Understand the typical applications of ultra-high vacuum and high vacuum technologies(2)

Vacuum Pumps

To address the future energy crisis, the international cooperation project on plasma nuclear fusion devices led by seven countries—namely the European Union, China, Japan, the United States, Russia, India, and South Korea—will, by 2025, complete the operation of the ITER prototype nuclear fusion device. This device will generate nuclear fusion energy with an electrical output of 500MW (for 7 minutes). Compared with the 50MW power input, the ITER device will achieve self-sustaining heating. By 2036, a more powerful nuclear fusion device, DEMO, will be built, with a nuclear power output of 4000MW, realizing commercial demonstration applications.

The attainment of the high vacuum operating environment at the level of 10⁻⁴Pa in nuclear fusion devices is mainly achieved by pumps capable of reaching ultra-high vacuum, such as cryogenic pumps and turbomolecular pumps. Based on different functional characteristics, there are 3 types of cryogenic pumps installed on the ITER device. The Cryostat cryogenic pump pumping system integrates two supercritical helium-cooled cryogenic adsorption pumps. 200 turbomolecular pumps need to meet the application requirements of different subsystems. Lower vacuum levels are achieved by screw pumps, Roots pumps, and scroll pumps.

To meet the environmental conditions of the ITER device, vacuum pump manufacturers including Edwards and Pfeiffer have made adaptive adjustments to the performance of their vacuum pumps. To broaden the magnetic field tolerance range of the turbomolecular pumps produced by Edwards, the magnetic field tolerance-enhanced nEXT400 turbomolecular pump adopts a number of new technologies, such as a martensitic stainless steel integral pump cavity structure with a 16mm wall thickness, an air-cooled structure for the heat-conducting base plate, ultra-high vacuum ceramic electrical feedthroughs and metal sealing devices, as well as optimization of radiation-resistant sealing components. These technologies enable reliable operation under a 160mT magnetic field.

Ultra-high vacuum and high vacuum valves are classified according to the range of vacuum degree. For different application scenarios, it is also necessary to describe and define the characteristic attributes of valves from different dimensions. Additional conditions such as high gas pressure, strong magnetic field, low leakage, particle-free state (the state with the minimum number of particles obtained), valve plate cooling, valve body heating, valve body conductivity, corrosion resistance, metal dust, and high-temperature radiation put forward higher requirements for valve performance.

Vacuum valves in the field of advanced manufacturing processes for integrated circuits are advanced and typical. Valve products from companies such as VAT, MKS, and VTES can meet the usage requirements of vacuum application equipment for deposition and etching: "no particle" generation (an extremely small amount of rubber and metal particles), no vibration (high-precision transmission), and precise control (no leakage, conductance adjustment). Particle-free valves are the foundation of high-end vacuum application equipment, distinguishing them from conventional vacuum valves: the metal valve body adopts high-vacuum brazing and dehydrogenation processes; the transmission seal uses metal bellows; the rubber is firmly combined with the valve plate and then undergoes vulcanization treatment; the rubber bears one-way sealing pressure without frictional movement.

In ultra-high vacuum systems, some components may generate particles during operation. Gases, liquid media (such as water, alcohol, or acid) or membrane filters are used for fine filtration to distinguish particles. Usually, particles with a particle size larger than 0.3μm are filtered out. Even in a vacuum environment without turbulence, it takes a long time for particles to settle on the surface. For vacuum devices exposed to ambient air (without a filter), after several hours of vacuum pumping, the floating particles (larger than 0.2μm) in the non-turbulent flow state take about 80 hours to adhere to the surface and no longer be transported. When a vacuum valve is opened at 10mbar, particles will fall off from the adsorption surface due to vibration and turbulence and be transported in the system along with the gas. To obtain a particle-free vacuum system, measures should be taken to avoid obvious vibration when the pressure is 1mbar.

The cleanliness level is classified according to the size and quantity of particles in the air, with reference to the international standard ISO14644-1, as shown in Table 1. Accelerator components usually adopt environmental conditions of ISO Class 4 and ISO Class 5. During the assembly process, clean rooms use Filter Fan Units (FFUs) to filter the air; in mobile clean rooms (tents), a local ISO Class 5 environment can be established