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Contrastive learning has recently narrowed the gap between self-supervised and supervised methods in image and video domain. State-of-the-art video contrastive learning methods such as CVRL and $\rho$-MoCo spatiotemporally augment two clips from the same video as positives. By only sampling positive clips locally from a single video, these methods neglect other semantically related videos that can also be useful. To address this limitation, we leverage nearest-neighbor videos from the global space as additional positive pairs, thus improving positive key diversity and introducing a more relaxed notion of similarity that extends beyond video and even class boundaries. Our method, Inter-Intra Video Contrastive Learning (IIVCL), improves performance on a range of video tasks., Comment: Accepted to the ICLR 2023 Workshop on Mathematical and Empirical Understanding of Foundation Models

Biopharmaceutical proteins have attracted increasing interest and been developed for treatment of various diseases due to their low toxicity, high affinity, and high efficacy. Subcutaneous injection, the most common delivery route for therapeutic proteins, requires high protein concentration formulations due to limited injection volume, resulting in challenges in manufacture, delivery, and stability of the drug products. Limited availability hinders the effective characterization of the physical properties of therapeutic proteins, especially during the early stage of development, where a large number of candidate formulations across a huge compositional space need to be screened. In this thesis, a droplet-based microfluidic device is innovated by incorporating a variety of sensing methods to quantify different physical properties of the concentrated protein solutions accurately and efficiently over a wide range of process-relevant parameters. Multiple particle tracking microrheology, birefringence, and turbidity are applied as the sensing methods to investigate viscosity, crystalline phase transitions, and phase separation of the protein solutions, respectively. The microfluidic technique provides data with high compositional resolution, and only requires a small sample volume. This work has demonstrated the potential of the developed novel technique to rapidly screen candidate formulations and provide guidance to facilitate the design of protein formulations to achieve desired properties. This technique can also be potentially applied as a complementary approach for computational methods to predict behaviors of high concentration formulations and facilitate understanding of the underlying mechanisms.

The surface chemical composition of nitrogen doping Nb samples used for the fabrication of superconducting radio frequency (SRF) cavities, followed by the subsequent successive EP with different amounts of material removal, has been studied by XPS. The chemical composition of Nb, O, C and N was presented before and after Gas Cluster Ion Beams (GCIB) etching. No signals of bad superconducting nitrides NbNx was found in any doped and un-doped samples before etching. However, in the depth range greater than 30nm, the content of N elements is below the XPS detection precision scope even in the samples directly after nitrogen doping treatment., Proceedings of the 18th Int. Conf. on RF Superconductivity, SRF2017, Lanzhou, China

The concept of micro-pulse electron gun(MPG) was proposed decades ago. It can provide electron beam with high current, short pulse and low emittance. But it is still not put into practical use as electron source because of its unsteady operating state. This paper presents an experimental result of the steady running of MPG which can operate stably for more than ten hours. The energy spread of the electron beam is also measured, the peak is located at near 20eV and half width is less than 15eV ., Proceedings of the 18th Int. Conf. on RF Superconductivity, SRF2017, Lanzhou, China

The recent studies of laser annealing on niobium films on copper are reported. Annealing is normally used to deal with the surface, reducing defects and even chang-ing the microstructure of the coating film. Short pulse laser can produce a sharp step temperature field on the film thickness scale (μm), which anneals the surface without substrate heated. The laser annealing experi-ments of niobium thin film sample have been carried out, and according to SEM and FIB results, Nb films melted and recrystallization occurred. Grains growing up can be observed while the power density of laser pulse in-creased., Proceedings of the 29\textsuperscript{th} Linear Accelerator Conf., LINAC2018, Beijing, China

X-ray is now widely used in many areas of physics, biology, chemistry and materials. And how to achieve emission, monochrome, and focusing of x-ray is of great significance to study. Micro-pulse electron gun (MPG) is a new type of electron source, with characteristics of high repetition frequency, short-pulse and low cost. Generating x-ray with better monochromaticity is one of the potential applications of MPG. And a preliminary scheme of X-ray based on MPG is proposed in this paper. The scheme is designed by comparing different anode materials and the thickness of filters. The simulation results based on the software MCNP5 show that the proposed scheme can effectively improve the monotonicity of the generated X-rays., Proceedings of the 18th Int. Conf. on RF Superconductivity, SRF2017, Lanzhou, China

Our work is mainly focused on the fabrication methods of Nb3Sn films on Cu substrates and film's properties. There are diffraction peaks of Nb3Sn in the X-ray diffraction patterns in which without diffraction peaks of copper compounds. Scanning electron microstructures of Nb3Sn film reflect its nice compactness and binding force between film and substrate., Proceedings of the 18th Int. Conf. on RF Superconductivity, SRF2017, Lanzhou, China

Nb3Sn for SRF cavities has been coated on copper samples by DC magnetron Sputtering. Pure Nb target and pure Sn target were installed separately in the magnetron sputtering device. Nb3Sn precursor was coated on copper in the Ar atmosphere of 0.5 Pa. The Nb3Sn precursor was annealed in the vacuum furnace whose pressure is 10⁻⁴ Pa. The XRD results demonstrate the exist of Nb3Sn crystal, and MPMS results show superconductivity of Nb3Sn. The highest critical temperature obtained is 15K., Proceedings of the 18th Int. Conf. on RF Superconductivity, SRF2017, Lanzhou, China

Micro-pulse electron gun (MPG) is a novel electron source which can produce narrow-pulse, high-repetition rate electron current. Theoretical and experiment work have been done to study physical properties and steady operating conditions of MPG. Proof-of-principle work has been finished and the next work is to research the parameters of the MPG electron beam and master the MPG work property deeply. Thus, a high voltage accelerating platform which can supply 100 kV direct voltage was designed. Furthermore, electromagnetic and mechanism designs were operated to adapt the high voltage platform and measure beam parameters., Proceedings of the 38th Int. Free Electron Laser Conf., FEL2017, Santa Fe, NM, USA

Nitrogen doping study on Niobium samples used for the fabrication of superconducting radio frequency (SRF) cavities was carried out. The samples' surface treatments were attempted to replicate that of the cavities, which included heavy electropolishing (EP), nitrogen doping and the subsequent successive EP with different amounts of material removal. The magnetization curves of both doped and un-doped samples have been measured, from which the lower critical field Hffp (First Flux Penetration, ffp) and upper critical field Hc2 was extracted. The thermodynamic critical field Hc, superheating field Hsh and superconducting parameters of samples with different treatments was calculated from the determined reversible magnetization curves. Hsh of doped samples is obviously smaller than that of un-doped samples, which may be a possible reason for the reduction of achievable accelerating gradient in SRF niobium cavities after nitrogen doping treatments., Proceedings of the 18th Int. Conf. on RF Superconductivity, SRF2017, Lanzhou, China

Direct observation of hydrides precipitates formation on both nitrogen doped and un-doped Nb samples at 80K has been carried out using Scanning Electron Microscope (SEM) with Cold Stand. We have found that, under our experimental conditions, when the subsequent EP removal is less than 7μm, the amounts of hydrides formed on the surface of doped samples can be effectively reduced. When the subsequent material removal is larger than 9μm, the amounts of precipitated hydrides increased with the EP removal. When the EP removal is 7-9μm, the amounts of hydrides can still be effectively reduced. Also, more hydrides were precipitated on the surface of un-doped samples. The amounts of hydrides of doped samples may be reduced to varying degrees with different amounts of material removal., Proceedings of the 18th Int. Conf. on RF Superconductivity, SRF2017, Lanzhou, China

Nitrogen doping treatment with the subsequent electropolishing (EP) of the niobium superconducting cavity can significantly increase the cavity's quality factor up to a factor of 3. But the process of the EP removal may reintroduce hydrogen in the cavity surface, which may influence the cavity's radio frequency performance. Plasma cleaning study on niobium samples with gas mixtures of argon and oxgen intended to remove contaminations (hydrocarbons and micronicdust particles) from cavity surface to avoid field emission, was performed in Peking University. The niobium samples have been analyzed using the time of flight secondary ion mass spectrometry (TOF-SIMS) to measure the depth profiles of H, C, O, F, P and Nb. The measuring results show that the plasma cleaning with gas mixtures of argon and oxgen and conditions of about 20Pa and 100W can remarkably reduce the contents of impurity elements in the depth of about 30 nm without introducing hydrogen in the cavity surface. So plasma cleaning has been proposed to be a new possible treatment for niobium superconducting cavity after nitrogen doping.

For the application of high intensity continuous wave (CW) proton beam acceleration, a new superconducting accelerating structure for extremely low β proton working in TE210 mode has been proposed at Peking University. The cavity consists of eight electrodes and eight accelerating gaps. The RF frequency is 162.5MHz, and the designed proton input energy is 200keV. A peak field optimization has been performed for the lower surface field. The accelerating gaps are adjusted by phase sweeping based on KONUS beam dynamics. Solenoids are placed outside the cavity to provide transverse focusing. Numerical calculation shows that the transverse defocusing of the KONUS phase is about three times smaller than that of the conventional negative synchronous RF phase. The beam dynamics of a 10mA CW proton beam is simulated by the TraceWin code. The simulation results show that the beam’s size is under effective control. Both the simulation and the numerical calculation show that the cavity has a relatively high effective accelerating gradient of 2.6MV/m. Our results show that this new accelerating structure may be a possible candidate for superconducting operation at such a low energy range., Proceedings of the 17th International Conference on RF Superconductivity, SRF2015, Whistler, BC, Canada

Nitrogen doping treatment with the subsequent electropolishing (EP) of the niobium superconducting cavity can significantly increase the cavity’s quality factor up to a factor of 3. The nitrogen doping experiment has been successfully repeated and demonstrated. But the mechanism of the nitrogen doping effect remains unclear. Nitrogen doping study on niobium samples was carried out in Peking University. The niobium samples were manual processed to avoid heat generation. The experiment condition is close to that of the Fermilab. After the nitrogen doping treatment, the samples were mildly electropolished with the thickness of 1.3μm, 1.9μm, 3.3μm, 4.2μm, 5.1μm, 5.9μm and 7.0μm. The time of flight secondary ion mass spectrometry (TOF-SIMS) measurements show that the samples directly after nitrogen doping have a much higher nitrogen concentration in the depth of about 90nm. When the EP removal is larger than 1.3μm, the samples’ impurity elements is remarkably reduced and their distribution is similar to each other. Also the measured results to some extent prove that EP removal can introduce H to the niobium surface., Proceedings of the 17th International Conference on RF Superconductivity, SRF2015, Whistler, BC, Canada