1. Fusion neutron research in Novosibirsk including experiments
“Piero Caldirola” International Centre for the Promotion of Science
and International School of Plasma Physics
Fusion neutron research in Novosibirsk including experiments
A. Ivanov
Budker Institute, Novosibirsk

2. Layout of the talk Brief description of the approach
GDT as a Neutron Source for materials testing and Hybrid
Experiments:
Electron temperature measurements with extended NBs
MHD and micro stability of high- plasma
Observation of AIC instability
axial confinement ambipolar plugs
Conclusions
WORKSHOP ON FUSION FOR NEUTRONS AND SUB-CRITICAL NUCLEAR FISSION Villa Monastero, Varenna, Italy, September , 2011 2

3. Gas Dynamic trap – general layout
The Gas-Dynamic Trap is a version of a standard simple mirror whose characteristic features are
– a very high mirror ratio, R , in the range of a few tens;
– a relatively large length, L , exceeding an effective mean free path,
ii lnR /R, with respect to scattering into the loss cone.
The warm target plasma is almost Maxwellian
– behaves like an ideal gas in a container with a pinhole leak
MHD-stable even though system is fully axially symmetric
– non-negligible amount of plasma in the regions beyond the mirror throats, where magnetic field has favorable curvature
– MHD ballooning/interchange modes limit stability at  40-60%
The electron neat flux to the end walls is suppressed by potential drop in expanders which develops if H mirror / H wall exceeds ~ 40
WORKSHOP ON FUSION FOR NEUTRONS AND SUB-CRITICAL NUCLEAR FISSION Villa Monastero, Varenna, Italy, September , 2011 3

4. Requirements to VNS for fusion materials testing
Fusion neutron spectrum
About 2MW/m2 neutron flux or higher for accelerated tests
Small enough gradient of neutron flux density
Continuous operation
More than 70% availability
Reasonably small tritium consumption
WORKSHOP ON FUSION FOR NEUTRONS AND SUB-CRITICAL NUCLEAR FISSION Villa Monastero, Varenna, Italy, September , 2011 4

5. Layout of GDT-based neutron source
Neutron flux density as a function of electron
temperature for injection energy 65 keV
Power consumption, MW 60
D/T beam energy (keV)
65/65
NB power/trapped (MW)
36.3/27.2
Mirror-to-mirror length (m)
11.4
Electron temperature (keV)
0.65
Plasma density (m-3)
2 x1020
Plasma radius at the center (m)
0.08
Mirror ratio 10
Central field (T)
1.3
Injection angle (deg.) 30
Max. neutron flux (MW/m2)
1.8
WORKSHOP ON FUSION FOR NEUTRONS AND SUB-CRITICAL NUCLEAR FISSION Villa Monastero, Varenna, Italy, September , 2011 5

6. Neutron shield & Testing zone arrangement
WORKSHOP ON FUSION FOR NEUTRONS AND SUB-CRITICAL NUCLEAR FISSION Villa Monastero, Varenna, Italy, September , 2011 6

7. Status of GDT-NS development
Conceptual design is completed for a version of GDT-NS with 1.8MW/m2 neutron flux, 60MW power consumption (BINP, Efremov, Snejinsk)
Plasma physical model based on Monte-Carlo approach is developed (BINP, FZR)
Feasibility of neutron shield is proven by numerical calculation (FZR, ENEA, Snejinsk)
26T, 90mm bore mirror coil design is developed (Efremov)
Small specimen test technology is proposed (KFK, BINP)
Application of GDT-NS for MA burner is considered (FZR, BINP)
WORKSHOP ON FUSION FOR NEUTRONS AND SUB-CRITICAL NUCLEAR FISSION Villa Monastero, Varenna, Italy, September , 2011 7

8. Conceptual parameters for GDT-based applications
PMI-HP-NS
GDT-NS
Hybrid
Length, m 8 10 30
Fusion power, MW - 2
200
Radius, m
0.2 1
Magnetic field, T
0.3
1.0
1.3
2.5
Beam energy, keV 20 40 65 80
Beam power, MW 4
100

9. Experimental model of GDT
View before and after
upgrade of neutral
beams
WORKSHOP ON FUSION FOR NEUTRONS AND SUB-CRITICAL NUCLEAR FISSION Villa Monastero, Varenna, Italy, September , 2011 9

10. VORTEX plasma CONFINEMENT IN GDT
U ~ Te
Plasma flow lines
for m=1 mode with vortex.
Steep potential gradient at periphery causes differential plasma rotation
The limiter biasing considerably improved plasma confinement
M=1 mode nonlinearly saturates
Limiter biasing produces radial electric field and plasma rotation at periphery
Stability of two-component plasma against interchange MHD modes was maintained in an axially symmetric magnetic configuration with the cusp end cell by making use of a set of biased radial limiters. With the cusp end cell only and without biasing of the radial limiters plasma energy and beta were limited by growth of MHD activity. The limiter biasing considerably improved plasma stability for higher betas. Formation of the sharp gradient of the electrostatic potential at plasma periphery caused differential rotation with high velocity shear. This effectively suppresses interchange modes with the azimuthal numbers m>1 and large scale drift turbulence. Theoretical consideration also showed non-linear saturation of amplitude of m=1 (rigid displacement) mode and formation in plasma core a region with closed flow lines, so that radial plasma transport become to be small. The nature of this phenomenon is similar to formation of an internal transport barrier in tokamak plasmas. a b
Potential profile Plasma decay a) with vortex, b) no vortex
WORKSHOP ON FUSION FOR NEUTRONS AND SUB-CRITICAL NUCLEAR FISSION Villa Monastero, Varenna, Italy, September , 2011

11. STEADY STATE IS ACHIEVED WITH PLASMA REFUELING
No gas puff
Gas puff with 5mc, 3.5 MW beams
WORKSHOP ON FUSION FOR NEUTRONS AND SUB-CRITICAL NUCLEAR FISSION Villa Monastero, Varenna, Italy, September , 2011 11

12. AXIAL RE-DESTRIBUTION OF HIGH- PLASMA PRESSURE
Loop data Plasma diamagnetism
WORKSHOP ON FUSION FOR NEUTRONS AND SUB-CRITICAL NUCLEAR FISSION Villa Monastero, Varenna, Italy, September , 2011 12

13. NB attenuation data –spontaneous excitation of m=2 mode in high- plasma
Oscillations

14. Findings in GDT experiments
Issues addressed
Some important results
Factors controlling electron temperature
Equilibrium and stability of anisotropic fast ions
Steady state operation
Ballooning instability threshold
Effect of ambipolar fields on confinement
Effect of plasma rotation/vortex barrier formation
Non-paraxial effects due to high β
Te is determined by balance between fast ion drag power and collisional end losses
Fast ion relaxation is classical
Skew NBI provide fast ion density peaks at turning points
High-β (>0.5) MHD – stable plasma in axisymmetric field
Suppression of axial electron heat conduction to the end wall by decreasing magnetic field
Plasma is sustained during several characteristic times with extended neutral beams
WORKSHOP ON FUSION FOR NEUTRONS AND SUB-CRITICAL NUCLEAR FISSION Villa Monastero, Varenna, Italy, September , 2011 14

15. PLASMA PARAMETERS IN GDT EXPERIMENT
Injected (Pinj) and trapped (Ptr) NBI power
Fast ion energy content
Linear DD yield near fast ion the mirror point.
Neutron scintillation detector and single particle proton detector near fast ions mirror point allow to control DD activity in each shot.
Analysis of DD reaction yield and diamagnetism of fast ions in the experiments with deuterium and hydrogen target plasma shows that fast ions collisions are defined
with DD reactions. Fusion reactions between fast component and target plasma give low contribution into total number of reactions.
Presented axial distribution is compared with results of numerical simulation (blue line).
DD reaction yield: axial profile radial profile

16. PLASMA PARAMETERS IN GDT EXPERIMENT- ELECTRON TEMPERATURE

17. PLASMA PARAMETERS IN GDT EXPERIMENT- PLASMA BETA
On-axis magnetic field depression in turning point region vs energy
accumulated in fast ions
B/B variation across plasma at the turning point
2<i
Magnetic field depression and local diamagnetism vs time
WORKSHOP ON FUSION FOR NEUTRONS AND SUB-CRITICAL NUCLEAR FISSION Villa Monastero, Varenna, Italy, September , 2011

18. “Saw teeth” relaxations
Spectrum of RF noise
Axial broadening of fast ion
reflection region
WORKSHOP ON FUSION FOR NEUTRONS AND SUB-CRITICAL NUCLEAR FISSION Villa Monastero, Varenna, Italy, September , 2011

19. Experiment with additional mirror cell
Compact
mirror
cell
GDT central cell
Internal cell: Magnetic field: Background plasma:
L=30 cm, d =70 cm. B0=2.4 T, Bm=5.2 T hydrogen, n0 ≈ 1019 m-3, Te ≈ 70 eV, a =9 cm.
NBI: H0 or D0 , E0=20 keV, θ=90º, Pinj ≈ 1 MW, τinj=4 ms
WORKSHOP ON FUSION FOR NEUTRONS AND SUB-CRITICAL NUCLEAR FISSION Villa Monastero, Varenna, Italy, September , 2011 19

20. Fast ion density in mirror cell
WORKSHOP ON FUSION FOR NEUTRONS AND SUB-CRITICAL NUCLEAR FISSION Villa Monastero, Varenna, Italy, September , 2011 20

21. Experimental observation of aic instability in mirror cell
HF oscillations threshold:
n > 2.5·1019 m-3, A ≈ 35, β┴ = 0.02,
сi/аp ≈ 0.23.
The following slide presents the results of investigation of microinstabilities in the anisotropic
synthesized hot ion plasmoid (SHIP). Plasmoid was located in a small mirror section that is installed at one side of
the GDT. To define the type and the parameters of the developing microinstability a set of high-frequency electrostatic and magnetic probes
was used. The observed microinstability was determined as the Alfven ion cyclotron instability (AIC), because of small azimuthal wave numbers,
magnetic field vector rotating in the direction of ion gyration and oscillation frequency below the actual ion cyclotron frequency. AIC instability threshold was
registered at the following plasma parameters: fast ion density n > 2.5 х 1013 cm-3, ratio of ion pressure to magnetic field pressure β ≈ 0.02, anisotropy A = 35, ai/Rp ≈ 0.23,
where ai is the ion gyroradius and Rp is the plasmoid radius.
Br, arb. u.
Bφ, arb. u.
Polarization
Main frequency f0 < fci
The magnetic field vector of the wave rotates in the direction of ion gyration.
Azimuthal mode number m = 1-2
AIC
instability
— anisotropy in velocity space
WORKSHOP ON FUSION FOR NEUTRONS AND SUB-CRITICAL NUCLEAR FISSION Villa Monastero, Varenna, Italy, September , 2011

22. Axial confinement with ambipolar end plugs
WORKSHOP ON FUSION FOR NEUTRONS AND SUB-CRITICAL NUCLEAR FISSION Villa Monastero, Varenna, Italy, September , 2011

23. Ambipolar plugging experiment at GDT.
Central cell
NBI
Plasma dump
Plug cell
Fas ions
Plasma source
Expander
Magnetic coils
Limiter
with plugging
w/o plugging
difference
with plugging
w/o plugging
difference
with plugging
one side plugging
w/o plugging
x 2 on axis
Radial profile of the plasma potential.
Radial profile of the plasma density.
Linear plasma density time evolution

24. Observation of AIC instability in local cell
The probe measured potential fluctuations show the presence of waves having small azimuthal mode numbers m=1,2.
The oscillation frequency is below local ion-cyclotron frequency.
Magnetic fluctuation probes show that the mode is nearly left-circularly (direction of ion gyration) polarized.
These properties are all consistent with an Alfven-like wave generated by AIC instability.
The AIC instability threshold is observed
WORKSHOP ON FUSION FOR NEUTRONS AND SUB-CRITICAL NUCLEAR FISSION Villa Monastero, Varenna, Italy, September , 2011 24

25. Conclusions
Electron temperature achieved already at the GDT experiment corresponds to ~0.4MW/m2 neutron flux for GDT-NS
Below some limit in pressure plasma behavior is classical. No critical issues were found preventing from further improvement of plasma parameters
Reduction of axial losses with ambipolar plugs is demonstrated
Plasma steady state conditions are planned to be achieved at the next step device at higher electron temperature
Conceptual design of GDT-NS for fusion materials and sub-components development is completed
Possible application of GDT-NS as a driver for fission/fusion hybrids is under consideration