Analysis of C-2W electrode arcing experimental data

Radiative Measurements in C2-W
Timothy DeHaas, Anton Bondarenko, Matt Tobin, Anton Tkachev, and the TAE Team
ABSTRACT
TAE Technologies’ current experimental device, C-2W, is an advanced, beam-driven, field-reversed configuration (FRC) with advanced divertors, end bias electrodes, and an active plasma control system [1]. The emergence of C-2W’s high performance regime has highlighted the need for further expansion of diagnostic systems to understand equilibrium behavior, global stability, and power loss mechanisms. To that end, the experiment has been fitted with an ensemble (over 300 channels) of XUV and soft X-Ray sensing diodes. The ensemble contains a mixture of both collimated and uncollimated views with broad spatial coverage. The collection of measurements yields the total radiated power from the plasma and has already been used to demonstrated the mitigation of impurity radiation to below 200 kW [2]. Recently, additional units have been deployed to provide axial coverage of the machine, yielding an axial emission profile. The system has been integrated into the active plasma control system to produce estimates of density and axial position control. Similarly, the diagnostic is fitted with differing thin, metallic, optical filters for coarse spectral resolution. Several Be filters have been added with the express purpose of identifying fast electron production. Initial observation of plasma emission profiles and x-ray production will be presented. [1] H. Gota et al, Nucl. Fusion 59, 112009 (2019). [2] T. DeHaas, A. DuBois, APS-DPP 2019 Poster UP10.00131
COMPACT SUITE OF BOLOMETERS
RECONSTRUCTION OF RADIATED POWER
HIGH PERFORMANCE REGIME
The C-2W (Norman) experimental program an advanced, beam driven, field-reversed configuration. The experiment is TAE’s fifth generation fusion device and over the last three years has radically improved in performance. To that end, it has achieved its experimental goals: A steady-state plasmas for 30 ms duration with 3 keV total temperature. Externally ramped magnetic pressure balanced by internally growing fast ion pressure (magnetic energy more than doubles over the 30 ms shot). Fast ion accumulation as recorded by neutron signal from D + D fusion.
Axial Bolometer Array
Fit to Flat-Top Distribution
MODEL PROFILE OF FRC
∝ 𝑛 !"
THREE-DIMENSIONAL MODELING OF
Wide Collection Bolometers
ESTIMATESOFRADIATEDPOWER
Shot=114534
Increasing Energy
Stable FRC Plasma for 30 ms
Max radiated power helps track progress
High-Performance Regime
ABOVE: Losses are reduced and confinement improved through a combination of experimental techniques, effective electrode biasing, targeted fueling, and Ti-arc and cryo gettering. Peak radiation below 200 kW.
These instruments are specifically designed to measure light from Vacuum UltraViolet (VUV) through soft X-ray, which necessitates in-vacuum detectors and related optics. Optical filters are also placed in-vacuum to provide coarse spectral resolution. Light that reaches the solid-state detectors used by these instruments (AXUVHS1) is converted to a small electrical current.
N
Axial Bolometer Array
Aided by the small design and modularity of the diagnostic, a number of units are strategically placed through the confinement vessel. Over 600 channels of data are collected and processed for every shot. The data provides for the radial profile, axial profile, and total radiated power within a certain spectral range. This information is used to generate a 3D radiation model of the radiated power.
𝐵!
We estimate the radRiaAtedDIpAowTeIOr ∝N𝑛!", a valid estimate confirmed by profile inversion. We combine ALL detectors together to generate a 3D model of radiation (left) to estimate the total radiated power (right).
𝑛!
CORE SOL BKG
BOLOMETER EXPANSION INTO SOFT X-
A Ridged Rotor Model is assumed: "
𝑢 = 2𝑟 ⁄𝑅$ − 1
"
Ramped D+D Flux
Thermal
Electron 300 eV
Neutron
𝑝 𝑟 ∝ 𝑛 !" ~ s e c h # 𝐾 𝑢
S O L CORE
Radical improvement in T over Norman Lifetime
Deconstructed Compact Bolometer
S
Temperature >
RAY BAND
Sn 250 nm
Al 750 nm
Be 1000 nm
Mo 300 nm
Pd 150 nm
UPCOMING ELECTRON BEAM INSTALLATION
R2 =
Proposed soft x-ray filter, Be 5 um thickness, to identify hot electrons
0.989 (i) 0.903 (a)
e
A new electron beam (Left) will be installed on Norman with energies up to 30 keV at 200 A. The purpose of the electron beam is to provide additional heating of electrons
Wide Collection Bolometers
OPTICAL FILTERS
Plasma Gun / Electrodes
in Outer Divertors
Divertors cooled with LN2 and gettered until base pressure < 10-9 Torr
C2-W
DC Magnets & Pulsed Power Theta-pinch
Neutral Beam Injectors
Inner Divertor to Improve Electron Confinement
The diagnostic is fitted with a series of thin, modular, metallic, optical filters for coarse spectral resolution. Continual improvements in the performance of C2-W have led to radical increases in electron temperature outside the bandpass of optical filters on the bolometers. To accommodate the increases in Te, new Be filters have been implemented to track electrons > 500 eV.
REAL TIME DENSITY RECONSTRUCTION
Spectrum for 30 keV beam on Mo Target
HVPS VOLTAGE HVPS CURRENT
X-RAY SIGNAL
BEAM PATH
The electron beam will increase the temperature of the plasma, leading to the need for a filtered bolometer array which function at higher energies. The the beam will also terminate on a Mo beam dump which must be monitored.
The predicted spectrum for 30 keV electrons on a Mo beam dump. To help verify the spectrum as power deposition, a new x-ray bolometer prototype was installed on electron beam test stand. The bolometer looks directly at dump. Once installed Norman, the bolometer will be used to identify both coupled and lost power from the beam as part of power balance analysis.
(NORMAN)
TAE Technologies latest generation in fusion devices is
Outer Divertor with Mo electrodes
named after our late intellectual co-founder, Dr. Norman Rostoker. The device was unveiled in July 2017. It has currently performed over 23,000 experiments.
Raw Bolometer Data
Hidden Layers
Processed FIR Data
The Real Time Plasma Control System (RTPSC) is a multivariate control system which regulates the current and voltage waveforms for magnet and electrode power supplies. The system can regulate waveforms with sub millisecond speeds.
The RTPSC utilizes hundreds of data nodes from the total radiation bolometers, the linear bolometers array, the bremsstrahlung radiation array, and the pyro- bolometers. These 284 signals are used as input to a neural net used to reconstruct plasma profiles in real time. To do this, machine learning algorithms are trained prior to the experiment using bolometers (among many other) as input nodes and interferometry as the output comparison. The two show good agreement.
Transient Period
Machine Learning Predicted
1 ms/div
5 kV/div 50 A/div
Continual improvement of Losses