Dongsheng Zhang Ph.D. Graduate Student Mechanical Engineering Room 1327 ERB dzhang25@wisc.edu HOMETOWN: Hualin, China Thesis: Model and test features of the cryopump being designed for use in ITER

Background The project is to model and test features of the cryopump being designed for use in ITER. After the fusion reaction in ITER, a mixture of hydrogen isotopes and helium flow through the cryopump. The gas mixture enters the pump at 80 K, while the walls of the cryopump are maintained at 4 K. At such low temperatures, hydrogen isotopes can be adsorbed on the pump surface, while helium atoms just flow through the pump. As a result, a separation of the hydrogen isotopes and helium is achieved. Figure 1: ITER in South of France

Objectives The research goal is to understand the energy load, mass transfer rate, and pumping capacity, etc. The project is interdisciplinary, and touches areas such as surface chemistry, physics, and cryogenics, in addition to mechanical engineering. H2 (D2) molecules are adsorbed on pump surface H2 (D2) density decreases because of chemisorption and physisorption effect Figure 2: mixture of H2 (D2) and He flowing through a cryopump

Molecule-surface interaction The figure below shows molecule surface interaction and energy transferred. E1 = S (Ek + Eint + ε - kTs) ; S is sticking coefficient E2 = Esub; Esub is sublimation energy E3 = α (Ek – 2kTs + Eint); α ≡ (T - Tf) / (T - Ts) E4 = [α (Ek – 2kTs + Eint)]He Figure 3: H2 (D2) and He impinge on pump surface and energy is transferred