Electron-positron pair plasmas represent a unique state of matter, whereby there exists an intrinsic and complete symmetry between negatively charged (matter) and positively charged (antimatter) particles. These plasmas play a fundamental role in the dynamics of ultra-massive astrophysical objects and are believed to be associated with the emission of ultra-bright gamma-ray bursts. Despite extensive theoretical modelling, our knowledge of this state of matter is still speculative, owing to the extreme difficulty in recreating neutral matter-antimatter plasmas in the laboratory. Here we show that, by using a compact laser-driven setup, ion-free electron-positron plasmas with unique characteristics can be produced. Their charge neutrality (same amount of matter and antimatter), high-density and small divergence finally open up the possibility of studying electron-positron plasmas in controlled laboratory experiments.

/pdf/generation-of-neutral-and-high-density-electron-positron-46rgg0oxh0.pdf

Generation of neutral and high-density electron-positron pair plasmas in the laboratory

This paper presents a review of the experimental achievements related to the development of high-power gyrotron oscillators for long-pulse or CW operation and pulsed gyrotrons for many applications. In addition, this work gives a short overview on the present development status of frequency step-tunable and multi-frequency gyrotrons, coaxial-cavity multi-megawatt gyrotrons, gyrotrons for technological and spectroscopy applications, relativistic gyrotrons, large orbit gyrotrons (LOGs), quasi-optical gyrotrons, fast- and slow-wave cyclotron autoresonance masers (CARMs), gyroklystrons, gyro-TWT amplifiers, gyrotwystron amplifiers, gyro-BWOs, gyro-harmonic converters, gyro-peniotrons, magnicons, free electron masers (FEMs), and dielectric vacuum windows for such high-power mm-wave sources. Gyrotron oscillators (gyromonotrons) are mainly used as high-power millimeter wave sources for electron cyclotron resonance heating (ECRH), electron cyclotron current drive (ECCD), stability control, and diagnostics of magnetically confined plasmas for clean generation of energy by controlled thermonuclear fusion. The maximum pulse length of commercially available 140 GHz, megawatt-class gyrotrons employing synthetic diamond output windows is 30 min (CPI and European KIT-SPC-THALES collaboration). The world record parameters of the European tube are as follows: 0.92 MW output power at 30-min pulse duration, 97.5% Gaussian mode purity, and 44% efficiency, employing a single-stage depressed collector (SDC) for energy recovery. A maximum output power of 1.5 MW in 4.0-s pulses at 45% efficiency was generated with the QST-TOSHIBA (now CANON) 110-GHz gyrotron. The Japan 170-GHz ITER gyrotron achieved 1 MW, 800 s at 55% efficiency and holds the energy world record of 2.88 GJ (0.8 MW, 60 min) and the efficiency record of 57% for tubes with an output power of more than 0.5 MW. The Russian 170-GHz ITER gyrotron obtained 0.99 (1.2) MW with a pulse duration of 1000 (100) s and 53% efficiency. The prototype tube of the European 2-MW, 170-GHz coaxial-cavity gyrotron achieved in short pulses the record power of 2.2 MW at 48% efficiency and 96% Gaussian mode purity. Gyrotrons with pulsed magnet for various short-pulse applications deliver Pout = 210 kW with τ = 20 μs at frequencies up to 670 GHz (η ≅ 20%), Pout = 5.3 kW at 1 THz (η = 6.1%), and Pout = 0.5 kW at 1.3 THz (η = 0.6%). Gyrotron oscillators have also been successfully used in materials processing. Such technological applications require tubes with the following parameters: f > 24 GHz, Pout = 4–50 kW, CW, η > 30%. The CW powers produced by gyroklystrons and FEMs are 10 kW (94 GHz) and 36 W (15 GHz), respectively. The IR FEL at the Thomas Jefferson National Accelerator Facility in the USA obtained a record average power of 14.2 kW at a wavelength of 1.6 μm. The THz FEL (NOVEL) at the Budker Institute of Nuclear Physics in Russia achieved a maximum average power of 0.5 kW at wavelengths 50–240 μm (6.00–1.25 THz).

/pdf/state-of-the-art-of-high-power-gyro-devices-and-free-5avfirj4z9.pdf

State-of-the-Art of High-Power Gyro-Devices and Free Electron Masers

In recent years, there has been a wealth of new science involving low-energy antimatter (i.e., positrons and antiprotons) at energies ranging from 10 to less than 10 eV. Much of this progress has been driven by the development of new plasma-based techniques to accumulate, manipulate and deliver antiparticles for specific applications. This article focuses on the advances made in this area using positrons. However many of the resulting techniques are relevant to antiprotons as well. An overview is presented of relevant theory of single-component plasmas in electromagnetic traps. Methods are described to produce intense sources of positrons and to efficiently slow the typically energetic particles thus produced. Techniques are described to trap positrons efficiently and to cool and compress the resulting positron gases and plasmas. Finally, the procedures developed to deliver tailored pulses and beams (e.g., in intense, short bursts, or as quasi-monoenergetic continuous beams) for specific applications are reviewed. The status of development in specific application areas is also reviewed. One example is the formation of antihydrogen atoms for fundamental physics [e.g., tests of invariance under charge conjugation, parity inversion and time reversal (the CPT theorem), and studies of the interaction of gravity with antimatter]. Other applications discussed include atomic and materials physics studies and study of the electron-positron many-body system, including both classical electron-positron plasmas and the complementary quantum system in the form of Bose-condensed gases of positronium atoms. Areas of future promise are also discussed. The review concludes with a brief summary and a list of outstanding challenges. ∗ Contact information: jrdanielson@ucsd.edu † Contact information: ddubin@ucsd.edu ‡ Contact information: rgreaves@fpsi.edu § Contact information: csurko@ucsd.edu

https://link.aps.org/accepted/10.1103/RevModPhys.87.247

Plasma and trap-based techniques for science with positrons

Wendelstein 7-AS was the first modular stellarator device to test some basic elements of stellarator optimization: a reduced Shafranov shift and improved stability properties resulted in β-values up to 3.4% (at 0.9 T). This operational limit was determined by power balance and impurity radiation without noticeable degradation of stability or a violent collapse. The partial reduction of neoclassical transport could be verified in agreement with calculations indicating the feasibility of the concept of drift optimization. A full neoclassical optimization, in particular a minimization of the bootstrap current was beyond the scope of this project. A variety of non-ohmic heating and current drive scenarios by ICRH, NBI and in particular, ECRH were tested and compared successfully with their theoretical predictions. Besides, new heating schemes of overdense plasmas were developed such as RF mode conversion heating—Ordinary mode, Extraordinary mode, Bernstein-wave (OXB) heating—or 2nd harmonic O-mode (O2) heating. The energy confinement was about a factor of 2 above ISS95 without degradation near operational boundaries. A number of improved confinement regimes such as core electron-root confinement with central Te ≤ 7 keV and regimes with strongly sheared radial electric field at the plasma edge resulting in Ti ≤ 1.7 keV were obtained. As the first non-tokamak device, W7-AS achieved the H-mode and moreover developed a high density H-mode regime (HDH) with strongly reduced impurity confinement that allowed quasi-steady-state operation (τ ≈ 65 · τE) at densities (at 2.5 T). The first island divertor was tested successfully and operated with stable partial detachment in agreement with numerical simulations. With these results W7-AS laid the physics background for operation of an optimized low-shear steady-state stellarator.

Major results from the stellarator Wendelstein 7-AS (Review Article)

The optimized superconducting stellarator device Wendelstein 7-X (with major radius $R=5.5\,\mathrm{m}$, minor radius $a=0.5\,\mathrm{m}$, and $30\,\mathrm{m}^3$ plasma volume) restarted operation after the assembly of a graphite heat shield and 10 inertially cooled island divertor modules. This paper reports on the results from the first high-performance plasma operation. Glow discharge conditioning and ECRH conditioning discharges in helium turned out to be important for density and edge radiation control. Plasma densities of $1-4.5\cdot 10^{19}\,\mathrm{m}^{-3}$ with central electron temperatures $5-10\,\mathrm{keV}$ were routinely achieved with hydrogen gas fueling, frequently terminated by a radiative collapse. Plasma densities up to $1.4\cdot 10^{20}\,\mathrm{m}^{-3}$were reached with hydrogen pellet injection and helium gas fueling. Here, the ions are indirectly heated, and at a central density of $8\cdot 10^{19}\,\mathrm{m}^{-3}$ a temperature of $3.4\,\mathrm{keV}$ with $T_e/T_i=1$ was accomplished, which corresponds to &#13; $nT_i(0)\tau_E=6.4\cdot 10^{19}\,\mathrm{keVs}/\mathrm{m}^3$ with a peak diamagnetic energy of $1.1\,\mathrm{MJ}$. The discharge behaviour has further improved with boronization of the wall. After boronization, the oxygen impurity content was reduced by a factor of 10, the carbon impurity content by a factor of 5. The reduced (edge) plasma radiation level gives routinely access to higher densities without radiation collapse, e.g. well above $1\cdot 10^{20}\,\mathrm{m}^{-2}$ line integrated density and $T_e=T_i=2\,\mathrm{keV}$ central temperatures at moderate ECRH power. Both X2 and O2 mode ECRH schemes were successfully applied. Core turbulence was measured with a phase contrast imaging diagnostic and suppression of turbulence during pellet injection was observed.

https://iopscience.iop.org/article/10.1088/1741-4326/ab03a7/pdf

Overview of first Wendelstein 7-X high-performance operation

Thermo-mechanical analysis of Wendelstein 7-X plasma facing components

An overview is given on the status of the demo-cryostat project for the WENDELSTEIN 7-X stellarator. Construction and assembly of the prototype are finished, and the test period is near completion. The intention of this project was to get experience with design and construction of W7-X-components, as well as with assembly of this complex system. The goal is now practically achieved, and it could be demonstrated that the W7-X cryostat can be built with reasonable effort. Many of the solutions found can be adopted directly for W7-X, or are starting points for further improvements. A short description is given of the cryostat, its assembly, and of the most important tests which were performed so far.

Assembly and test of the W7-X demo-cryostat

The narrow support elements (NSEs) between the casings of the superconducting coils of the Wendelstein 7-X magnet system consist of pads made of Al–bronze, which can slide and tilt against stainless-steel counter-faces. Static friction peaks or stick–slip events during sliding may cause disturbances to the magnet system and have to be avoided. Because the NSEs are operated at a temperature of ≈4 K and in vacuum at a residual pressure of p −6  mbar, only solid lubricants, in particular MoS 2 , are applicable. Therefore, the static and dynamic friction behaviour of MoS 2 -coatings was tested in liquid helium at 4.2 K and in vacuum at room temperature under high contact pressures and very low sliding velocities. In model tests with small Al–bronze pins, sliding in reciprocating motion against a PVD–MoS 2 -coating on a steel substrate, very stable sliding behaviour without coating failure, and a friction coefficient below 0.1 was observed. However, tests with downscaled samples in original geometry showed stable sliding in liquid nitrogen, but unexpected stick–slip behaviour in liquid helium at 4.2 K. For further investigations and because in actual service the NSEs are not immersed into a liquid cryogen, tests under vacuum at low temperatures are being prepared.

Cryogenic vacuum tests of scaled down narrow support elements for the W7-X coil system

https://pure.mpg.de/pubman/item/item_2056722_7/component/file_2407074/Warmer_Implementation.pdf

Implementation and verification of a HELIAS module for the systems code PROCESS

The paper reports on finite element (FE) method analyses aiming at the determination of the static load bearing capacity of critical components in the support structure of superconducting coils of Wendelstein 7-X (W7-X). These components are made of austenitic stainless steel that at 4 K is known to exhibit instable plastic flow – the so-called serration effect, i.e. a scattering of the stress under increasing strain. This material behavior may significantly reduce the load bearing capacity of the components. The developed modeling approach uses a special procedure to simulate the serration effect in a conservative way. The expected reduction of the material properties due to the welding process is also taken into account. It is demonstrated that the models are capable to determine the limit loads for the critical components in a flexible and efficient way.

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Felix Schauer

Papers

Thermo-mechanical analysis of Wendelstein 7-X plasma facing components

Assembly and test of the W7-X demo-cryostat

Cryogenic vacuum tests of scaled down narrow support elements for the W7-X coil system

Implementation and verification of a HELIAS module for the systems code PROCESS

Limit analysis of W7-X critical magnet system components with consideration of material serration effect