Behavior of Compact Toroid Injected into the C-2U Confinement Vessel

Abstract
The C-2U experiment offers a unique plasma environment combining a high beta field reversed configuration (FRC) embedded in a low beta magnetic mirror with high power neutral beam injection. The beams are injected tangentially into a modest magnetic field so that the orbits of the resulting fast ions encircle the entire plasma. These large orbit particles sustain1 and stabilize2 the plasma and suppress turbulence.
Measurements of magnetic fluctuations at the edge of the plasma reveal the presence of three coherent beam driven modes: a low frequency, chirping mode, a mode near the ion cyclotron frequency, and a high frequency compressional Alfven mode. Remarkably, none of these modes are observed to have a deleterious effect on global plasma confinement. In fact, the cyclotron mode has the beneficial effect of dramatically enhancing the DD fusion reaction rate by drawing a trail from the plasma ion energy distribution on a sub-collisional timescale.
In this presentation, we experimentally characterize the beam driven modes in the C-2U FRC with data from multiple diagnostics including magnetics, interferometry, neutral particle analyzers and fusion product diagnostics. Results are compared to a particle-in-cell simulation in a simplified geometry.
1 M. W. Binderbauer, et. al., AIP Conference Proceedings 1721, 030003 (2016) 2 M. Tuszewski et. al, Phys. Rev. Lett 108, 255008 (2012)
Overview
§ Unique plasma configuration: high beta, toroidal plasma (with no or very small toroidal magnetic field) embedded in a magnetic mirror
Sustainment and stabilization
Discrete beam driven modes
Mode control
Ion cyclotron mode amplitude and enhanced fusion rate is reduced by staggering beam energies
Experimental indication that df/dv is free energy source for cyclotron mode
Chirping mode (the staircase) is reduced by elongating FRC through active profile control
Elongated FRC has more high frequency Alfven activity, indicating that fast ions are present, despite the absence of the staircase
Measurements of fusion neutron flux during deuterium NBI indicate that total fast ion content is the same in the elongated and standard cases
Proton measurements indicate that there is a larger axial density gradient in shots with larger chirping mode activity
Experimental indication that dn/dx is free energy source for chirping mode
§ Unique fast ion environment: high NB power density and low magnetic field results in substantial population of large orbit particles
§ NB power density and ratio of fast ion orbit size (ρ) to machine minor radius (a) more akin to mirror machine than a tokamak
• At least three distinct fast ion modes are observed in magnetic fluctuation spectrograms
during hydrogen NBI
§ Measured neutron rate is consistent with neutron 0
• None are fatal to the plasma • In C-2U, E /T =30. In future
0
400 500
f (kHz) f (kHz)
0
600 700
t (μs)
§ Identified mode reproduces tail
bi
devices, this ratio is smaller.
§ Density fluctuation amplitude is larger in the scrape off layer (SOL) plasma than in the core
8×10-5 8×10-5
§
§
PIC simulations predict much stronger mode activity in the SOL plasma relative to the core
Mode is identified as Ion Bernstein mode based on dispersion relation
generation and fusion enhancement
§ See Poster by A. Necas for discussion of PIC simulation (this session)
Fast ion mode activity is likely to decrease in next generation experiment
4×10
Device
ITER DIII-D 2XIIB C-2U GDT
Class
tokamak tokamak
mirror machine FRC
mirror machine
Cyclotron mode
NBI (MW/m3) ρ/a 0.04 0.005
1.4 0.03 5 0.7
10 1
30 1.5
Alfvenic mode
5
rate calculated from f(E) as measured by NPA 1.0 0.8
0.6 diagnostic and measurements of suprathermal tail 0.4
t = 0 μs t = 8 μs
1.0
10
beam plane mid-plane
Phenomenology of beam driven modes in the field reversed configuration
Chirping modes
6×10-5
§ § §
High power neutral beam injection (NBI) sustains the FRC plasma for the duration of the beam pulse
NBI suppresses broadband magnetic turbulence while driving fluctuations near f
400
0 MW
600
f (kHz)
measured thermonuclear
3 MW
800
1000
4
§
§
Fusion neutron and proton production is observed to be several orders of magnitude larger than the expected thermonuclear rate
10
10
0 0.0
§
§
See poster by R. Clary for details on NPA
(this session) 0.2
-2
Cyclotron fluctuations also appear in electron density, measured with Far Infrared interferometry (FIR)
54××1100 4×10-5
density (r=30cm)
R. M. Magee, R. Clary, A. Necas, S. Korepanov, A. Smirnov, M. Thompson, T. Tajima, and the TAE Team
TRI ALPHA ENERGY, INC., P.O. Box 7010, Rancho Santa Margarita, CA 92688-7010
ci
measured thermonuclear
Suprathermal neutron production (100x thermonuclear)
is correlated with cyclotron fluctuations 108
9 MW
01234012340123 t (ms) t (ms) t (ms)
40
Ion energization from continuo3u0 s beam driven mode
20
4
5 4
3
2
1
0 -1
5 4
3
2
1
0 -1
1.0
Measurements with mass resolved NPA reveal
emergence of population of high energy deuterium 10
deuterium
measured
calculated
600
5 6 7 8 9 105 6 7 8 9 105 6 7 8 9 10
5×10-5 -5-5
density (r=0cm)
density (r=15cm)
1×10-6 -76
-5 3×10-5
density (r=0cm)
-7 8×10-7
3×10 2×10-5
density (r=45cm; /10)
6×10 6×10-7
-5 2×10-5
-5 6×10 -5
-5 4×10 -5
2×10 -5
10-5
10-6 200
1010 109
0 MW
3 MW
9 MW
measured thermonuclear
§
§
§ § §
High frequency Alfven modes are interrupted by bursts of low frequency fluctuations leading to discrete drops in internal pressure (aka “the staircase”)
Burst frequency decreases with increasing beam energy (right); burst frequency increases with increasing magnetic field (not pictured)
See Poster by B. Deng for discussion of low frequency modes (this session)
Bursts of neutron flux evidence of discrete ion energization (right)
Phase velocity of high frequency mode equal to local Alfven velocity (below)
32 30 28 26 24
2.5
8 6
4
2
0 0
8 2.5
26.0
1.5 4
1.4
1.2
1.0
0.8
0.6
0.4
0.2 0.0
0
peaked
flat
1011
2
10
0.5
0.0
10 10-3
0
0 200 -0.10 -0.05
400 600 0.00
800 0.05
1000 0.10
10 15 8 10
hydrogen
9
time relative to burst (ms)
10 7
50 2 4 6 8 10
fAlfven (r=7cm) 1400f
δB
fAlfven(r=20cm)
δn (r=0cm)
δn (r=15cm)
neutrons 10 protons 106 thermonuclear (calc.)
1200 1000
0 15
t (ms)
fAlfven(r=60cm) 800
012345 10 t (ms)
0.5
5
v (10
6
m/s)
1.5
15
density (r=15cm)
magnetic (edge)
density (r=30cm)
density (r=45cm; /10)
magnetic (edge)
-7
Summary
4×10 4×10-7
-7 10 000
1×10
1×10-5 2×10-7
2×10 400 500 600 700
0
C-2U FRC plasmas are sustained and stabilized by high power NBI Large population of fast ions drive multiple modes:
§ Ion cyclotron mode driven by df/dv creates high energy tail of ions
§ Chirping mode driven by dn/dx leads to drop in internal pressure and burst of ion energization
§ Alfvenic mode heavily damped by plasma
PIC simulations reproduce key features of ion energization by beam driven mode Modes are controlled by reducing free energy source
2×10 0
-20 0 20 40 60 80
0 r (cm)
-20 0 20 40 60 80
r (cm)
t (ms)
t (ms)
PIC Simulation
100 10-1
-4 0.0
40 18×10 30
20
t (ms)
0 -5
-10 2π -15
3π -20
Alfvenic mode chirping mode
800 peaked 700
1.8 1.5 1.2 0.9 0.6 0.3 0.0
den. fluct. amp. (δn/n) den. fluct. amp. (δn/n)
rel. den. fluct. (δn/n) rel. den. fluct. (δn/n)
fusion rate (s-1)
rel.rmel.amg.afglu.cftlu. c(δt.B(/δB)/B)
fusion enhancement factor
f(v)
neutrons (a.u.)
energy (keV)
energy (keV)
r∆φ (cm)
neutron rate (s-1)
b~ spectral power (G2)
signal (a.u.)
signal (a.u.)
f (kHz)
neutron rate (1010 s-1) burst frequency (kHz)
burst frequency (kHz)
r∆φ (cm)
neutron rate (a.u.)
f(v)
f (kHz)
f (kHz)
phase (radians)
b2 (10-4 G2)
0 100 200 300 400 coil angle (degrees)
beams terminated
3.0 3.5 4.0 4.5 5.0 t (ms)
5 10 15 20 beam energy (kV)
500 400 800 700 600 500 400
B (G) e
§
§
§
§
§
§
§
§ §
§ §
1.2
1.0 flat
peaked
flat
0.8
0.6
0.4
0.2
600
velocity (106 m/s) t (ms)
1 2 3 t (ms)
standard FRC
elongated FRC
elongated FRC
standard FRC
0.0
0.0 0.5 1.0 1.5 2.0 0 1 2 3 4