. It is composed of high frequency cavity, power source, low electrical level and computer control system, with characteristics of high voltage gradient (20kV/m), wide frequency band (no tuning), and fast response speed (less than 10μs). Nanocrystalline soft magnetic alloy loading cavity high frequency system is a new technology developed in the last 20 years. Magnetic alloy materials have the characteristics of high permeability, high saturation magnetic induction intensity, excellent Curie temperature characteristics, low Q value and fast response. Using outstanding performance and large size magnetic alloy ring as loading material of high frequency cavity, as well as high power broadband pulse power source and high speed digital low level control system, it can not only improve the acceleration voltage gradient and response speed, broaden the working frequency band, but also reduce the complexity of the whole structure of high frequency system. Magnetic alloy loaded cavity has been used by KEK in Japan, GSI in Germany, FNAL in the United States. HIAF adopts liquid-cooled large-size magnetic alloy loaded high-frequency cavity, dual-tube push-pull dynamic load high-power broadband pulse power source, as well as full digital low level control system to realize high-precision phase-locking, amplitude stabilization and multi-harmonic beam load effect compensation for the first time in China. The development of magnetic alloy loaded cavity at home is in its infancy, which also confronts the sale prohibition of foreign key materials and technology blockade. In order to meet the requirements of heavy ion accelerator strong flow to the high frequency system, the team aims at magnetic alloy loaded cavity, large size high performance magnetic alloy ring, valve power source and the key technology of low level control system to research and develop, which has very important significance to fill the domestic technical blank in this field and to break the foreign blockade on materials and technology.

5. Ultra-thin wall ultra-high vacuum chamber with ceramic lining

The project of ceramic lined thin-walled vacuum chamber proposed by the international initiative has completely solved the technical problem of thin-walled vacuum chamber of heavy ion accelerator. At present, thin-wall stiffened structure technology scheme is widely used in thin-wall vacuum chamber in the world. However, this scheme occupies a large size of magnet air gap, which greatly increases the cost of magnet, power supply, operation and maintenance. Based on the above reasons, the team proposes the ceramic lining scheme, which lines the zirconia ceramic ring (thickness: 2~3mm) in the 0.3mm thin-walled stainless steel vacuum chamber, using the high bending strength of ceramics to resist atmospheric pressure, to ensure that the maximum deformation of the thin-walled vacuum chamber meets the physical requirements. At the same time, zirconia ceramic Au film plating technology is developed, which effectively solves the problems of beam impedance and inspiratory load caused by ceramic ring lining in the operation of high current heavy ion accelerator. This design scheme provides a new technical scheme for the development of ultra-high thin-wall vacuum chamber of accelerator in the future, which is of great significance to the development of ultra-high vacuum thin-wall vacuum chamber of international high-current heavy ion accelerator.

6. BRing fast pulse diode magnet

In order to accelerate a variety of heavy ions from low energy to high energy rapidly, thus avoiding ion loss due to the avalanche effect caused by space charge effect and dynamic vacuum effect, a fast pulse diode magnet is needed. This not only requires that the magnetic field uniformity and harmonics in the whole magnetic field be broadened less than or equal 3×10^-4 during the whole acceleration process, but also requires that the magnetic field rise rate reach 12T/s within the whole magnetic field variation range of 0.047t~1.58t. At present, the pulse-running ring magnet with the largest magnetic field rising rate is the SIS18 fast pulse diode magnet of GSI, which has the maximum magnetic field rising rate of 10T/s in 1.2t and the magnetic field rising rate drops to 4T/s in 1.2t ~1.8T. Under the fast pulse operation mode, the iron core, coil and vacuum pipe of the magnet produce eddy current effect, which causes the magnetic field quality deterioration, magnetic field delay, magnet power loss increase and thermal effect, etc. At the same time, under the action of pulse current, the electromagnetic force received by the magnet coil also changes correspondingly, which is easy to cause the fatigue and loosening of the coil fixing device, resulting in coil damage. Therefore, based on the above problems, our team carried out detailed optimization design and structural analysis, and determined the structure and process plan of the magnet. The magnet adopts straight iron scheme, while the main body of the iron core is effectively blocked the eddy current in the direction of the beam by the way of 0.5mm silicon steel sheet full adhesive stacking. And the end plate, side plate, drawplate and vacuum pipe adopts 304 stainless steel with low conductivity to reduce the influence of eddy current effect. Through the detailed force analysis and anti-fatigue calculation, the combination of cable-stayed fixing and column top fixing is considered to ensure the stability of the coil running under fast pulse. In view of the rapid change of magnetic field, the static and dynamic measurement system was designed and constructed in detail to explore the influence of eddy current.

7. High precision multipole combined superconducting coil technology

By using the latest high-precision composite superconducting multipole magnet technology with multi-layer nesting of DCT (Discrete Cosine Theta) coil and CCT (Canted Cosine Theta) coil, the difficulties such as large volume weight, high power consumption, high cost and poor linearity of magnetic field of traditional large-aperture magnet are solved. Due to the need for efficient transmission of the radioactive secondary beam with large angle and large momentum dispersion, HFRS has high requirements for good field aperture (φ320 mm), field gradient (11T/m) and uniformity (plus minus 3×10^-4). The currently widely used low temperature superconducting magnets based on cold iron core structure (such as A1900 of MSU in the United States, BigRIPS of Riken in Japan and SuperFRS of FAIR in Germany, etc.) can achieve the above indicators, but they have problems of large volume, large cold mass, large liquid helium consumption and high cost. In view of the above problems, our team proposed a scheme of combining high-precision multipole DCT and CCT superconducting coil. The room-temperature yoke was adopted to reduce magnetic leakage and enhance the central magnetic field, avoiding the use of cold iron core and greatly reducing the cold mass, the cooling time of the system, the scale and cost of the low-temperature system. The main innovations include the following aspects: firstly, the DCT coil with high excitation efficiency is used to generate the main quadrupole field, supplemented by the CCT coil with low magnetic field error and easy machining as the magnetic field padding coil, which solves the contradiction between excitation efficiency and magnetic field accuracy; secondly, the radial combination of eight, four, six and two correction coils is realized by G10 skeleton multi-layer nesting and in-groove coil winding process, which greatly reduces the length of HFRS beam line; thirdly, A number of special technologies, such as "6+1" insulated superconducting cable technology, laser scanning and correction technology of wire positioning error, have been put forward to solve the machining and measurement problems of high-precision multi-pole combined superconducting coil; finally, innovatively proposing the adjustable nonlinear Quench-back technology to solve the problem of overshoot protection and ensure the reliable and safe operation of the magnet.

8. Coating the Corner Septum on two planes with low beam loss

Innovating the electrostatic deflector plate with the structure of "miniaturization", "multi-electrode" and "ceramic frame". Oblique electrostatic deflector plate is used for bidirectional smear injection of BRing in heavy ion accelerator. In order to reduce the probability of loss of returned beam at the position of electrostatic cutting plate (anode), the size of electrostatic cutting plate should be strictly required, the width should not be greater than 9mm, and the thickness should not be greater than 0.1mm. According to the physical requirements, the good field area of electric field is a circle with a diameter of 8mm, which is close to the width of the anode cutting plate. However, a single high voltage electrode cannot meet the uniformity of electric field within the beam range, which becomes the difficulty of electric field optimization. At the same time, it is urgent to solve the fixation method of electrode under the condition of strict space limitation. In view of the above problems, our team innovated the electrostatic deflector plate with the structure of "miniaturization", "multi-electrode" and "ceramic frame". The cutting plate is made of alloy strip 3mm wide and 0.1mm thick. The interval between strips is 1mm, and the linear distance of the strip is 9mm. Both ends are bent backwards and clamped on the fixed frame, and the overall structure tends to be "miniaturized". In order to satisfy the intensity uniformity, "more electrode" design is adopted. The main electrode is used to provide the average electric field within the range of good field, while the small electrode is used to compensate for the small field with large local deviation in the good field. Through the optimization design of electric field, the width of the main electrode is similar to the size of the cutting plate, and the small electrode adopts the filament with a diameter of 1mm. The problem of fixing and insulating the small electrode can be solved by using the special-shaped "ceramic frame" with good insulation performance and placing the small electrode in it.