Magnetron Gauge Research Articles

A 3D-printed miniature magnetron gauge for ultra-high vacuum environments

Miniaturized magnetron gauges are necessary for monitoring vacuum level in miniature vacuum chambers. This paper addresses the challenges of designing the magnetron cavity and configuring the electric and magnetic fields within it while meeting the contradictory constraints of small size and high vacuum level that make electron confinement and accumulation difficult. The resulting gauge has an internal volume of 0.3 cm3 and is designed for and fabricated by advanced 3D printing – including direct metal laser sintering of 316 L stainless steel. Additionally, it is capable of directly attaching to a standard vacuum electrical feedthrough. A particle tracing simulation is used to assess the electron confinement for a range of operating conditions. Experimental characterization shows that the operating pressures extend from 10 nTorr (1.33 μPa) to 1 mTorr (133 mPa). The gauge current is repeatable over the characterized pressures at various values of supply source voltage, VS, from −1000 V to −2500 V, and ranges from ≈1 nA to ≈100 μA over these pressures and voltages. The effects of VS and pressure are examined and compared to prior work. This gauge is at least 30× smaller than commercially available magnetron gauges (e.g., a typical inverted magnetron transducer has an internal volume of 15 cm3).

A low-outgassing-rate carbon fiber array cathode**Project supported by the National Natural Science Foundation of China (Grant No. 61671457).

In this paper, a new carbon fiber based cathode — a low-outgassing-rate carbon fiber array cathode — is investigated experimentally, and the experimental results are compared with those of a polymer velvet cathode. The carbon fiber array cathode is constructed by inserting bunches of carbon fibers into the cylindrical surface of the cathode. In experiment, the diode base pressure is maintained at 1×10−2 Pa–2×10−2 Pa, and the diode is driven by a compact pulsed power system which can provide a diode voltage of about 100 kV and pulse duration of about 30 ns at a repetition rate of tens of Hz. Real-time pressure data are measured by a magnetron gauge. Under the similar conditions, the experimental results show that the outgassing rate of the carbon fiber array cathode is an order smaller than that of the velvet cathode and that this carbon fiber array cathode has better shot-to-shot stability than the velvet cathode. Hence, this carbon fiber array cathode is demonstrated to be a promising cathode for the radial diode, which can be used in magnetically insulated transmission line oscillator (MILO) and relativistic magnetron (RM).

Outgassing rate analysis of a velvet cathode and a carbon fiber cathode

In this paper, the outgassing-rates of a carbon fiber array cathode and a polymer velvet cathode are tested and discussed. Two different methods of measurements are used in the experiments. In one scheme, a method based on dynamic equilibrium of pressure is used. Namely, the cathode works in the repetitive mode in a vacuum diode, a dynamic equilibrium pressure would be reached when the outgassing capacity in the chamber equals the pumping capacity of the pump, and the outgassing rate could be figured out according to this equilibrium pressure. In another scheme, a method based on static equilibrium of pressure is used. Namely, the cathode works in a closed vacuum chamber (a hard tube), and the outgassing rate could be calculated from the pressure difference between the pressure in the chamber before and after the work of the cathode. The outgassing rate is analyzed from the real time pressure evolution data which are measured using a magnetron gauge in both schemes. The outgassing rates of the carbon fiber array cathode and the velvet cathode are 7.3 ± 0.4 neutrals/electron and 85 ± 5 neutrals/electron in the first scheme and 9 ± 0.5 neutrals/electron and 98 ± 7 neutrals/electron in the second scheme. Both the results of two schemes show that the outgassing rate of the carbon fiber array cathode is an order smaller than that of the velvet cathode under similar conditions, which shows that this carbon fiber array cathode is a promising replacement of the velvet cathode in the application of magnetically insulated transmission line oscillators and relativistic magnetrons.

Effects of externally-generated electrons on pressure measurements of inverted magnetron gauges at Diamond

Diamond Light Source (Diamond) utilises a large number of Inverted Magnetron Gauges (IMGs) for both monitoring the vacuum and as a source of interlocks for protection of the machine. During storage ring operation it has been found that, in the presence of the stored electron beam, charged particles, assumed to be electrons, impinging on some of the gauges cause the pressures indicated by the gauges to drop to zero which causes problems with the operation of the storage ring. Diamond has designed and fitted a magnetic filter at the neck of each affected gauge port to eliminate this problem. This paper reports on and discusses experiments and proposed experiments with an electron flood gun and IMGs from two different manufacturers (MKS Instruments and Pfeiffer Vacuum) aimed at obtaining a better understanding of the effect. Externally-generated electrons from the flood gun can cause serious errors in the pressures indicated by the gauges; the error can be positive or negative depending on the gauge current-measuring circuit. Methods aimed at eliminating these spurious pressure readings, such as magnetic filters, bias voltages within the gauges and the use of earth meshes and baffles are also discussed.

Contamination deposited on mass standards in vacuum from an inverted magnetron gauge

Stable mass measurements in vacuum are essential for several metrological reasons. The gravimetric determination of air density using artefacts requires the mass difference between the two artefacts to be measured in vacuum, and all the potential routes to a redefinition of the kilogram also require weighing of masses in vacuum. Any contamination of an artefact will increase the uncertainty with which its mass can be determined. This paper describes work to determine the effect of contamination generated by an inverted magnetron gauge during the calibration of artefacts in vacuum.

Inverted magnetron manometer with enhanced operating time

If a cold-cathode ionization gauge is operated in a gas atmosphere with a high content of hydrocarbons at high pressures, the pressure reading of the gauge is disturbed by the contamination after a certain time of operation. Since in the most cases it is not possible to eliminate these disturbing residual gas components, one has to demand that the measuring process may not be substantially influenced by these gas components. To enhance the time of reliable pressure reading of a cold-cathode gauge we have equipped the entrance of an inverted magnetron gauge head with an ion baffle. By this measure the time of a reliable operation of the gauge is increased at least by a factor 3. The construction of this ion baffle is rather simple. It does not require any additional voltage supply or additional magnets. An additional feedthrough is only necessary for the measurement of the discharge current of the measuring system. The gauge has a range of pressure measurement from 10 −9 to 3×10 −3 mbar.

Paul Aveling Redhead

Paul Aveling Redhead, one of the founders of vacuum technology and surface science, died on 9 July 2005 in Ottawa, Canada, after a long battle with heart disease. Born on 25 May 1924 in Brighton, England, Paul received his BA from Cambridge University in 1944 and joined the British Department of Naval Ordnance, where he worked on tubes for microwaves and proximity fuses. After World War II, he undertook research on experimental vacuum tubes for the Services Electronics Research Laboratories. He was awarded an MA from Cambridge in 1948 for that work; in 1969 he received his PhD from Cambridge.In 1947 Paul joined the division of radio and electrical engineering of the National Research Council of Canada in Ottawa to develop key radar components. Until 1970 he led a distinguished team at NRC in establishing the methods and basis of both ultrahigh vacuum technology—which was to have such impact in many areas, including particle accelerators—and the field of surface science. After a three-year stint in research planning for NRC, he became director of the division of physics in 1973. He believed that the management of science was an important responsibility of scientists. In his last three years at NRC before retirement, he was a key member and secretary of the science and technology policy committee. He also served for many years on the NRC management committee, its tactical committee, the advisory board of TRIUMF, and the advisory committee for fusion-related research. He officially retired from NRC in 1986 but continued to work from an office there.Paul will be remembered for his groundbreaking research in the generation, measurement, and utilization of ultrahigh vacuum, research that paved the way for many other developments in surface science. With NRC colleagues Peter Hobson and Ernie Kornelsen, he coauthored the classic book The Physical Basis of Ultrahigh Vacuum (Chapman and Hall, 1968), the defining book in that field. He developed the magnetron gauge for reliable measurement down to pressures of 10−14 torr, and one version of that gauge went to the Moon in the Apollo missions. He also developed thermal desorption spectroscopy and laid the foundation for electron stimulated desorption. In 1975, he received the American Vacuum Society’s Medard W. Welch Award “in recognition of outstanding current research in the fields of vacuum science and technology, vacuum metallurgy, thin films, and surface science.” He also received the Queen Elizabeth Jubilee Award in 1977 and the Medal for Lifetime Achievement in Physics in 1989, both from the Canadian Association of Physicists. Paul served as president of AVS in 1967–68 and as editor of the Journal of Vacuum Science and Technology from 1970 to 1975. In 1970, he was awarded honorary membership in AVS. He continued to serve the society in many roles, including as both editor and author in recording historical events that he considered important: Vacuum Science and Technology: Pioneers of the 20th Century (AIP, 1994), History of the American Vacuum Society 1953–1994 (AVS, 1995), and 50 Years of Science, Technology, and the AVS (1953–2003), a special issue of the Journal of Vacuum Science and Technology A, 2003.The depth of the impact that Paul had over his career is so remarkable that he has left a tangible imprint on a wide range of scientific and technical endeavors involving the use and understanding of high vacuum. There are entire fields of science and generations of scientists that are dependent on Paul’s contributions to the technology of making high vacuum in the laboratory.During his prolific career as a scientist, technical manager, and historian, Paul had the opportunity to be a mentor for many of his colleagues. We count ourselves privileged members of that group because we had the opportunity to learn from him as our paths crossed in various conferences, committees, collaborations, coauthored papers, and unforgettable chats over a glass of beer. We were very fortunate to have Paul join us in a celebration of his contributions to our scientific community in a special “Symposium to Honor Paul Redhead” organized for the annual AVS symposium in October 2000. He agreed to that celebration if we promised to avoid anything that bordered on being pompous. We could never be pompous when talking about Paul—only grateful. Paul Aveling Redhead NRCPPT|High resolution© 2006 American Institute of Physics.

Pressure/current characteristics of a magnetron cold cathode gauge

The gauge characteristics for a magnetron gauge in the pressure range 10 −8–10 −5 mbar with nitrogen as a test gas have been studied. The discharge current versus pressure was measured at different magnetic fields from 0.10 to 0.15 T and anode voltages from 4.0 to 7.0 kV. The results showed that a change of slope appeared at pressures between 5×10 −8 and 5×10 −7 mbar. At higher magnetic fields, a change of slope appeared at lower pressures. The slope of the curve below the inflection was higher than that above the inflection. At the lowest magnetic field (0.10 T), a divergence of the curves was observed at pressures below the inflection. The slope of the curve showed no clear dependence on applied voltage.

Gas Puffing into the AMBAL-M Solenoid Plasma

The central solenoid of AMBAL-M was filled with a turbulent plasma stream generated by a source located outside the entrance magnetic throat, the plasma ~0.4 m in diameter, with density ~1.5·1013 cm-3, electron temperature ~50 eV and ion energy ~200 eV was obtained.Additional hydrogen puffing allowed plasma density increase. The plasma with a cold component from ionized gas and charge exchange ions was heated by electrostatic oscillations produced by the working source. At optimized gas puffing the plasma density was increased to 5·1013 cm-3 without substantial reduction of the ion temperature. No big differences in plasma properties were found between gas puffing through a gas-box and a ceramic tube.The plasma density increment was shown to depend only on the total amount of the injected gas. The experimental optimization was made for different values of solenoid magnetic field taking the diamagnetism into account.Neutral hydrogen distribution in the solenoid vacuum chamber and recycling rate were estimated from data of fast inverse magnetron gauges constructed in BINP.

A Hot Cathode Magnetron Gauge using a Divergence-Type Magnetic Field for Electric Field Control

A new sensitive type of magnetron gauge has been developed in order to obtain a linear response of ion current, Ii, to pressure, P, with a high sensitivity over a wide range. The gauge is equipped with two ion collectors. The ratio of the ion currents of the two ion collectors is measured and kept constant by controlling the filament voltage, in order to keep the electric field constant within the anode over a wide range of pressure measurement. When a divergent-magnetic field is applied to this hot-cathode magnetron gauge, the slope of the plot of the filament voltage vs the ratio of the ion current of the two separate two ion collectors is improved from -0.28 to +1.1–+3.9 (V-1). When the diameters of the ion collectors were investigated, it was found that the current ratio η=1 was attained when the outer diameters of the ion collectors were identical, and their inner diameters were 2 and 8 mm respectively. With this configuration, ion current was directly proportional to pressure in the range of 1×10-8–1×10-4 Pa.

磁石内蔵形正マグネトロン真空計

Aiming at elimination of the sudden change and inflection of the discharge current at a pressure, five type magnetron gauges containing a magnet inside of the electrode are investigated. One is a hand drum type magnetron gauge. Others are normal cylindrical magnetron gauges of various sizes. It is found that a normal cylindrical magnetron gauge, which contains rather big magnet inside of the cathode with the magnetic flux density of 0.091 [T] at 1.8 mm from the cathode surface, has wider stable pressure range and discharge voltage range, and shows good linearity of the pressure vs the discharge current characteristics in log-log plots. The sensitivity I/P is 3.96 (at 1 × 10-4 [Pa]) and 0.861.06 (at 1 × 10-6 [Pa]) [10-2 A/Pa], the slope of the pressure vs the discharge current characteristics in the log-log plots is 1.097 (at 1 × 10-4 [Pa]) and 1.684 (at 1 × 10-6 [Pa]) at the discharge voltage of 6 [kV] without any sudden change and inflection of the discharge current. The others also have good linearity of the pressure vs the discharge current characteristics in log-log plots at the discharge voltage of 6 [kV]. But, these gauges have sudden changes and inflections, except for 6 [kV] of the discharge voltages.

Ionization gauge errors at low pressures

Factors affecting the accuracy of ionization gauge measurements at low pressures are reviewed. In hot-cathode gauges these include electron-stimulated desorption at the electron collector, forward and reverse x-ray effects, the potential of the gauge envelope, outgassing, and various controller-related errors. In cold-cathode gauges they include nonlinearities below the “magnetron knee,” plasma instabilities, and background currents. Case studies are given to illustrate many of these sources of error and their elimination. The case studies were gathered in the course of long-term stability measurements on over 30 ionization gauges at pressures ranging from the 10−8 to 10−11 Torr ranges. The investigation included Bayard–Alpert (both conventional and modulated), extractor, inverted magnetron, double inverted magnetron, and magnetron gauges. Errors caused by outgassing from virtual leaks in all-metal seals are also considered. By far the largest and most unpredictable source of error proved to be electron-stimulated desorption in hot-cathode gauges, avoidable by operating the grid at a sufficiently high temperature. It is concluded that, with proper precautions, better then 10% reproducibility in the 10−10 Torr range is easily achievable over periods of many months with either hot-cathode or cold-cathode gauges. Over shorter periods, reproducibility of better than 1% is obtainable with well-designed hot-cathode gauges.

電界制御熱陰極マグネトロン真空計

Poisson's equation, ∇2φ=-ρ/e0, shows that if the electric potential distribution or electric field in the space in which ions are generated is held constant, the distribution of the electric charge density is constant. By dividing the ion collectors of the gauge into two sections, and controlling the distribution ratio of ion current at a constant level against pressure, a constant distribution of the electric charge density is secured. Based upon this idea, a prototype of the magnetron gauge is built.It is found that the ion collector current is directly proportional to the pressure in a range of 1×10-52×10-9 [Pa].

電界制御熱陰極マグネトロン真空計の開発

We have developed a new type of magnetron gauge to obtain linear response of the ion current to the pressure in a wider range with a higher sensitivity. The gauge is equipped with two ion collectors while the ratio of the ion currents is measured and kept constant with a feed-back control of the filament heating voltage. The constant ratio, e.g. 0.4 in a range between 3×10-2 and 2×10-9 Pa, resulted from a geometrically similar generation of ions, hence from a spatial electron distribution independent of the pressure. In this report we describe the principle, the design, and results of the new type of magnetron gauge, by comparing them with those of conventional types and of B-A gauges.

Reliability and accuracy of total and partial pressure measurements in the uhv pressure range under real experimental conditions

Comparative tests of Bayerd-Alpert and inverted magnetron gauges from diverse manufacturers were carried out in the laboratory. Commercial residual gas analysers from four different producers were tested to determine (i) linearity, stability and sensitivity of output signals on the pressure response ; (ii) high-pressure performance ; (iii) hysteresis ; (iv) fragment ion and electron stimulated desorption (ESD) peaks ; (v) detection of small signals. Measurements were performed at different experimental conditions over the pressure range 10 −11–10 −6 mbar for gases H 2, N 2, Ar and CH 4. The pumping system which allows to measure total pressures according to the current in the sputter-ion pump was calibrated and tested during Elettra commissioning. It was observed that the real experimental conditions, e.g. the presence of the radio frequency, radiation, strong magnetic fields can change the interpretation of total and partial pressure measurements significantly.

卵形冷陰極マグネトロン真空計の研究

Aiming at increase of the discharge volume of crossed electric and magnetic field domain, egg type magnetron gauge cathodes, a surface along the magnetic force line from a magnetic pole to the other pole, are investigated. Three egg type magnetron gauges, one with magnetic end plates, one with small pole pieces and one with large pole pieces are compared to study the effects of increasing the discharge volume of the field domain. It is found that the sudden changes of the discharge current are eliminated except the egg type one without any pole pieces at lower discharge voltage of 1 to 3 kV, and good lineariy is achieved. The gauge with small pole pieces which has magnetic flux density of 0.108 T at 1.8 mm from cathode surface, shows the best characteristics, the sensitivity I/P=2.4 and 2.7 [×10-2 A/Pa], and slope n [log I2/I1/logP2/P1] =1.26 and 1.20 at discharge voltage of 6 kV and 4 kV, respectively.

Starting delays in cold-cathode gauges at low pressures

A cold-cathode gauge does not immediately start when power is applied. Starting is generally assumed to be triggered by a cosmic ray or other energetic particle. The starting behaviors of two magnetron, two inverted magnetron, and five double inverted magnetron gauges were studied at pressures ranging from 2×10−7 down to 10−10 Torr. The effects of various starting devices (beta, alpha, field-electron, and photon emitters) were also evaluated. The median starting time for a given gauge was found to be approximately inversely proportional to pressure and consisted of two distinct components: a dormant interval and a growth period. The dormant interval is inherently nonreproducible, being governed by the statistical laws of random events. It can only be quantified by taking the median of a large number of measurements. It is normally by far the larger component and can amount to hours or days at 10−10 Torr. The growth period, on the other hand, is highly reproducible. It typically has a value of about 1 min at 10−10 Torr. For most of the growth period the discharge current is below the limit of detection, making it necessary to use special experimental techniques to determine the true growth period. A double inverted magnetron with radioactive triggering to reduce the dormant interval to less than a second at 10−10 Torr is described. Sources of error in starting-time measurements are discussed.

Enhanced ignition of cold cathode gauges through the use of radioactive isotopes

The Relativistic Heavy Ion Collider (RHIC) at Brookhaven National Laboratory comprises vacuum systems that range in pressure from 10−4 to 10−11 Torr. For cost savings associated with commonality, it is desirable to use one ultrahigh vacuum (UHV)-compatible high vacuum gauge throughout the accelerator. Work was done with radioactive isotopes to enhance ignition of inverted magnetron gauges at very low pressures (i.e., <10−10 Torr). Tests were initially conducted using cobalt-coated copper washers with a nickel flash to prevent oxidation. Solid nickel washers were also activated and tested. Problems with these sources will be discussed. Finally, an americium source was tested. The UHV-compatible americium sources yielded average ignition times of about 10 min at 4×10−11 Torr. The addition of the americium source produced a gauge suitable for operation at UHV pressures, and as a consequence for all of the RHIC.

ヌード型冷陰極マグネトロン真空計の開発

A nude type magnetron gauge has been developed, which is highly simplified and small. The gauge has only a cathode which is comprised of a thin vacuum chamber containing a magnet, and magnetic pole pieces.The characteristics of the gauge are investigated in the pressure range of 10-110-8 Pa. Current I vs pressure P in a log-log plot has good linearity. This linearity is sensitive to the distance between cathode and vacuum chamber as an anode d. In the case of a short d of 10mm, the gauge shows good linearity in a wide pressure range of 6 decades, or 10-110-6Pa, with 2 kV of voltage applied to the cathode. This pressure range is very wide compared with that of a conventional penning discharge gauge. In the case of a long d of 19.5 mm, 5 decades, or 10-110-5 Pa, at 3 or 4 kV. With increase or decrease of the applied voltage, the range of linearity will be narrowed. The optimum applied voltages are 2 kV at d-10 mm, and 34 kV at d= 19.5 mm.It is found that the gauge shows a sudden change of several times in discharge current at a pressure Pc, when the pres-sure is decreased or increased. Pc is 10-510-6 Pa for d = 19.5 mm with an applied voltage of 1.56.0 kV, and 10-310-6 Pa for d = 10 mm with an applied voltage of 1.56.0 kV.This gauge is very useful for application in rocket and balloon experiments, and for pressure measurement in the deep portion of a vacuum chamber.

ヌード型冷陰極逆マグネトロン真空計の特性

A nude type inverted magnetron gauge which consists of an anode, pole pieces, and a cathode with a thin vacuum chamber containing a magnet is developed. Relatively good uniformity of the magnetic flux density in the central space of the cathode is obtained. The discharge current vs pressure characteristics are measured over a wide pressure range of 10-8 to 10-2 Pa. It is found that the gauge shows good linearity in the discharge current vs pressure plot in the range 10-8 to 10-2 Pa. However, the characteristic curves typically show an abrupt change of slope at 10-5 Pa. Sudden change in the discharge current, as found for a planar magnetron or a cylindrical magnetron with a magnet inside the cathode, is not observed when the pressure is varied.