Ionization Gauge Research Articles

Verification of lifetime and mechanical load aspects for ITER diagnostic pressure gauges based on ZrC emitter.

Hot cathode ionization gauges will measure the neutral gas pressure in the vacuum vessel of ITER. Overall, 52 gauge heads based on the concept of the ASDEX pressure gauge but using novel ZrC emitters are located in the divertor, in equatorial ports and in pumping ducts. The initial lifetime tests of the novel gauge design have raised the concern that the pyrolytic graphite used to indirectly heat the ceramic ZrC emitter erodes too fast during operation. This effect might limit the lifetime of the gauges below the limits acceptable according to the inherent availability (ignoring operational delays) requirement of 99.8% for the diagnostic system over the lifetime of ITER. Long term operation tests have been performed using 0.1mm thick W, Ir, and Mo foils as an interlayer between the pyrolytic graphite and the ZrC within a constant atmosphere of 20Pa H2. The longest lifetime of 860h has been achieved with the Ir foil. Subsequent vibration tests applying accelerations on a shaker with magnitudes typical for seismic and ITER disruption loads demonstrated that the emitter can survive such demanding loads even after far-progressed erosion of the pyrolytic graphite. Additional ON/OFF cycle tests demonstrated that the gauge is still fully operational. In combination with a revision and consolidation of the operational plan and the reliability, accessibility, maintainability, and inspectability (RAMI) analysis, ITER's inherent availability requirement for the diagnostic system could be demonstrated successfully.

Cylindrical hot cathode ionisation gauge – Construction and testing

Hot cathode ionisation gauges are indispensable in measuring high and ultra-high vacuum. In the present work the construction details and the metrological evaluation of a novel gauge design — the cylindrical ionisation gauge is presented. With the aim of enhancing accuracy and stability both in the short and long term, details of the mechanical assembling of electrodes and precautions to minimize leakage currents influencing ion current measurement are described. Performance evaluations demonstrate high short-term stability, with linearity error, reproducibility, and repeatability values consistently below 1 %, placing the gauge at the same level as the most stable ionisation gauges. Operating across a pressure range of at least 6 orders of magnitude, the gauge exhibits low linearity error from high to ultra-high vacuum ranges. Its capability to measure pressures below 10−9 mbar remains open and requires further investigation.

Developing a penning ionization gauge ion source with a filament for a compact ion microbeam system

We present the development of a penning ionization gauge ion source with a filament (F-PIG) for producing submicron ion beams in a compact ion microbeam system. The conventional penning ionization gauge ion source (PIG) represents a viable choice for microbeam system due to its compact size and low power consumption. However, directly utilizing a PIG for generating submicron ion beam poses challenges, primarily from significant energy spread, causing substantial aberrations. Considering this, this study proposes a solving the generation of low-voltage discharge plasma, achieved using low-energy electrons from a filament by focusing on the development of F-PIG for this purpose. Experimental results demonstrate that the F-PIG exhibits considerable promise an ion source to forming submicron beam within a compact ion microbeams system. These findings indicate that F-PIG can effectively reduce the width of the beam energy spread, making it a valuable asset for microbeam applications.

Development of a novel Internal Radio-frequency Ion Source for Cyclotrons (IRISC)

IRISC (Internal Radio-frequency Ion Source for Cyclotrons) is an innovative design of an internal ion source, primarily for producing H− ions, in which 2.45 GHz radio-frequency power is supplied to its electrodes for achieving electron emission. Theoretical work indicates that lower electrode sputtering and higher extraction efficiency of H− may be achieved as compared to its standard cold-cathode direct current (DC) Penning Ionization Gauge (PIG) counterparts. If achieved, IRISC would arise as a ground-breaking ion source with significant importance for compact cyclotrons. IRISC dimensions have been matched to CIEMAT’s AMIT cold-cathode DC PIG ion source so that, in the future, side-by-side testing can be carried out and, eventually, both types can be used in AMIT cyclotrons. This contribution presents IRISC key design aspects, outcome from simulations, its dedicated test bench and first low-power test results from a prototype.

Cold atom technology applied to ultra-high vacuum (UHV) measurements

Accurate measurements in ultrahigh vacuum (UHV) are increasingly critical for scientific research and industrial applications. Here, we report an experimental study of UHV measurements performed on a 7Li cold atom vacuum measurement setup. The setup enables cold atom vacuum measurements in the magneto-optical and magnetic traps (CAVM-MOT and CAVM-MT), respectively. The precision of the vacuum measurements in both traps was effectively improved after optimisation of the excited-state fraction data processing for CAVM-MOT and the initial loading control method for CAVM-MT. A comparison of the measurement methods was carried out in combination with an ionisation gauge, revealing the good linearity and accuracy of the CAVM. We believe that CAVM in the two potential traps have their own application situations and development directions, respectively.

Development of Penning ion gauge for in-situ measurement of neutral pressure in VEST

A Penning Ion Gauge (PIG) has been developed to measure the responsive temporal changes of neutral pressure inside the magnetic fusion devices, and successfully operated in Versatile Experiment Spherical Torus (VEST). To check its reliabilities during in-situ operation, feasibility tests are conducted by specifically investigating both the penning discharge mode transition and the effect of magnetic field alignment on the penning discharge. The mode transition of penning discharge is clearly observed, confirming the developed PIG is applicable to the operation regime of VEST. The PIG is also designed to measure discharge current stably during magnetic field angle variation of upto 15∘. In addition, it has been verified that the operation range of PIG can be expanded by increasing inverse aspect ratio of the gauge cell geometry. Finally, the developed PIG is installed in VEST, measuring a millisecond-order temporal changes in neutral pressure within a range of 3×10−5 mbar to 1×10−3 mbar, under the magnetic field angle variation of up to 15∘. Finally, it is successfully measured rapidly evolving neutral pressures resulting from the plasma wall interaction during ohmic discharges in VEST.

Characteristics of the pressure profile in the accelerator on the RF negative ion source at ASIPP

Neutral beam injection (NBI) systems based on negative hydrogen ion sources—rather than the positive ion sources that have typically been used to date—will be used in the future magnetically confined nuclear fusion experiments to heat the plasma. The collisions between the fast negative ions and neutral background gas result in a significant number of high-energy positive ions being produced in the acceleration area, and for the high-power long-pulse operation of NBI systems, this acceleration of positive ions back to the ion source creates heat load and material sputtering on the source backplate. This difficulty cannot be ignored, with the neutral gas density in the acceleration region having a significant impact on the flux density of the backstreaming positive ions. In the work reported here, the pressure gradient in the acceleration region was estimated using an ionization gauge and a straightforward 1D computation, and it was found that once gas traveled through the acceleration region, the pressure dropped by nearly one order of magnitude, with the largest pressure drop occurring at the plasma grid. The computation also revealed that the pressure drop in the grid gaps was substantially smaller than that in the grid apertures.

Study of a low-energy collimated beam electron source and its application in a stable ionisation gauge

Stable ionisation vacuum gauges obtain stable sensitivity by controlling both the well-defined electron trajectory and electron energy in the ionisation volume. To achieve this goal, a collimated electron beam and large ionisation cross-section are required as the electrons enter the ionisation volume. In this paper, a low-energy electron source based on carbon nanotubes is designed to realise a collimated electron beam by the addition of specialised deceleration and focus electrodes. The experimental transmission is approximately 25%, which is in good agreement with the simulation results. Furthermore, a simulation study is carried out combining the electron source with the components of a novel stable ionisation gauge. The numerical simulation results show a sensitivity of 0.250 Pa-1. It provides an important reference for the development of stable ionisation gauge based on the field emission cathode.

Tailoring On-Surface Molecular Reactions and Assembly through Hydrogen-Modified Synthesis: From Triarylamine Monomer to 2D Covalent Organic Framework.

Relative to conventional wet-chemical synthesis techniques, on-surface synthesis of organic networks in ultrahigh vacuum has few control parameters. The molecular deposition rate and substrate temperature are typically the only synthesis variables to be adjusted dynamically. Here we demonstrate that reducing conditions in the vacuum environment can be created and controlled without dedicated sources─relying only on backfilled hydrogen gas and ion gauge filaments─and can dramatically influence the Ullmann-like on-surface reaction used for synthesizing two-dimensional covalent organic frameworks (2D COFs). Using tribromo dimethylmethylene-bridged triphenylamine ((Br3)DTPA) as monomer precursors, we find that atomic hydrogen (H•) blocks aryl-aryl bond formation to such an extent that we suspect this reaction may be a factor in limiting the ultimate size of 2D COFs created through on-surface synthesis. Conversely, we show that control of the relative monomer and hydrogen fluxes can be used to produce large self-assembled islands of monomers, dimers, or macrocycle hexamers, which are of interest in their own right. On-surface synthesis of oligomers, from a single precursor, circumvents potential challenges with their protracted wet-chemical synthesis and with multiple deposition sources. Using scanning tunneling microscopy and spectroscopy (STM/STS), we show that changes in the electronic states through this oligomer sequence provide an insightful view of the 2D COF (synthesized in the absence of atomic hydrogen) as the end point in an evolution of electronic structures from the monomer.

Testing of ITER diagnostic pressure gauges in preparation of final design review

Hot cathode ionization gauges will measure neutral gas pressures in the vacuum vessel of ITER. Overall 52 gauge heads are distributed in the divertor, the equatorial ports and pumping ducts. Present design and prototype testing of diagnostic pressure gauge (DPG) heads is presented. Focus is on the development of a novel type of cathode, which is able to overcome major issues that emerged during the preliminary design phase. In comparison with the formerly employed yttria coated tungsten–rhenium filament, the operational stability of the present cathode, which uses an indirectly heated plate of zirconium carbide as the electron emitter, is enhanced in terms of needed heating current and accuracy. A major issue of filament deflection by electromagnetic forces was overcome through the rigid setup, which still allows for the compensation of differential thermal expansion and provides a high reproducibility in manufacturing. Reliable functioning for long periods was proven successfully in a superconducting magnet, covering the required pressure range. Prototype testing revealed an issue during permanent operation in hydrogen at high pressures: erosion can lead to limited life-time of the cathode. The introduction of interlayer foils can retard but not completely inhibit the process. A possible mechanism based on reaction and diffusion of carbon is proposed.

Precise quantum measurement of vacuum with cold atoms.

We describe the cold-atom vacuum standards (CAVS) under development at the National Institute of Standards and Technology (NIST). The CAVS measures pressure in the ultra-high and extreme-high vacuum regimes by measuring the loss rate of sub-millikelvin sensor atoms from a magnetic trap. Ab initio quantum scattering calculations of cross sections and rate coefficients relate the density of background gas molecules or atoms to the loss rate of ultra-cold sensor atoms. The resulting measurement of pressure through the ideal gas law is traceable to the second and the kelvin, making it a primary realization of the pascal. At NIST, two versions of the CAVS have been constructed: a laboratory standard used to achieve the lowest possible uncertainties and pressures, and a portable version that is a potential replacement for the Bayard-Alpert ionization gauge. Both types of CAVSs are connected to a combined extreme-high vacuum flowmeter and dynamic expansion system to enable sensing of a known pressure of gas. In the near future, we anticipate being able to compare the laboratory scale CAVS, the portable CAVS, and the flowmeter/dynamic expansion system to validate the operation of the CAVS as both a standard and vacuum gauge.

Comparison of ionization vacuum gauges close to their low pressure limit

Parts of CERN’s accelerator complex and experiments, especially in the antimatter field, require a vacuum in the 10−12mbar range or better. Thus gauges are needed to reliably measure XHV during experimental operation and in order to study the vacuum science needed for those experiments. We therefore built a setup to reach 1⋅10−13mbar in order to simultaneously compare different hot cathode ionization gauges with the ability to measure high UHV and XHV close to their lower pressure limit: Barion extended, Extractor IE514, a modulated Bayard–Alpert gauge and two Improved Helmer gauges. All gauges but the Extractor behave similarly with respect to small pressure variations around the limit pressure, while the Extractor seems to overestimate high UHV hydrogen pressure. We show how gauge operation determines our ultimate achievable pressure due to outgassing, which was comparable for all gauges and in the order of Q∼10−10mbarls−1. Further we show the disturbances caused in the static system due to gauge pumping (visible only as electronic pumping), and report some of the possible difficulties and origins of noise when measuring pressures in the XHV range, including the thermoelectric effect.

Particle orbit optimization in the beam extraction region of a cold cathode PIG negative ion source for electrostatic tandem accelerators

When using a PIG (Penning Ion Gauge) type ion source for production of negative ions, these ions must be extracted from a hole on the side of the anode cylinder, perpendicularly to the magnetic field for plasma confinement. As a result, unnecessary deflection force works on the extracted beam. This study aims to improve the ion extraction efficiency of a cold-cathode PIG negative-ion source. The idea is to reduce the beam loss by cancelling the stray magnetic field from the source by placing small dipole magnets in the extraction electrode. Through the measurement of the magnetic field distribution and beam trajectory calculation, it was numerically confirmed that the beam trajectory can be corrected by this small dipole magnet with a peak magnetic field of about 50 mT under the present experimental condition.

Effect of the magnetic field on the operation of ionisation gauges

Hot cathode ionisation gauges are the only reliable pressure measurement devices suitable for both high and ultra-high vacuum measurement. These devices are characterized by low accuracy that is hardly better than 20%. The sources of loss of accuracy are documented in the literature although their quantification and details are not fully understood. In the present work, simulations of Bayard-Alpert and extractor gauges were performed under the influence of uniform magnetic fields up to 30 Gs. Results are in qualitative agreement with measurements taken previously with the Bayard-Alpert gauge. Both gauges are sensitive to low intensity magnetic fields which may be present in vacuum chambers. However, the extractor gauge proved to be more stable in the presence of a magnetic field due to its ring-shaped cathode.

Experimental investigation of current jumps in linear geometry hot cathode ionization gauges in strong magnetic fields

In magnetic confinement nuclear fusion, neutral gas in the plasma edge region plays an important role for plasma fuelling and plasma wall interaction and thus influences the overall plasma behaviour. The ASDEX pressure gauge is a diagnostic instrument designed to provide local and temporally highly resolved measurements of the neutral gas flux density within the strong magnetic fields of fusion devices. It is a hot cathode ionization gauge with a linear geometry. Its function principle relies on the measurement of an ion current, caused by oscillating electrons in the gauge which ionize the neutral gas. Anomalous jumps in the ion current of up to 30% have been observed during gauge operation in the tokamak ASDEX Upgrade and the stellarator Wendelstein 7-X, which introduces an error in the measurement. This paper describes for the first time the characterization of these jumps over the broad operational space. Parameter dependencies on pressure, electron current, electrode potentials and magnetic field angle are discussed. The appearance of electrostatic oscillations with frequencies in the range of 160MHz to 350MHz which are associated with the current jumps has been discovered. The frequency of these oscillations was found to scale with the oscillation frequency of the electron motion inside the gauge. Potential physical mechanisms to explain the jumps are discussed.

Investigating the interactions of hydrogen and nitrogen with the tantalum cathode of a novel ionization vacuum gauge with a straight path of electrons

The outgassing characteristics of a tantalum disk cathode mounted inside a novel ionization gauge were investigated in comparison to an yttrium-coated iridium-filament cathode mounted inside an extractor gauge. Both gauges were exposed to nitrogen or hydrogen at a pressure of 2.4 × 10−5 mbar, and the outgassing rates were measured periodically during subsequent pumping. For the novel ionization gauge, two contributions to the outgassing, i.e., the thermal outgassing from the cathode and the electron-stimulated desorption from the Faraday cup, were studied separately. The tantalum cathode showed rapid outgassing of nitrogen after exposure, while the outgassing after exposure to hydrogen was much lower. Hydrogen is mainly released from the Faraday cup when electrons collide with it. The extractor gauge showed a more-than-3-times higher outgassing rate after hydrogen exposure than after nitrogen exposure. The tantalum cathode and the Faraday cup in the novel ionization gauge showed absorption equilibrium after just 30 min.

Novel ionisation vacuum gauge suitable as a reference standard: Influence of primary electron trajectories on the operation

A detailed study of the operation of a novel ionisation gauge, recently proposed as a reference standard by the consortium of the European project 16NRM05, is performed with focus on the influence of electron trajectories and the interaction of electrons with electrode surfaces. The strategy behind the gauge design was to provide well defined primary electron trajectories that form an electron beam. This investigation seeks to correlate experimental investigations of improper primary beam focusing, electron beam profiling, electron stimulated desorption and outgassing, with charged particle optics simulations that include space charge effects, and a detailed study of secondary electron emission. The main outcome of the research is that control of the primary electron trajectories, quantified by electron transmission efficiency, is essential for its proper operation mainly as a pressure measurement device but also as a vacuum gauge sensitivity reference standard. Once all primary electrons end their trajectories in a Faraday cup, being one of the gauge electrodes, two additional problems are identified: escape of a very small fraction of electrons from the Faraday cup and electron stimulated desorption of neutrals. Their contribution is of low significance when using this device as a sensitivity reference standard in the high vacuum range, which meets the major goal of the 16NRM05 project. However, these effects might represent an obstacle for the gauge application, mainly as a measurement device, at lower pressures. Approaches to mitigate the mentioned potential problems are discussed.

Low-pressure gas flow properties of sintered stainless steel microporous media

The gas flows through the low permeable porous media are of great interest, especially in vacuum technology for filtering, separation processes, protection, and flow control. They can combine a high mass flow rate and a high level of rarefaction. The transient experimental technique, developed previously for the mass flow rate measurements through the microchannels [Rojas Cardenas et al., Phys. Fluids 23, 031702 (2011)], is generalized to obtain the permeability and conductance directly from the pressure variation measurements. The present experimental methodology, allowing for step by step data verification, leads to higher accuracy than the similar and commonly used method such as “pulse-decay” techniques [W. F. Brace and R. J. Martin, Int. J. Rock Mech. Min. Sci. 5, 415–426 (1968)]. The measured data are fitted according to the exponential function with the pressure relaxation time as a single fitting parameter. The permeability and conductance of the sintered stainless steel microporous medium are measured for different gases. By using the methodology proposed by Johansson et al. [Int. J. Heat Mass Transfer 142, 118412 (2019)], the characteristic pore size of the sample was determined to be equal to 1.5 micrometers. It was found that free molecular flow constant conductance is achieved at a pressure below 30 mbar for helium. This property makes this kind of porous media particularly suitable as a leak element by taking advantage of the constancy of conductance in the free molecular flow regime, for example, for calibration of ionization gauges or mass spectrometers [Yoshida et al., Measurement 45(10), 2452–2455 (2012)].

Vacuum pressure measurement based on <sup>6</sup>Li cold atoms in a magneto-optical trap

Ultra-high vacuum measurement and extremely high vacuum (UHV/XHV) measurement play an important role in high-tech fields such as deep space exploration, particle accelerators, and nanoscience; with the continuous extension of the lower limit of measurement, especially when it reaches the order of 10<sup>–10</sup> Pa, higher requirements are placed on the accuracy of the measurement. At present, in the field of UHV/XHV measurement, ionization gauges based on the principle of neutral gas ionization are commonly applied to the vacuum measurement. However, traditional ionization vacuum gauges during use can create electronic excitation desorption effects, soft X-rays, and the effect of hot cathode outgassing, thereby affecting the accuracy of measurement and limiting the lower limit of measurement. Compared with the traditional measurement technology, this method uses the relationship between the loss rate and pressure caused by the collision of cold atoms trapped in the trap depth with the background gas to calculate the gas density and inversely calculate the vacuum pressure. Based on the intrinsic quantum mechanical properties of cold atom collisions, this method is expected to be developed into a new vacuum traceability standard. In this paper, based on the small-angle approximation and impulse approximation under the quantum scattering theory, the loss rate coefficient of the collision of <sup>6</sup>Li cold atoms with background gas molecules is calculated. According to the ideal gas equation, the pressure inversion formula is obtained. The collision loss rate is extracted by accurately fitting the loss curve of the cold atom. In order to improve the accuracy of vacuum inversion and reduce the influence of quantum diffractive collision on loss rate measurement, the trap depth under the conditions of a certain cooling laser intensity, detuning, and magnetic field gradient is determined by the photoassociation method. Finally, in a range of 1 × 10<sup>–8</sup>–5 × 10<sup>–6</sup> Pa, the inverted pressure value is compared with the measured value of the ionization meter, proving that this method has good accuracy and reliability in the inversion of vacuum pressure. At present, the main factor restricting the improvement of accuracy is the influence of the collision between the excited atoms in the magneto-optical trap and the background gas on the loss rate measurement. In the future, with the proportion of excited atoms and the excited state <i>C</i><sub>6</sub> coefficient to be precisely determined, the uncertainty of vacuum pressure measurement can be further reduced.

Precise control of time-varying effusion cell flux in molecular beam epitaxy

We outline a method to control time-varying effusion cell flux in molecular beam epitaxy. This can be used to grow 2D nanostructures with precise continuously graded alloys, and we explore the specific case of controlling an Al cell for the growth of continuously graded Al xGa 1 − xAs parabolic quantum wells. We develop a simple model of the Al cell’s thermal dynamics, including the effects of shutter transients. This model’s parameters can be empirically derived using a beam flux monitoring ion gauge. Though simple, this model can already be used to generate near-arbitrary time-varying fluxes (within the physical limits of the cell). For growths that require higher precision than the simple model can achieve, we introduce an iterative correction scheme to eliminate any lingering errors. Using this method, we control the composition in a 3 THz Al xGa 1 − xAs parabolic quantum well array to a root mean squared error of just ± 0.18 %Al. This accuracy is achieved at the standard growth rates necessary for thick structures (0.15–0.25 nm/s). The approach is quite generally applicable to other composition profiles or other situations where time-dependent flux control is necessary, and it could be extended to other effusion cells.