Differential Pumping System Research Articles

Optical ionization effects in kHz laser wakefield acceleration with few-cycle pulses

We present significant advances in laser wakefield acceleration (LWFA) operating at a 1 kHz repetition rate, employing a sub-TW, few-femtosecond laser and a continuously flowing hydrogen gas target. We conducted a comprehensive study assessing how the nature of the gas within the target influences accelerator performance. This work confirms and elucidates the superior performance of hydrogen in LWFA driven by few-cycle, low-energy laser pulses. Our system generates quasimonoenergetic electron bunches with energies up to 10 MeV, bunch charges of 2 pC, and angular divergences of 15 mrad. Notably, our scheme relying on differential pumping enables continuous operation at kHz repetition rates, contrasting with previous systems that operated in burst mode to achieve similar beam properties. Particle-in-cell simulations explain hydrogen's superior performances: the ionization effects in nitrogen and helium distort the laser pulse, negatively impacting the accelerator performance. These effects are strongly mitigated in hydrogen plasma, thereby enhancing beam quality. This analysis represents a significant step forward in optimizing and understanding kHz LWFA. It underscores the critical role of hydrogen and the imperative need to develop hydrogen-compatible target systems capable of managing high repetition rates, as exemplified by our differential pumping system. These advances lay the groundwork for further developments in high-repetition-rate laser plasma accelerator technology. Published by the American Physical Society 2024

In situ mass spectrometric investigation to probe GeSn growth dynamics and mechanisms in the chemical vapor deposition processes

GeSn has attracted increasing attention due to its tunable bandgap from indirect to direct resulting in unique electronic and optoelectronic capabilities. Chemical vapor deposition (CVD) is well acknowledged as an advanced growth method for GeSn, demonstrating its capability with grown materials for infrared lasers and detector development. As an in situ diagnostics of the CVD process, a residual gas analyzer (RGA) could enable the detection of all gaseous species during growth, thus probing the growth dynamics and mechanism. Therefore, it is highly desirable but is lacking in the (Si)GeSn research community. This work utilized an RGA equipped with a specialized differential vacuum pumping system to analyze the mass spectra of the GeSn CVD precursors of SnCl4, GeH4, and their combination. The spectra of SnCl4 collected from gas samples at a standard base pressure of 0.3 Torr display consistent fragments with central mass-to-charge ratios (m/z) of 78, 120, 155, 225, and 260, corresponding to Sn+, SnCl2+, SnCl+, SnCl2+, SnCl3+, and SnCl4+, respectively. These profiles closely resemble those observed in the National Institute of Standards and Technology and Wiley and Matsumoto et al. The simultaneous introduction of SnCl4 and GeH4 at a combined pressure of several tens of Torr yields spectra, indicating a chemical reaction that produces GeCl4 at room temperature in the vacuum chamber and tubing. Utilizing Gaussian16 and ORCA codes, the ab initio and density functional thermochemistry computations were employed to predict potential essential reactions and validate the experimental findings.

The integration and tests of the shattered pellet injection system on EAST

A SPI system specifically designed for EAST has been integrated and commissioned to carry out plasma rapid shutdown. This system is capable of injecting pellets with quantities ranging from 7 to 14 Pa m3 and at velocities between 150 and 400 m/s, as determined after conducting a series of bench tests. A two-stage differential pumping system designed to remove the propellant gas has had its efficiency assessed through the application of pumping equations and the fourth-order Runge-Kutta routine. For the purpose of facilitating remote control of the SPI system, a control system based on EPICS has been developed. This system enables an automated workflow for pellet formation and launch, which includes stages such as pellet formation, strengthening pellet, awaiting trigger, launching pellet, baking gun barrel, and cooling cold head, with the entire process taking 600 s. Furthermore, rapid shutdown experiments using Ne shattered pellet injection have been conducted, demonstrating the system's high reliability and reproducibility on EAST.

Novel differential pumping system with extended sampling range: Design and application in the EAST tokamak

A novel differential pumping system (DPS) with an adjustable sampling rate has been developed and implemented in the Experimental Advanced Superconducting Tokamak (EAST). This system is equipped with a high-resolution quadrupole mass spectrometer (QMS), covering a mass range of 1–16 atomic mass units (AMU), alongside a conventional QMS with a broader range of 1–200 AMU. The DPS features a dynamic pressure sampling capability from 10−2 to 105 Pa, utilizing a gas dosing valve and an angle valve, enabling a wide gas sampling ratio range of 10−9 to 3 × 10−2. The high-resolution QMS (QMG700) is adept at directly quantifying the partial pressures of deuterium and helium within mixtures, while the conventional QMS (RGA200) offers indirect measurement of D2 partial pressures above 2 × 10−5 Pa, complemented by direct wide-range mass analysis. During the EAST tokamak campaigns of 2023 and 2024, the DPS was effectively utilized for glow discharge cleaning (GDC) at a pressure of 3 Pa and ion cyclotron wall conditioning (ICWC) at approximately 3 × 10−3 Pa, accurately distinguishing deuterium pressure from the helium environment in the wide pressure range of vacuum vessel. Moreover, the DPS was successfully employed to trace the vacuum leak location in EAST campaign with a pressure of up to hundreds Pa. This DPS is a key tool for gas ingredient analysis and leak detection in the EAST campaigns, providing a valuable reference for the vacuum operation of future fusion reactors.

Closed-cycle noble gas recycling system for high-repetition rate high-harmonic generation.

We present a compact closed-loop recycling system for noble and inert gases. It has been developed for an extreme-ultraviolet (XUV) frequency comb based on high-harmonic generation at 100 MHz repetition rate. The system collects gas injected at several bars of backing pressure through a micrometer-sized nozzle into the laser-interaction region with a differential pumping system comprising turbomolecular pumps, and subsequently compresses the gas to a pressure of up to 200bar. By drastically reducing the waste of expensive gases such as xenon and krypton, it enables the long operation times needed for spectroscopic measurements, as well as for continuous operation of the XUV frequency comb.

Implementation of extreme ultraviolet spectroscopy on a sheared-flow-stabilized Z pinch

A diagnostic for extreme ultraviolet spectroscopy was fielded on the sheared-flow-stabilized (SFS) fusion Z-pinch experiment (FuZE-Q) for the first time. The spectrometer collected time-gated plasma emission spectra in the 5–40 nm wavelength (30–250 eV) range for impurity identification, radiative power studies, and for plasma temperature and density measurements. The unique implementation of the diagnostic included fast (10 ns risetime) pulsed high voltage electronics and a multi-stage differential pumping system that allowed the vacuum-coupled spectrometer to collect three independently timed spectra per FuZE-Q shot while also protecting sensitive internal components. Analysis of line emission identifies oxygen (N-, C-, B-, Be-, Li-, and He-like O), peaking in intensity shortly after maximum current (>500 kA). This work provides a foundation for future high energy spectroscopy experiments on SFS Z-pinch devices.

Characteristic test of dual shattered pellet injection systems of KSTAR

Dual shattered pellet injection (SPI) systems are installed in KSTAR (Korea Superconducting Tokamak Advanced Research) to study disruption mitigation in support of the ITER DMS (Disruption Mitigation System) design activities. KSTAR has two identical pellet injectors that can shoot pellets simultaneously from opposite toroidal locations. Each injector can shoot up to of three different diameter pellets. The pellet integrity and velocity after formation have been measured using microwave cavity (MWC) systems since 2019. However, this diagnostic provides only limited information on the pellet integrity, especially for partially broken pellets. Therefore, an optical pellet diagnostic (OPD) system monitoring the pellet has been installed in the first stage viewport of the differential pumping system and is operational since 2021. The KSTAR SPI systems offer a wide parameter range that must be characterized before the experimental campaign. To satisfy the experimental requirements, a proper procedure for pellet formation and acceleration must be commissioned. We carry out the commissioning by varying pellet parameters such as formation duration, temperature, heating, propellant gas pressure, and pulse length. The resulting pellet velocity, integrity, and tilt can be measured with the OPD system. These tests can be performed without plasma operation because no additional information is needed from the plasma response to the injection. This paper describes the results of the characteristic tests of single and multiple injections of dual SPI.

Deviation of photoelectron intensity from Beer-Lambert law in near-ambient pressure hard x-ray photoelectron spectroscopy

In near-ambient-pressure photoelectron spectroscopy, the photoelectron intensity is assumed to follow the Beer-Lambert law, that is, the intensity decreases exponentially with distance d between the sample and the aperture cone. In this study, the gas pressure dependence of photoelectron intensity is systematically studied in a wide range of d values from 0.3 up to 5 mm. The experimental results were reproduced by replacing d with d + do (do is a constant) in the Beer-Lambert law. The do value was evaluated as ∼1 mm, which is considerably larger than the normal d value of 0.3 mm. Fluid dynamics simulation results obtained using a structural model with a size close to that of the actual differential pumping system suggested that the residual gas in the long pumping path caused a large do value.

A compact gas attenuator for the SwissFEL ATHOS beamline realized using additive manufacturing.

Gas attenuators are important devices providing accurate variation of photon intensity for soft X-ray beamlines. In the SwissFEL ATHOS beamline front-end the space is very limited and an innovative approach has been taken to provide attenuation of three orders of magnitude up to an energy of 1200 eV. Additive manufacturing of a differential pumping system vacuum manifold allowed a triple pumping stage to be realized in a space of less than half a meter. Measurements have shown that the response of the device is as expected from theoretical calculations.

(Invited) On Several Atmospheric Pressure Plasma Sources

In this talk, firstly, several plasma jets developed in our group are presented, including a nobble gas plasma jet for root canal treatment and four air plasma jet devices [1, 2] . Secondly, several interesting physics phenomena of plasma jets are introduced, such as the repeatability of the plasma bullet, the plasma interruption behavior, the plasma transfer dielectric phenomenon, and the appearance of multiple plasma bullet per voltage pulse [2-4] . Thirdly, the state-of-the-art on the measurement of VUV emission of atmospheric pressure air plasma is discussed and our results by using differential pumping system as EUV window are presented [5] . Fourthly , several different configurations of nonequal gap distance DBD are described [6, 7] . Finally, an atmospheric pressure glow discharge in air stabilized by a magnetic field and its application on nitrogen fixation are rep orted. Li, J. Liu, X. Lu, A large atmospheric pressure nonequilibrium open space air plasma based on a rotating electrode,P lasma Sources Sci. Technol. 29, 045015 (2020)Lu*, G. Naidis, M. Laroussi, K. Ostrikov, Guided ionization waves: Theory and experiments. Physics Reports , 540,123-166 (2014)Lu*, and Kostya (Ken) Ostrikov, Guided ionization waves: The physics of repeatability, Appl. Phys, Rev ,5, 031102 (2018)Liu, L. Nie, Y. Xian, X. Lu*, The twisted behavior of a rotating electrode atmospheric-pressure argon plasma jet, J. Phys. D: Appl. Phys. 54, 185201 (2021)F Liu, L Nie, X Lu, J Stephens and K Ostrikov, Atmospheric plasma VUV photon emission, Plasma Sources Sci. Technol. 29, 065001(2020)Li, S. Jin, Y. Xian, L. Nie, D. Liu, X. Lu*, A non-equal gap distance dielectric barrier discharge: between a wedge-shaped and a plane-shaped electrode, Plasma Sources Sci. Technol. 30, 065026 (2021)Jin, Z. Li, Y. Xian, L. Nie, X. Lu*, A non‐equal gap distance dielectric barrier discharge: Between cone‐shape and cylinder‐shape electrodes, High Voltage , 1-9 (2021)

Development of a double-barreled Tracer Encapsulated Solid Pellet (TESPEL) injection system for LHD

For performing a study of impurity transport efficiently by using a Tracer-Encapsulated Solid Pellet (TESPEL) in Large Helical Device (LHD), a new double-barreled TESPEL injection system was recently developed and installed on the LHD. The TESPEL in each injection line can be injected with different timing and acceleration pressure which can be set individually. To prevent the high-pressured gas for the TESPEL acceleration from getting into the vacuum vessel of the LHD, a 3-stage differential pumping scheme was applied. Since the available space is very limited, the 3-stage differential pumping system is shared with each TESPEL injection line. And for the same reason, one of the guide tubes in one injection line of the new TESPEL injection system was purposely bent (bending radius of about 40 m). The guide tubes in the other injection line of the new TESPEL injection system are straight. We performed a laboratory test to check the TESPEL transferring capability with the guide tubes in the new injection system. The points of TESPEL impact on the target foil are found to be within the circles defined by a possible pellet divergence full angle, 2 degrees. Therefore, we confirmed that the TESPEL can pass through both the slightly bent and the straight injection lines of the new injection system without any problem. The blank shot test shows that the increment of the total pressure even by using both the injection lines simultaneously is much below the base pressure of the LHD vacuum vessel during the discharge. This result successfully demonstrated the capability of the applied 3-stage differential pumping scheme in the new injection system.

Apparatus for generation of nanojoule-class water-window high-order harmonics.

In our recent study [Fu et al., Commun. Phys. 3(1), 92 (2020)], we have developed an approach for energy-scaling of high-order harmonic generation in the water-window region under a neutral-medium condition. More specifically, we obtained a nanojoule-class water-window soft x-ray harmonic beam under a phase-matching condition. It has been achieved by combining a newly developed terawatt-class mid-infrared femtosecond laser and a loose-focusing geometry for high-order harmonic generation. The generated beam is more than 100 times intense compared to previously reported results. The experimental setup included two key parts: a terawatt mid-infrared femtosecond driving laser [Fu et al., Sci. Rep. 8(1), 7692 (2018)] and a specially designed gas cell. Despite the dramatic drop in the optimal gas pressure for phase-matching due to loose-focusing geometry, it still reached the 1bar level for helium. Thus, we have designed a double-structured pulsed-gas cell with a differential pumping system, which enabled providing sufficiently high gas pressure. Moreover, it allowed reducing gas consumption significantly. A robust energy-scalable apparatus for high-order harmonic generation developed in this study will enable the generation of over ten-nanojoule water-window attosecond pulses in the near future.

Design and fabrication of a differential pressure MEMS capacitance diaphragm gauge based on heavily boron-doping technique

In this paper, a differential pressure MEMS CDG with a full range of (5–4000) Pa has been developed, and its pressure-sensing diaphragm is manufactured by boron-doping technique. The design principle and manufacturing process of the MEMS CDG were demonstrated in detail. To test the working performance of the CDG, a differential pressure high vacuum pumping system and a precision LCR meter are adopted to obtain the capacitance value of the vacuum gauge varying with pressure. The test results show that the MEMS CDG can work normally in the pressure range of (5–4000) Pa, and the maximum sensitivity is 87.2 fF/Pa. In addition, boron-doped diaphragm can effectively improve the sensitivity of vacuum gauge and further extend its measurement upper limit.

Construction and experimental tests of the high frequency pellet injection system on EAST tokamak

A high frequency deuterium pellet injection system based on extrusion and pneumatic acceleration has been recently developed and built for EAST tokamak, which is one of fully superconducting Tokamaks with ITER-like plasma configuration. It is mainly used for plasma edge localized mode control and can achieve 1–50 Hz injection rate. It has been completely installed on EAST up to now. A new differential pumping system and pellet transmission lines have been designed according to the actual condition of EAST. Recently we have tested the system systematically and injected multiple pellets into type-I ELMy H-mode plasma on EAST for the first time. The test and experimental results show that the system can inject continuous pellets into plasma stably, and ELMs can be triggered by deuterium pellets in H-mode plasma with negligible impact on density and stored energy. And it was also found that ELM trigger efficiency is less than 100 % sometimes. This lays an important foundation for further experimental study of ELM modulation using deuterium pellet injection on EAST.

Neutral gas analysis for JET DT operation

Neutral gas analysis, the measurement and evaluation of total and partial pressures, is a key technique to study the impact of neutral gas dynamics on retention, recycling and release processes of fuel or impurity species in fusion devices. At JET, the experiment closest to ITER in terms of operating parameters and size, various detectors and techniques for partial pressure and total pressure measurements are deployed together to characterise neutral gas dynamics during and after plasma operation on various toroidal and poloidal locations. An extensive modification of JET's sub-divertor neutral gas diagnostic system aims at retaining and extending established measurement capabilities in the forthcoming Deuterium-Tritium (DT) experiments (DTE2). To achieve DT compatibility, a separation of radiation-sensitive electronics from the sensor and adequate radiation shielding is required, as well as utilisation of a DT compatible differential pumping system with adjustable throughput to account for the strong pressure variation in the sub-divertor region. Finally, the sub-divertor neutral gas diagnostic will be equipped with multiple Residual Gas Analysers (RGAs), utilising quadrupole mass spectrometry and electrostatic ion-trap-principles, all operating with remote electronics located behind the biological radiation shield. These RGAs will record data in a fast selected discrete mass mode during plasma pulses (cycle ∼2 s) and will automatically switch back to continuous data recording (cycle ∼100 s) afterwards. They will be complemented by a newly improved Penning gauge spectroscopy configuration in particular supporting the He and D2 separation relevant for DT operation. The distance between these devices and their associated control unit is typically 15m . A newly developed RGA with a cable length of 80 m, compatible with the ITER environment, will also be employed for the first time. This set-up and its operation in DTE2 will provide vital input to the development of the ITER divertor RGA in the most relevant environment currently available.

Integration of the TESPEL injection system at W7-X

Since impurities determine the plasma performance in magnetically-confined fusion plasmas, analyzing their behavior and controlling their confinement is significantly important for a stable and steady-state operation of fusion devices. A comparison of the impurity transport in the core region of large stellarators, such as Wendelstein 7-X and Large Helical Device (LHD) is quite useful to gain a better understanding of the impurity transport in reactor-relevant stellarator plasmas. Therefore, a Tracer-Encapsulated Solid Pellet (TESPEL) injection system has been newly installed for the operational phase OP1.2b of W7-X. The injector was designed and manufactured by NIFS to achieve a similar TESPEL deposition location as in LHD. Technical guidelines and functional specifications, addressing the specific conditions at W7-X had been implemented in close collaboration between NIFS and IPP. This resulted in a compact 3-stage differential pumping system considering spatial restrictions due to existing and future diagnostics, material requirements and limitations due to the magnetic stray field in the vicinity of the turbo pumps. This contribution reports on the design and the integration of the new TESPEL injection system to W7-X and first achievements during the commissioning of the system.

Non-Evaporative Getter-Based Differential Pumping System for SRILAC at RIBF

Non-Evaporative Getter-Based Differential Pumping System for SRILAC at RIBF

Ambient Pressure XPS Studies of Liquid Solid Interfaces in Batteries

Liquid solid interfaces in batteries are not trivial to study in a realistic and detailed way. Often interfaces of cathodes or anodes in cycled batteries, are studied post mortem and at ultra-high vacuum to really determine the chemical composition in the outermost electrode/electrolyte layer. It is, therefore, a need to develop new characterisation methods which can allow for the study of dynamic reactions taking place at interfaces as a function of battery cycling. In this presentation, we will describe our attempts to develop electrochemical cells that can be used at ambient pressure (25 mbar). The cells are dependent on photoelectron spectrometers (PES) that have differential pumping systems and where the samples can be protected but still transferred without intermediate ultrahigh vacuum step of the load lock. These spectrometer exist now at synchrotron facilities but also as in house equipment. The challenge is how the electrochemical cell should be designed and how to study the liquid solid interfaces in detail and especially when the liquids are organic volatile solvents. Our first results will be presented as three examples. The first example is a comparison of the solid electrolyte interphase (SEI) on a silicon surface measured at ultrahigh vacuum with one measured at ambient pressure. Questions such as: will the chemical composition of the SEI be considerably different at ambient pressure and will the resolution of the analysis be high enough to reveal any differences in the chemical composition? The second example is a study of the surface of a droplet of a lithium-ion battery electrolyte on a metallic lithium substrate characterised at 2 mbar and 0.2 mbar, respectively, and studied with ambient pressure PES. The study was performed on the beam-line Hippie at MAXIV laboratory. Our third result is the cycling of an electrode with an electrolytes within electrochemical cell in the Hippie spectrometer. The implication of all these three studies is that a step has been taken to more realistic studies of surface reactions in a LIB context. The experimental challenges are also detailed in the presentation.

Beam thermalization in a large gas catcher

Thermalization of fast ions in a buffer gas provides a method to transform the high-energy, exotic beams produced by projectile fragmentation at the National Superconducting Cyclotron Laboratory (NSCL) into low-energy beams. The process includes slowing down the fast exotic beams in solid degraders combined with momentum compression and removal of the remaining kinetic energy by collisions with a buffer gas. The original beam thermalization area for mass measurements at the NSCL was reconfigured to accommodate a new momentum compression beam line, a large radio-frequency (RF) gas catcher constructed by Argonne National Lab and a low-energy beam transport system. A large variety of exotic isotopes produced by projectile fragmentation and selected by the A1900 fragment separator was thermalized in the 1.2 m long gas catcher filled with helium at approximately 100 mbar. The ions were guided to an extraction nozzle with a combination of electrostatic and RF potentials and ejected by the gas flow. A novel RF ion guide was used in a differential pumping system to remove the helium and transport the ions into ultrahigh vacuum. Finally, the ions were accelerated to 30 kV for transport to various experiments. The distribution of the thermalized ions among chemical adducts is one of the operational challenges. The important steps implemented to minimize the production of the chemical adducts in the gas catcher are discussed. The operational status of the facility and some example results from characterization of the gas catcher operation with 37K and 47K beams are presented.

A near ambient pressure photoemission electron microscope (NAP-PEEM)

Photoemission electron microscopy (PEEM) is a powerful surface technique for dynamic imaging of surface processes while PEEM studies have been performed under ultrahigh vacuum or high vacuum conditions. Here we report on a near ambient pressure PEEM (NAP-PEEM) instrument, which enables high resolution PEEM imaging in near ambient pressure (> 1 mbar) gases over a wide temperature range (150 – 1200 K). Installed with an electron gun near ambient pressure low energy electron microscopy (NAP-LEEM) can be achieved as well. The success of this new NAP-PEEM/LEEM instrument relies on the following key design concepts. First, a two-stage accelerating electric field consisting of a low field region between sample and intermediate electrode (“nozzle”) and a high field between nozzle and objective lens. Second, a three-stage differential pumping system allowing a near ambient pressure atmosphere at the sample surface while ultrahigh vacuum maintained in the imaging lens systems. Third, a unique NAP cell with gas inlet/outlet, light illumination, sample cooling/heating, and precise sample positioning. The new technique will have important applications in surface catalysis, thin film growth, and energy conversion devices under nearly realistic working conditions.