Page 1 - An Interesting Poster to look at from the Tri Alpha Energy Team in California

Near-infrared Bremsstrahlung Radiation Measurements in an Advanced Beam-driven FRC Plasma
 BACKGROUND & MOTIVATION
 PREVIOUS MEASUREMENTS: C-2 & C-2U
 Titanium gettering, however, introduced impurity lines that polluted Bremsstrahlung measurements near 523 nm
 Resulted in overestimated values of the effective ionic charge
NEAR-INFRARED VS. VISIBLE BREMSSTRAHLUNG
 NEAR-IR BREMSSTRAHLUNG DIAGNOSTIC: C-2W
 In C-2W, an upgraded diagnostic system will be deployed
to measure Bremsstrahlung near 1000 nm (FWHM: 10nm)  Silicon avalanche photodetectors (APD)
 Laser Components A-CUBE-S1500-01  Responsivity of ~45 A/W at 1000 nm  Ø1.5 mm detector chip
 System bandwidth of 1 MHz
 Active T compensation electronics
 NIR system will be paired with a Da system to remove contributions from pollutants (share the same view)
 Tangential mount will enable edge data collection
 Survey spectrometer capabilities extended to 1100 nm
 Pulsed NIR LED array with Lambertian diffuser used for system calibration
Marcel Nations, Deepak Gupta, Nathan Bolte, Matt Thompson, and the TAE Team
TAE Technologies, Inc., 19631 Pauling, Foothill Ranch, CA 92610
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In magnetically confined fusion plasmas, the effective ionic charge (𝑍𝑒𝑓𝑓) is a measure of the plasma contamination from impurities:
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Zeff FROM VISIBLE BREMSSTRAHLUNG EMISSION
 Survey spectrometer: 250-680 nm (Avantes Inc.)  12 optical chords
 1⁄2” collimator optics
 PMT detectors
 600 mm fiber optics  Bandpass filter:
 l0: 523.5 nm
 FWHM: ~1.5 nm
𝑛 𝑍2
𝑍𝑒𝑓𝑓=𝑖𝑖𝑖 (1)
𝑖 𝑛𝑖𝑍𝑖
where 𝑛𝑖 and 𝑍𝑖 are the density and charged state of
individual ionic species present in the plasma, respectively Such impurities can account for substantial radiative power
losses and thus knowledge of 𝑍𝑒𝑓𝑓 profiles is critical
One method to determine 𝑍𝑒𝑓𝑓 is to measure Bremsstrahlung continuum radiation over a small spectral range free from line radiation
 BREMSSTRAHLUNG = “BREAKING RADIATION”
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NIR spectrum doesn’t appear to have large line radiation found in the visible (green) region
NIR region is more consistent with theoretical Bremsstrahlung vs. wavelength than the visible (green) region
 Above 700 nm, it appears far more feasible that the local spectral minima are approaching the Bremsstrahlung baseline
electron-ion
Spectral Bremsstrahlung emissivity is given by the
following equation (assuming quasi-neutrality, 𝑛𝑒 = 𝑖 𝑛𝑖𝑍𝑖):
where 𝑛 is the electron density, 𝑇 is the electron 𝑒𝑒
(3)
 Continuous
arises from electron deceleration due to Coulomb collisions in high-temperature plasmas
(e−i) Bremsstrahlung
emission
𝑔 𝑛2𝑍 𝑒−h𝑐𝜆𝑇𝑒
𝜖𝑒−𝑖 𝜆 = 1.516 × 10−30 𝑓𝑓 𝑒 𝑒𝑓𝑓 [W/cm3/nm/sr] (2)
𝜆2 𝑇 𝑒
temperature (eV), 𝜆 is the wavelength, and 𝑔𝑓𝑓 is the free- free gaunt factor:
 Li wall-conditioning helped keep impurity levels low
 Emissivity profiles are obtained from chord measurements of plasma brightness
0.15
𝑔 =1.35𝑇 ; 0.1keV≤𝑇 ≤2.0keV 𝑓𝑓 𝑒 𝑒
 MEASUREMENT METHODOLOGY
 Optical mounts housing an array of focusing lenses are
installed near the center plane of the machine (“A-plane”)
 Quasi-cylindrical volumes of the plasma are sampled at multiple lines-of-sight and collected emission signals are focused into quartz optical fibers
 The optical fibers then rout measured light to a separate room where the detection system is located
Before reaching the detectors, light passes through bandpass filters which transmit only at a narrow spectral region free from line radiation
 Irradiance calibration is applied and measured line- integrated plasma brightness are Abel-inverted to get local emissivity profiles
 CHALLENGES
FromEq2)𝜖𝑒−𝑖 1000nm ≈0.27
 Time evolution of 𝑛 and 𝑇 profiles are obtained from 𝑒𝑒
Thomson scattering and, together with Eq. (2) and (3), are used to calculate 𝑍𝑒𝑓𝑓 𝑟, 𝑡
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Neutral Beam Injection (NBI) termination experiments:
 NIR: 1000 nm (10 nm FWHM); Visible: 523.5 nm (1.5 nm FWHM)
 Visible Bremss drops over twice as much as NIR Bremss
 Neutrals are involved in continuum electron-neutral Bremss (core) and the dissociation of molecular hydrogen (edge)
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𝜖𝑒−𝑖 523nm
 Higher NIR sensor detectivity needed
From Eq 2)  𝜖 ∝ 𝑇−0.35 𝑒−𝑖 𝑒
 𝑻𝑒 is expected to be 10x higher
 𝜖𝑒−𝑖 signal in C-2W should be approx.
half of that in C-2U
The new system must be optimized to keep SNR at a suitable level
 Larger collimating optics (Ø1”)  ~4x more signal
 Lowpass digital filter to remove high-frequency content