Plasma Physics Research Research Articles

Eqtools: Modular, extensible, open-source, cross-machine Python tools for working with magnetic equilibria

As plasma physics research for fusion energy transitions to an increasing emphasis on cross-machine collaboration and numerical simulation, it becomes increasingly important that portable tools be developed to enable data from diverse sources to be analyzed in a consistent manner. This paper presents eqtools, a modular, extensible, open-source toolkit implemented in the Python programming language for handling magnetic equilibria and associated data from tokamaks. eqtools provides a single interface for working with magnetic equilibrium data, both for handling derived quantities and mapping between coordinate systems, extensible to function with data from different experiments, data formats, and magnetic reconstruction codes, replacing the diverse, non-portable solutions currently in use. Moreover, while the open-source Python programming language offers a number of advantages as a scripting language for research purposes, the lack of basic tokamak-specific functionality has impeded the adoption of the language for regular use. Implementing equilibrium-mapping tools in Python removes a substantial barrier to new development in and porting legacy code into Python. In this paper, we introduce the design of the eqtools package and detail the workflow for usage and expansion to additional devices. The implementation of a novel three-dimensional spline solution (in two spatial dimensions and in time) is also detailed. Finally, verification and benchmarking for accuracy and speed against existing tools are detailed. Wider deployment of these tools will enable efficient sharing of data and software between institutions and machines as well as self-consistent analysis of the shared data. Program summaryProgram title: eqtoolsCatalogue identifier: AFBK_v1_0Program summary URL:http://cpc.cs.qub.ac.uk/summaries/AFBK_v1_0.htmlProgram obtainable from: CPC Program Library, Queen’s University, Belfast, N. IrelandLicensing provisions: GNU GPL v3No. of lines in distributed program, including test data, etc.: 27204No. of bytes in distributed program, including test data, etc.: 1217844Distribution format: tar.gzProgramming language: Python, C.Computer: PCs.Operating system: Linux, Macintosh OS X, Microsoft Windows.RAM: Several megabytes, depends on resolution of dataClassification: 19.4.External routines: F2PY [1], matplotlib [2], MDSplus [3], NumPy [4], SciPy [5]Nature of problem:Access to results from magnetic equilibrium reconstruction code, conversion between various coordinate systems tied to the magnetic equilibrium.Solution method:Data are stored in an object-oriented data structure with human-readable getter methods. Coordinates are converted using bivariate or trivariate splines.Running time:Coordinate transformations on a 66x66 point spatial grid take between 1 and 5 ms per time slice, depending on the transformation used and how many intermediate results have been stored.

Direct Writing Target Structures by Two-Photon Polymerization

Two-photon polymerization (2PP) offers an attractive option for direct writing micron- to millimeter-sized laser target components that support high-energy-density plasma physics research. 2PP was used to deterministically print a number of common targetlike structures including tubes, spatially periodic Rayleigh-Taylor–like surfaces, and low-density foams. The structures were printed using commercially available acrylic photoresins. The elemental compositions are reported for comparison with other polymers used for making target components. A number of foamlike structures ranging in size from tens to hundreds of microns and varying in density from 600 to 60 mg/cm3 were readily printed in times ranging from several seconds to a few hours depending on the size. In addition, direct printing was demonstrated to fabricate graded-density foam comprising 12 individual layers with a vertical density gradient of 600 to 80 mg/cm3. Control of shrinkage and deformation during development and subsequent drying remains a challenge for certain structures and a focus of ongoing research.

STARDUST-U experiments on fluid-dynamic conditions affecting dust mobilization during LOVAs

Since 2006 the Quantum Electronics and Plasma Physics (QEP) Research Group together with ENEA FusTech of Frascati have been working on dust re-suspension inside tokamaks and its potential capability to jeopardize the integrity of future fusion nuclear plants (i.e. ITER or DEMO) and to be a risk for the health of the operators. Actually, this team is working with the improved version of the “STARDUST” facility, i.e. “STARDUST-Upgrade”. STARDUST-U facility has four new air inlet ports that allow the experimental replication of Loss of Vacuum Accidents (LOVAs). The experimental campaign to detect the different pressurization rates, local air velocity, temperature, have been carried out from all the ports in different accident conditions and the principal results will be analyzed and compared with the numerical simulations obtained through a CFD (Computational Fluid Dynamic) code. This preliminary thermo fluid-dynamic analysis of the accident is crucial for numerical model development and validation, and for the incoming experimental campaign of dust resuspension inside STARDUST-U due to well-defined accidents presented in this paper.

Completion of the Commissioning of the EDIPO Test Facility

The main coils of the new European DIPOle (EDIPO) test facility, which is able to provide a background magnetic field up to 12.35 T, were commissioned at the Center for Research in Plasma Physics in July 2013. A field homogeneity of +/-1% was achieved over a 900-mm length, as compared to SULTAN's 10.9 T over 425 mm. To complete the commissioning of the test facility, the new superconducting transformer has been operated in 2015, and a high-current NbTi sample, which was earlier tested in the SULTAN test facility, has been tested in EDIPO under the same conditions as in SULTAN to benchmark the results. During the initial commissioning of the 100-kA transformer in 2013, the primary coil was found to be insufficiently cooled due to the debonding of the primary winding from its former that also served as the liquid helium cryostat. The NbTi primary coil was therefore modified to be in direct contact with liquid helium. At the next attempt of commissioning, in March 2014, a mechanical failure due to the imperfect alignment of the primary and secondary windings caused major damage. A replacement primary coil, which was wound in Summer 2014, was used in the third commissioning campaign in November 2014, but it failed due to the poor impregnation of the NbTi winding. Eventually, the original primary winding was repaired and commissioned in April 2015. The commissioning included a calibration of the current meter for the secondary winding and the high-current test, i.e., up to 95 kA at various background fields, of the “Trasek” conductor sample, which is a rectangular cable-in-conduit conductor made of 324 SnAg-coated NbTi strands. An excellent agreement was found between the results obtained from the two facilities. The sample holder unit is also equipped with a high-temperature superconducting (HTS) adapter and a counterflow heat exchanger to allow testing in EDIPO of HTS high-current samples over a broad range of operating temperatures, from 4.5 up to 50 K.

Design and Low Power Test of Pulse Forming Network for Klystron Modulator at Chiang Mai University

A new pulse forming network (PFN) for klystron modulator has been designed and tested at Plasma and Beam Physics Research Facility (PBP), Chiang Mai University. The PFN consists of six capacitor and inductor sections. The output waveform of the PFN has pulse width of 3.2 μs and a flat-top of 2.0 μs with ripple less than ± 2.0%. In this paper discusses the design, simulation and low power test results of the PFN.

RF Power Commissioning and an Electron Beam Diagnostic Station for a New RF-gun at Chiang Mai University

This paper presents results of the RF power commissioning for a new thermionic RF electron gun of the linear accelerator system at the Plasma and Beam Physics Research Facility, Chiang Mai University. The research focuses on generation and measurements of a radio-frequency wave for using as a power source for electron acceleration. The low-level RF power measurements were conducted to study the performance of the system. Then, the commissioning of the RF-gun with the high-level RF power obtained from a new 7-MW klystron tube was performed to study the proper condition for the RF-gun operation. The gun temperature was optimized and the proper temperature was about 33oC-34oC. In nominal operation, the RF system produces the RF pulses with the repetition rate of 10Hz for the proper RF pulse length of 3-4μs and the maximum peak power of ∼3.9 MW. The absorbed RF peak power is 2.7 MW, which leads to the average power of 81W. In order to investigate the influence of the asymmetric electromagnetic field distribution inside the RF-gun, an integrated electron beam diagnostic station has been developed and is installed in the accelerator beam transport line. The results of the RF commissioning and development of the diagnostic system for measurements of the electron pulse current, the transverse beam emittance and the beam energy are reported and discussed.

Thermal–Hydraulic Modeling of a Novel HTS CICC for Nuclear Fusion Applications

A thermal–hydraulic model has been developed for the novel cable-in-conduit conductor (CICC) recently proposed by Italian National Agency for New Technologies, Energy and Sustainable Economic Development (ENEA), which foresees the use of second-generation REBCO-based coated conductor tapes stacked into twisted grooves slotted into an Al core. A central hole in the Al core provides a low hydraulic impedance channel for the coolant, whereas direct cooling of the tape stacks is achieved through small channels at the bottom of the grooves and other small passages around the stack. The coolant could be either supercritical helium (SHe), for high-field/high-current applications, as in fusion magnets, or liquid nitrogen (LN2), for self-field/lower current requirements. The issues of the thermal–hydraulic performance and optimization of such a CICC are addressed for the first time in this paper. The 4C code is used to develop an adequate model for this complex 3D problem, taking advantage of the 1D nature of the coolant flow. The model, first calibrated and then validated using the results of thermal–hydraulic tests performed in cryogenic conditions at the Center for Research in Plasma Physics (CRPP), is used to evaluate the flow repartition among the different cooling paths in the CICC for SHe at 4.5 K and for LN2 at 77 K. The model is finally used to perform parametric analyses, varying, e.g., the central hole diameter, as well as the size of the grooves at the bottom of the tapes, within the limits imposed by structural constraints, and to assess the cooling capability of the cable.

Design Optimization of Round Strands Made by Twisted Stacks of HTS Tapes

A twisted stack of high-temperature superconducting (HTS) tapes soldered into copper profiles was used as a strand in the fabrication of the 60-kA/12-T HTS cable prototype at the Center for Research in Plasma Physics (CRPP). Considering the strand in general as a modular element for the high-current cables, design parameters of the strand need to be optimized in order to best fulfill requirements of appropriate application field. During the development program of the HTS fusion cable at CRPP, influence of the design parameters on the strand's performance was obtained as input data for the optimization process. In this paper, we summarize the results of the twisting, bending, and transverse pressure tests on strands of various tapes and profile's geometries. Finite-element model for the transverse pressure test was developed, validated with the test results, and used for the design proposals. Effect of the stack aspect ratio, tape properties, and anisotropy, as well as selection of HTS material for the strand, will be discussed.

EDIPO: The Test Facility for High-Current High-Field HTS Superconductors

The commissioning of the European DIPOle (EDIPO) test facility at the Center for Research in Plasma Physics in Villigen was completed in Spring 2015, and the first test of a 60-kA/12-T high-temperature superconductor (HTS) was carried out in June 2015. User specifications are issued to allow external users to prepare their own sample matching the interface of the test facility. EDIPO offers a background magnetic field up to 12.35 T in a rectangular test well with a cross section of 89 mm $\times$ 138 mm (sample dimension). The homogeneous high field length is 910 mm (±1%) or 680 mm (±0.5%) at full current. A set of small copper coils provides a superimposed ac field with amplitude up to ±0.3 T and frequency up to 1 Hz. A superconducting transformer provides steady-state operating current for the sample up to 100 kA. The cooling circuit of the sample supplies supercritical helium at 10-bar inlet pressure. The mass flow rate, which is adjustable by cryogenic valves, is up to 8 g/s in each branch of the sample at 4.5 K. The operating temperature ranges from 4.5 to 60 K (at high temperature, the range of mass flow rate is reduced). To test samples at temperature higher than 10 K, an HTS “adapter” connects the sample to the NbTi transformer plates, which must remain cold. In such case, the length of the sample is reduced from 3600 mm (typical low-temperature superconductor sample) to 2900 mm (HTS sample with adapter).

Benjamin Lax

Benjamin Lax, a pioneering solid-state and plasma physicist, an MIT professor of physics, and the long-time director of the MIT National Magnet Laboratory, died at age 99 on 21 April 2015 in Newton, Massachusetts.Benjamin LaxBACHRACH PHOTOGRAPHYPPT|High resolutionBorn on 29 December 1915 in Miskolc, Hungary, Ben moved with his family in 1926 to the US, where they settled in Brooklyn, New York. After completing Boys High School in Brooklyn, he graduated cum laude from the Cooper Union for the Advancement of Science and Art in 1941 with a degree in mechanical engineering. He wanted to do graduate work in mathematics at Brown University, but he was drafted into the US Army in 1942. Assigned to the famous MIT Radiation Laboratory, he worked on a crash program to develop a new radar system, named Little Abner, which was successfully tested in the field. After World War II ended, Ben went to MIT, where he completed his PhD in physics in 1949 under Sanborn Brown. For his thesis, “The effect of magnetic field on the breakdown of gases at high frequencies,” he utilized cyclotron resonance, a technique that he continued to pursue throughout his career.In 1951 Ben joined the MIT Lincoln Laboratory, where he made significant contributions to the understanding of semiconductors, particularly by using cyclotron resonance to study the semiconductors’ energy band structure. Ben and his coworkers employed cyclotron resonance absorption of microwave radiation to measure the effective mass of electrons, which was determined by the curvature of the energy bands. His studies of germanium and silicon were particularly important to the progress of semiconductor devices. Ben was also a coinventor on an early patent for the semiconductor laser. Because of his scientific advances and his abilities as a leader, Ben was made head of the Lincoln Laboratory’s solid-state division in 1958 and served as associate director of the lab in 1964–65. In 1960 he was recognized by the American Physical Society with the Oliver E. Buckley Condensed Matter Physics Prize “for his fundamental contributions in microwave and infrared spectroscopy of semiconductors.”In the late 1950s, Ben led a group of scientists and engineers in proposing that a high-magnetic-field laboratory be built on the MIT campus to conduct research in solid-state and plasma physics, magnetic resonance spectroscopy, and magnet engineering. The successful proposal led to the 1960 establishment of the National Magnet Laboratory (NML), later renamed the Francis Bitter Magnet Laboratory. Ben served as director for its first 21 years. In 1965 he also accepted a position as a professor in the MIT physics department.Under Ben’s leadership, the NML became known internationally for its wide-ranging research, including in such areas as the physics of solids in high magnetic fields; high-field nuclear magnetic resonance; biomagnetism; magnet technology, including magnetic levitation for trains; laser–plasma interactions in high magnetic fields; and laser–plasma diagnostics. He was also interested in the use of high magnetic fields to achieve better confinement of plasmas for fusion. The first high-magnetic-field tokamak confinement device, Alcator A, was constructed at the NML and successfully demonstrated the benefits of the high-field approach. But the research on fusion energy and plasma physics soon required larger facilities, which led to the creation of what is now the MIT Plasma Science and Fusion Center.Ben supervised 36 PhD students, many of whom went on to highly successful careers in physics and engineering. Ben retired from the NML in 1981 and from the physics faculty in 1986. But he continued working in physics for over 15 more years, including being a consultant at the Lincoln Laboratory. He was coauthor with Kenneth Button of the classic book Microwave Ferrites and Ferrimagnetics (McGraw-Hill, 1962) and author or coauthor of more than 300 journal articles and book chapters.Ben was also a talented athlete; he was a gymnast in his youth and began skiing at the age of 50. He brought to the athletic field the same intensity and enthusiasm he conveyed in the laboratory. He was a wonderful mentor to countless students, young scientists, and colleagues. Ben’s brilliance, his boundless energy, and his enthusiasm for physics will be greatly missed.© 2016 American Institute of Physics.

Efficient evaluation of collisional energy transfer terms for plasma particle simulations

Particle-based simulations, such as in particle-in-cell (PIC) codes, are widely used in plasma physics research. The analysis of particle energy transfers, as described by the second moment of the Boltzmann equation, is often necessary within these simulations. We present computationally efficient, analytically derived equations for evaluating collisional energy transfer terms from simulations using discrete particles. The equations are expressed as a sum over the properties of the discrete particles.

Smaller & Sooner: Exploiting High Magnetic Fields from New Superconductors for a More Attractive Fusion Energy Development Path

The current fusion energy development path, based on large volume moderate magnetic B field devices is proving to be slow and expensive. A modest development effort in exploiting new superconductor magnet technology development, and accompanying plasma physics research at high-B, could open up a viable and attractive path for fusion energy development. This path would feature smaller volume, fusion capable devices that could be built more quickly than low-to-moderate field designs based on conventional superconductors. Fusion’s worldwide development could be accelerated by using several small, flexible devices rather than relying solely on a single, very large device. These would be used to obtain the acknowledged science and technology knowledge necessary for fusion energy beyond achievement of high gain. Such a scenario would also permit the testing of multiple confinement configurations while distributing technical and scientific risk among smaller devices. Higher field and small size also allows operation away from well-known operational limits for plasma pressure, density and current. The advantages of this path have been long recognized—earlier US plans for burning plasma experiments (compact ignition tokamak, burning plasma experiment, fusion ignition research experiment) featured compact high-field designs, but these were necessarily pulsed due to the use of copper coils. Underpinning this new approach is the recent industrial maturity of high-temperature, high-field superconductor tapes that would offer a truly “game changing” opportunity for magnetic fusion when developed into large-scale coils. The superconductor tape form and higher operating temperatures also open up the possibility of demountable superconducting magnets in a fusion system, providing a modularity that vastly improves simplicity in the construction, maintenance, and upgrade of the coils and the internal nuclear engineering components required for fusion’s development. Our conclusion is that while tradeoffs exist in design choices, for example coil, cost and stress limits versus size, the potential physics and technology advantages of high-field superconductors are attractive and they should be vigorously pursued for magnetic fusion’s development.

Design Optimization of Nested Bitter Magnets

An optimization technique that utilizes a genetic algorithm is used to minimize the electrical power produced by the conducting disks in resistive Bitter magnets. The resulting values from the optimization give the conditional design parameters necessary for a preliminary magnet design. This precursor design method can be employed for either split or single solenoid nested designs. This method is used to design a 10 T, 10 s magnet at the University of Maryland, Baltimore County for plasma physics research. The design parameters from the optimization are used to generate computer-aided design models that are examined with finite element analysis software to verify safety and performance of the design. The magnet design then progresses to a series of thermal and mechanical tests to ensure safe operating conditions.

Globus-M plasma physics research for fusion application and compact neutron source development

During the past decade, plasma physics research promoting the physics base of ITER and developing novel concepts such as a compact fusion neutron source has been conducted on the Globus-M spherical tokamak (ST) (R = 36 cm, a = 24 cm, Ip ⩽ 250 kA, BT ⩽ 0.4 T). Tokamak reconstruction is imminent. The upgraded tokamak Globus-M2 will have the same vacuum chamber and an enhanced magnetic system to provide BT = 1 T and Ip = 500 kA. In this paper we outline the most important research directions and the main results obtained on Globus-M and make some predictions about the possibilities and parameters of Globus-M2.

APPA at FAIR: From fundamental to applied research

FAIR with its intense beams of ions and antiprotons provides outstanding and worldwide unique experimental conditions for extreme matter research in atomic and plasma physics and for application oriented research in biophysics, medical physics and materials science. The associated research programs comprise interaction of matter with highest electromagnetic fields, properties of plasmas and of solid matter under extreme pressure, density, and temperature conditions, simulation of galactic cosmic radiation, research in nanoscience and charged particle radiotherapy. A broad variety of APPA-dedicated facilities including experimental stations, storage rings, and traps, equipped with most sophisticated instrumentation will allow the APPA community to tackle new challenges. The worldwide most intense source of slow antiprotons will expand the scope of APPA related research to the exciting field of antimatter.

Mechanism of Fast Current Interruption inp−π−nDiodes for Nanosecond Opening Switches in High-Voltage-Pulse Applications

Pulsed-power applications supporting research in fusion, plasma, and particle physics require delivery of high voltages over very short times (nanoseconds, or less)---no mean feat of engineering. The authors study current interruption in a step-recovery diode (SRD), which could be a fast-opening switch for high-voltage pulsed electronics, and find the main governing parameters to be the diode's reverse current density and base doping concentration. This understanding allows the design of improved structures, taking advantage of modern semiconductor growth technology.

Simulations and Experiments to Reach Numerical Multiphase Informations for Security Analysis on Large Volume Vacuum Systems Like Tokamaks

Dust re-suspension as a consequences of loss of vacuum accident (LOVA) or loss of coolant accident (LOCA) situations inside a nuclear fusion plant (ITER-like) is an important issue for the workers’ safety and for the security of the plant. The dust size expected inside tokamaks like ITER is of the order of microns (0.1–1000 μm). Analysis of the thermo fluid-dynamics and transport phenomena involved during an accidental pressurization transitory is necessary in order to set up and operated tokamaks with careful consideration of the potential risks. Computational fluid dynamics (CFD) study of LOVA scenario is a challenging task for today numerical methods and models because it involves 3D large vacuum volumes, multiphase flows ranging from highly supersonic to nearly incompressible and heat transfer simultaneously. Present work deals with development and experimental validation of CFD model, which simulates the complex thermo fluid-dynamic field and gives some indication about internal hazardous dust mobilization phenomena during vessel filling at near vacuum conditions, for supporting first instant of LOVA safety analysis. The research activity had been carried out in the framework of EURATOM–ENEA Association—University of Rome Tor Vergata Quantum Electronics Plasma Physics and Materials Research Group.

Plasma turbulence, suprathermal ion dynamics and code validation on the basic plasma physics device TORPEX

The TORPEX basic plasma physics device at the Center for Plasma Physics Research (CRPP) in Lausanne, Switzerland is described. In TORPEX, simple magnetized toroidal configurations, a paradigm for the tokamak scrape-off layer (SOL), as well as more complex magnetic geometries of direct relevance for fusion are produced. Plasmas of different gases are created and sustained by microwaves in the electron-cyclotron (EC) frequency range. Full diagnostic access allows for a complete characterization of plasma fluctuations and wave fields throughout the entire plasma volume, opening new avenues to validate numerical codes. We detail recent advances in the understanding of basic aspects of plasma turbulence, including its development from linearly unstable electrostatic modes, the formation of filamentary structures, or blobs, and its influence on the transport of energy, plasma bulk and suprathermal ions. We present a methodology for the validation of plasma turbulence codes, which focuses on quantitative assessment of the agreement between numerical simulations and TORPEX experimental data.

Shadowgraph Technique Applied to STARDUST Facility for Dust Tracking: First Results

The problem of dust resuspension in case of Loss Of Vacuum Accident (LOVA) in a nuclear fusion plant (ITER or DEMO like) is an important issue for the safety of workers and the security of environment. The Quantum Electronics and Plasma Physics Research Group has implemented an optical set-up to track dust during a LOVA reproduction inside the experimental facility STARDUST. The shadowgraph technique, in this work, it is applied to track dark dust (like Tungsten). The shadowgraph technique is based on an expanded collimated beam of light emitted by a laser (or a lamp) transversely to the flow field direction. Inside STARDUST the dust moving in the air flow causes variations of refractive index of light that can be detected by the means of a CCD camera. A spatial modulation of the light-intensity distribution on the camera can be measured. The resulting pattern is a shadow of the refractive index field that prevails in the region of the disturbance. The authors use an incandescent white lamp to illuminate the vacuum vessel of STARDUST facility. The light-area passes through the test section that has to be investigated and the images of the dust shadows are collected with a fast CCD camera. The images are then elaborated with mathematical algorithms to obtain information about the velocity fields of dust during the accidents reproduction. The experimental set-up together with a critical analysis of the first results are presented in this paper.

Microwave remote plasma enhanced-atomic layer deposition system with multicusp confinement chamber.

A microwave remote Plasma Enhanced-Atomic Layer Deposition system with multicusp confinement chamber is established at the Plasma and Beam Physics research facilities, Chiang Mai, Thailand. The system produces highly-reactive plasma species in order to enhance the deposition process of thin films. The addition of the multicusp magnetic fields further improves the plasma density and uniformity in the reaction chamber. Thus, the system is more favorable to temperature-sensitive substrates when heating becomes unwanted. Furthermore, the remote-plasma feature, which is generated via microwave power source, offers tunability of the plasma properties separately from the process. As a result, the system provides high flexibility in choice of materials and design experiments, particularly for low-temperature applications. Performance evaluations of the system were carried on coating experiments of Al2O3 layers onto a silicon wafer. The plasma characteristics in the chamber will be described. The resulted Al2O3 films-analyzed by Rutherford Backscattering Spectrometry in channeling mode and by X-ray Photoelectron Spectroscopy techniques-will be discussed.