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

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 12 - 15, 2011 2

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 12 - 15, 2011 3

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 12 - 15, 2011 4

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 12 - 15, 2011 5

Neutron shield & Testing zone arrangement WORKSHOP ON FUSION FOR NEUTRONS AND SUB-CRITICAL NUCLEAR FISSION Villa Monastero, Varenna, Italy, September 12 - 15, 2011 6

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 12 - 15, 2011 7

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

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 12 - 15, 2011 9

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 12 - 15, 2011

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 12 - 15, 2011 11

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 12 - 15, 2011 12

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

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 12 - 15, 2011 14

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

PLASMA PARAMETERS IN GDT EXPERIMENT- ELECTRON TEMPERATURE

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 12 - 15, 2011

“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 12 - 15, 2011

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 12 - 15, 2011 19

Fast ion density in mirror cell WORKSHOP ON FUSION FOR NEUTRONS AND SUB-CRITICAL NUCLEAR FISSION Villa Monastero, Varenna, Italy, September 12 - 15, 2011 20

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 12 - 15, 2011

Axial confinement with ambipolar end plugs WORKSHOP ON FUSION FOR NEUTRONS AND SUB-CRITICAL NUCLEAR FISSION Villa Monastero, Varenna, Italy, September 12 - 15, 2011

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

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 12 - 15, 2011 24

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