arXiv subjects

Oliver Schmitz

Publications and source records attributed to Oliver Schmitz.

10 recordsLinked to original sources

arXiv2026

Determining neutral fueling response to RMPs in MAST using a multi-reservoir particle balance model and EMC3 EIRENE

The application of certain configurations of resonant magnetic perturbations (RMPs) has been shown to cause a plasma pump-out in both L- and H-mode discharges at MAST. In this paper we discuss the impact of neutral fueling on this density reduction. The neutral fueling and average particle confinement time $τ_p$ of the main ion species were calculated using a 0-D particle balance analysis. When the RMPs were applied, it was found that there was an increase in ionizations and a 15$\%$ reduction in $τ_p$ for L-mode discharges and a similar result for inter-ELM (edge localized modes) periods of H-mode discharges. A time-dependent global multi-reservoir particle balance (MRPB) was developed, which included atomic and molecular reservoirs, to further investigate the role neutrals had on the density change. We discuss how this model was able to accurately reproduce the experimentally measured density reduction and ionization increase due to either a reduction in $τ_p$ or a reduction in particle fueling efficiency. Results from EMC3-EIRENE modeling indicate this change could be attributed to neutral particle fueling occurring in locations with now-opened field lines due to the chaotic edge-region from RMP applications.

physics.plasm-ph↗

arXiv2025

Density and Particle Sourcing Optimization in a Helicon Plasma Source Prototype For Wakefield Accelerator Applications

Helicon plasmas are being considered as plasma sources for wakefield accelerators, subject to strict density requirements. We present various mechanisms to increase axial density homogeneity in a helicon plasma for implementation in such an accelerator. We consider various background neutral flow configurations for helicons generated with first one antenna and then two identical antennas in a 2 meter long, 52 mm diameter plasma chamber with homogeneous magnetic field. In the case of a single antenna, the ionization source rate and density profiles are not significantly influenced by the background neutral flow. The use of a second antenna expands the plasma axially along the device, and results in an increased dependence of the axial density profile on the background neutral flow. We find an increase in axial homogeneity by a factor of two when there is no background neutral flow compared to when there is flow in either direction relative to the plasma. The minimum axial density deviation accomplished by optimization of RF antenna and neutral flow was 5%. This is still a factor of 20 above the nominal homogeneity requirement, but means to further improve this have been established in this study.

physics.plasm-ph↗

arXiv2025

An Innovative Heterodyne Microwave Interferometer for Plasma Density Measurements on the Madison AWAKE Prototype

The Madison AWAKE Prototype (MAP) is a high-power, high-density helicon plasma experiment. The project's main goal is to develop a scalable plasma source for use in a beam-driven plasma wakefield accelerator as part of the AWAKE project. We measure the plasma density with a new heterodyne microwave interferometer that features several improvements over traditional approaches. The design uses a single microwave source combined with an upconverter to avoid frequency drift and reduce overall cost. Elliptical mirrors focus the probe beam into the plasma and guide it back to the receiver. The transmitter and receiver along with the measurement electronics are co-located in a small enclosure and are assisted by two small mirrors on the opposite side of MAP. Both halves of the system move independently on computer-controlled motion platforms. This setup enables fast repositioning of the interferometer to measure at any axial location despite the magnets, wiring and structural supports that would block movement of a waveguide-based system. A high-speed, high-precision mixed signal circuit and FPGA analyze the probe signal directly in the enclosure which obviates the need for a digitizer or oscilloscope. The interferometer resolves phase shifts down to one hundredth of a fringe, resulting in a line-averaged resolution of $1.5\mathrm{\cdot 10^{17}\; m^{-3}}$. The system provides a real-time measurement every $5\;\mathrm{μs}$ up into the mid $\mathrm{10^{19}\; m^{-3}}$ density range with a noise level of $1.0\mathrm{\cdot 10^{17}\; m^{-3}}$.

physics.ins-det↗

arXiv2025

Exploration of Helicon Plasmas for Wakefield Accelerators at the Madison AWAKE Prototype

Plasma wakefield accelerators have the potential to revolutionize particle physics by providing lepton collision energies orders of magnitude beyond current technology. Crucially, these accelerators require a high-density, highly homogeneous, scalable plasma source. The Madison AWAKE Prototype (MAP) is a new plasma development platform that has been built as part of CERN's beam-driven wakefield accelerator project AWAKE. MAP uses a dual helicon antenna setup with up to 20 kW of RF power to create plasmas in the low $10^{20}\,\mathrm{m^{-3}}$ range in a highly uniform magnetic field. The project is supported by a range of diagnostics that allow non-invasive measurements of plasma density, ion and neutral flows, and temperatures, and a 3D finite element model that can calculate helicon wavefield and power deposition patterns. In this paper, we present an in-depth overview of MAP's design and construction principles and main physics results. We show that the plasma discharge direction is set by the combination of antenna helicity and field direction and linked to the well-known preference for right-handed helicon modes. We find that the plasma density depends dramatically on the direction of plasma and neutral flow. A detailed measurement of the ionization source rate distribution reveals that most of the plasma is fueled radially by recycling at the wall, a finding with strong implications for optimizing plasma homogeneity. Lastly, we describe how helicon antennas can be engineered to optimize power coupling for a given target density. Together these findings pave the way toward the practical use of helicon plasmas in wakefield accelerators.

physics.plasm-ph↗

arXiv2025

Computational and Analytical Optimization of Helicon Antennas with a Fast Full Wave Solver Exploiting Azimuthal Fourier Decomposition

Plasma wakefield accelerators (PWA), such as AWAKE, require homogenous high-density plasmas. The Madison AWAKE Prototype (MAP) has been built to create a uniform argon plasma in the $10^{20}\,\mathrm{m^{-3}}$ density range using helicon waves. Computational optimization of MAP plasmas requires calculating the helicon wavefields and power deposition. This task is computationally expensive due to the geometry of high-performance half-helical antennas and the small wavelengths involved. We show here for the first time how the 3D wavefields can be accurately calculated from a small number of 2D-axisymmetric simulations. Our approach exploits an azimuthal Fourier decomposition of the non-axisymmetric antenna currents to massively reduce computational cost and is implemented in the Comsol finite-element framework. This new tool allows us to calculate the power deposition profiles for 800 combinations of plasma density, antenna length, and radial density profile shape. The results show the existence of an optimally coupling antenna length in dependence on the plasma density. This finding is independent of the exact radial profile shape. We are able to explain this relationship physically through a comparison of the antenna power spectrum with the helicon dispersion relation. The result is a simple analytical expression that enables power coupling and density optimization in any linear helicon device by means of antenna length shaping.

physics.plasm-ph↗

arXiv2023

Analysis of the neutral fluxes in the divertor region of Wendelstein 7-X under attached and detached conditions using EMC3-EIRENE

This paper analyzes the neutral fluxes in the divertor region of the W7-X standard configuration for different input powers, both under attached and detached conditions. The performed analysis is conducted through EMC3-EIRENE simulations. They show the importance of the horizontal divertor to generate neutrals, and resolve the neutral plugging in the divertor region. Simulations of detached cases show a decrease in the number of generated neutrals compared to the attached simulations, in addition to a higher fraction of the ion flux arriving on the baffles during detachment. As the ionization takes place further inside the plasma during detachment, a larger percentage of the generated neutral particles leave the divertor as neutrals. The leakage in the poloidal and toroidal direction increases, just as the fraction of collected particles at the pumping gap. The fraction of pumped particles increases with a factor two, but stays below one percent. This demonstrates that detachment with the current target geometry, although it improves the power exhaust, is not yet leading to an increased particle exhaust.

physics.plasm-ph↗

arXiv2023

Preference of Right-Handed Whistler Modes and Helicon Discharge Directionality due to Plasma Density Gradients

Whistlers are magnetized plasma waves in planetary magnetospheres. Bounded whistlers, known as helicons, can create high-density laboratory plasmas. We demonstrate reversal of the plasma discharge direction by changing either antenna helicity or magnetic field direction. Simulations reproduce these findings only in the presence of a radial density gradient. Inclusion of such a gradient in the wave equation gives rise to azimuthal shear currents which for the first time consistently explains the preference of right- over left-handed whistlers and the discharge directionality in helicon plasmas.

physics.plasm-ph↗

arXiv2022

The AWAKE Run 2 programme and beyond

Plasma wakefield acceleration is a promising technology to reduce the size of particle accelerators. Use of high energy protons to drive wakefields in plasma has been demonstrated during Run 1 of the AWAKE programme at CERN. Protons of energy 400 GeV drove wakefields that accelerated electrons to 2 GeV in under 10 m of plasma. The AWAKE collaboration is now embarking on Run 2 with the main aims to demonstrate stable accelerating gradients of 0.5-1 GV/m, preserve emittance of the electron bunches during acceleration and develop plasma sources scalable to 100s of metres and beyond. By the end of Run 2, the AWAKE scheme should be able to provide electron beams for particle physics experiments and several possible experiments have already been evaluated. This article summarises the programme of AWAKE Run 2 and how it will be achieved as well as the possible application of the AWAKE scheme to novel particle physics experiments.

physics.acc-ph↗

arXiv2020

Heat and particle flux detachment with stable plasma conditions in the Wendelstein 7-X stellarator fusion experiment

Reduction of particle and heat fluxes to plasma facing components is critical to achieve stable conditions for both the plasma and the plasma material interface in magnetic confinement fusion experiments. A stable and reproducible plasma state in which the heat flux is almost completely removed from the material surfaces was discovered recently in the Wendelstein 7-X stellarator experiment. At the same time also particle fluxes are reduced such that material erosion can be mitigated. Sufficient neutral pressure was reached to maintain stable particle exhaust for density control in this plasma state. This regime could be maintained for up to 28 seconds with a minimum feedback control.

physics.plasm-ph↗

arXiv2019

First Results from an Event Synchronized -- High Repetition Thomson Scattering System at Wendelstein 7-X

The Wendelstein 7-X (W7-X) Thomson scattering (TS) diagnostic was upgraded to transiently achieve kilohertz sampling rates combined with adjustable measuring times. The existing Nd:YAG lasers are employed to repetitively emit "bursts", i.e. multiple laser pulses in a short time interval. Appropriately timing burst in the three available lasers, up to twelve evenly spaced consecutive measurements per burst are possible. The pulse-to-pulse increment within a burst can be tuned from 2 ms to 33.3 ms (500 kHz - 30 Hz). Additionally, an event trigger system was developed to synchronize the burst Thomson scattering measurements to plasma events. Exemplary, a case of fast electron density and temperature evolution after cryogenic H2 pellet injection is presented in order to demonstrate the capabilities of the method.

physics.plasm-ph↗