Interpretation of the charge exchange neutral energy spectrum of C-2W

Interpretation of the charge exchange neutral energy spectrum of C-2W
Abstract
In TAE Technologies’ current experimental device, C-2W (also called “Norman”) [1], record breaking, advanced beam-driven field reversed configuration (FRC) plasmas are produced and sustained in steady state utilizing variable energy neutral beams (15 – 40 keV, total power up to 20 MW), advanced divertors, end bias electrodes, and an active plasma control system. Diagnosis of fast ions, which are born from neutral beam injection and responsible for current drive and plasma heating, is critical for understanding the FRC behavior. Neutral Particle Analyzers (NPAs) are used to measure the energy spectrum of fast ions that charge exchange on background or beam neutrals and are lost from the plasma. The fast ion energy spectrum can be reconstructed by modeling the spatial distribution of fast ions and neutral particles. We present measurements made with an electromagnetic electromagnetic NPA which provides isotopic separation of beam and bulk plasma species. C-2W also has a lighter electrostatic NPA which can be steered with a 2-axis gimbal to access a wide range of pitch angles. Forward modeling of the spectra with a hybrid MHD-Monte Carlo code is used to examine the collisional processes of fast ion slowing down on electrons and charge exchange loss. Non-collisional ion acceleration by a beam-driven wave, similar to that observed on C-2U [2], is also observed in C-2W and modeled with a particle-in-cell code.
C-2W Neutral Particle Analyzers & Thermo-Optical Array  Electromagnetic NPA
G. Player, R. Clary, S. Dettrick, B. Fox, S. Korepanov, R. M. Magee, B. Nicks, T. Tajima, and the TAE Team
Classical Fast Ion Confinement in C-2W  Fast ions in C-2W are classically confined
 Strong agreement between observed CX spectrum, Monte Carlo, and the 0D Model
 The Monte Carlo and 0D Model predict a classical slowing-down distribution
 “Hollowing out” of NPA signal at higher energy due to increased ion scattering
Perturbative Modeling of Fast Ion Losses  2 major ion loss channels in the CV
 “First Orbit Loss” – beam neutrals which charge exchange too far into the edge plasma and are not confined
 “Prompt Loss” – fast ions which are lost within their first few orbits
 Perturbations don’t affect first orbit losses – scale with neutral density
 Perturbations scatter fast ions into the mirror’s “loss cone”
 Fast ion losses are dominated by scattering into the loss cone
Modeling Effects of Mirror Field on Fast Ion Confinement
 Increasing the mirror field increases fast ion accumulation in the FRC
 Increases fast ion end loss – larger population of slowed- down ions to pitch angle scatter
 Losses in CV are primarily near the beam injection energy
 CV losses are reduced by increasing mirror field
 End losses decrease with
increasing energy - 𝝂𝒃𝒊 ∝ 𝟏/𝑬𝟑/𝟐 𝒃
Modeling Effects of Mirror Field on Charge Exchange Spectra
 B-field perturbations cause anomalous fast ion transport, increasing high- energy charge exchange
 Increasing mirror ratio also decreases charge exchange
 Axial extent of Veff is reduced for fast ions, preventing them from interacting with edge neutrals
Initial Observation of Thermal Ion Acceleration
 In C-2U, R. Magee et. al. discovered the existence of thermal ion acceleration [2, 5]
 Recent observations have revealed the signatures of this process in C-2W
 These shots had hydrogen only beams and a deuterium thermal population in the plasma
 A strong deuterium signal was observed, peaking at 10-12 keV  The neutron yield increases by ~ 102 concurrently with spike in
deuterium signal
 Could be attributed to Ion-Bernstein waves with further work to determine precise nature of the acceleration [2, 4, 5]
Conclusions
 This work focused on the diagnosis of fast ions in C-2W, through both experimental and theoretical channels  Monte Carlo simulations were done to model the collisional processes of slowing down and charge exchange
 Comparing simulation results to experimental spectra strongly suggests the fast ion population in C-2W is mostly classically confined
 Increasing the mirror field improves fast ion confinement lifetime & decreases charge exchange losses  Additionally, experimental deuterium spectra and neutron data suggests the re-appearance of thermal ion
acceleration that was originally reported in C-2U [2, 4, 5]
References [1] H. Gota et al, Nucl. Fusion 59, 112009 (2019)
[2] R. M. Magee et al, Nature Physics 15 (2019)
[3] E. Granstedt et al, DPP19 APS Meeting (2019)
[4] T. Tajima and A. Necas, Physics of Plasma-Driven Accelerator and Accelerator-Driven Fusion (2016) [5] B. Nicks, Phys. Rev. Let., Submitted (2019)
 36 energy channels per isotope, sees deuterium and hydrogen
 Heavy and slow to move, limited dynamic range  Electrostatic NPA
 16 energy channels, no isotope discrimination
 Lightweight and mounted on a fast-moving, high dynamic range gimbal  Thermo-Optical Array
 4 cables – 2 at mirror points, 1 axial, 1 azimuthal near midplane  Measures local temperature rise per shot
 Gives spatially-resolved measurement of energy deposition
Modeling to Reconstruct Fast Ion Spectra
 The charge exchange spectrum in C-2W does not provide a direct measurement of the fast ion spectrum
 Fast ions sample a wide variation of plasma parameters over the course of one orbit
 𝒇𝒄𝒙 E ∝𝒇𝒇𝒊 𝒙,E ∗𝒏𝟎 𝒙 ∗𝝈𝒄𝒙(E)
 Monte Carlo simulations can reconstruct the fast ion population
 Fast ions simulated against a stiff background
 Neutral density inferred from camera measurements using
DEGAS2 code [3]
 Background plasma & magnetic field calculated using Bayesian reconstruction of experimental data
 Applied simple perturbations of the form 𝜹𝑩 ∝ 𝝃 ∗ 𝒄𝒐𝒔 𝒏𝜽  As a secondary confirmation, a simple 0D model was built
 Utilizes a simple rigid rotor profile of B-field and plasma parameters  Charge exchange and electron slowing-down only
2D Orbit Averaging
Initial Particle Energy Spectra
Electron Slowing Down & Charge Exchange Losses
Confined Fast Ion Spectrum & CX Loss Spectrum