Possible further steps for upgrading the GDT device. T.D. Akhmetov , A.A. Ivanov, and V.V. Prikhodko. Budker Institute of Nuclear Physics, Novosibirsk, Russia. Outline. Current parameters of Gas Dynamic Trap (GDT) Why upgrade? to increase electron temperature and hot ion energy content
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T.D. Akhmetov, A.A. Ivanov, and V.V. Prikhodko
Budker Institute of Nuclear Physics, Novosibirsk, Russia
Magnetic field: centerup to 0.33T
mirror up to 15 T
Mirror ratio:up to 35
Injection duration: 5 ms
NBI power: up to 5 MW
(2-3)1013 cm-3, 200 eV
Hot ions (H+, D+):
up to 5·1013cm-3, <E>≈10 keV
Injection energy Einj = 2225 keV
Hot ion diamagnetism with D0 injection into D plasma (B0= 0.33 T, R = 32)
dWf /dt 0.4 kJ/ms
By the end of injection n 51013cm3andTe 180 eV
for deuterons ei 4 ms
No steady state yet
Te , eV
No steady state yet
Experiment: Wf and Te are not saturated at 5 ms NBI
Proposal: extend injection up to 20 ms to increase Wf and Te
“Optimistic” estimation without limit: max(Wf) 0.4 kJ/ms 20 ms 8 kJ
Zero-order (space-averaged) numerical model includes
The model was adjusted to reproduce Te(t) and Wf(t) for 5 ms injection in the current experiments.
Calculation: ne = 1014 cm3, Pinj = 4 MW
Te , eV
Our simple numerical model gives qualitative agreement with experiments for small and large gas puffing for 5 ms NBI.
Now the model is developed to better account for cold halo plasma and balance of neutral gas in order to proceed to 20 ms regime.
60% already and storage of hot ions will be limited soon by ballooning instability.
Therefore, extension of the injection pulse together with magnetic field increase should allow accumulation of significantly greater hot-ion energy content which in turn should allow for greater Te.
Plasma near the turning point
Estimation from magnetic field depression: max 0.6
Hot ion density estimation near the turning point
< e > = 10keV n51013 cm3
Value of is close to the ballooning instability limitin GDT (crit ~ 0.70.8) and will limit hot ion accumulation and electron heating.
Can we decrease near the turning point keeping the same or even larger Wf ?
Since phi /B2, to increase Wf phidV ,
one hasto increase B or reduce hot-ion pressure near the turning point.
Let us change angle by and calculate the shift of the turning point
For n~51013 cm3, Te~200 eV, Ei ~ 20 keV
ion energy loss
ms for H+ and 4.8 ms for D+
ms for H+
Thus, scattering can be neglected during the whole plasma pulse length.
In simple estimations we will neglect also deceleration of ions on electrons
Hot-ion (neutral beam) distribution function is taken in the form
0 injection energy
0 pitch-angle of injection
Peaking of density and pressure near the turning point relative to the central plane
For << 0~1
In GDT 0=45 p(zs)/p(0) ~ 5.2 1/2 [degree]and for=5: p(zs)/p(0) ~ 2.3
Multiplier for the coil current
It should increase the hot-ion energy content Wf possible for the given magnetic field strength.
Assumptions:=8p/B2 << 1; axial symmetry; paraxial limit, a2/L2<<1
Plasma is stable if variation of potential energy of perturbations is positive
For radially localized perturbations and for sharp boundary plasma
(M.N.Rosenbluth, C.L.Longmire, 1957)
– field line curvature
paraxial limit (fails in the turning region)
small (fails in the turning region)
applicable only for small-scale modes or for p(r)= const and sharp boundary
We will use this criterion as a starting point for estimations of MHD stability
For p(z)=const, |W| is minimal for[Bushkova, Mirnov, Ryutov, 1986]
Magnetic field was originally optimized for p=const
Now pressure is strongly anisotropic due to sloshing ions
unfavorable curvature, r''<0
R=2, turning point for ions injected at 45
Magnetic field should be corrected to reduce unfavorable curvature. It will improve MHD stability.
Minimization of potential energy W with p=p(B) for sloshing ions by shifting several coils reduces W by a factor of 2.7 compared to the present GDT system
Relatively simple adjustment of coils can improve MHD stability
Additional coils from AMBAL-Mwith I=26.3 kA placed optimally to provide the same B(z) profile as in GDT, increase magnetic fieldin the central cell 1.36 times over the length 260<z<260 cm (turning points zt = 190 cm) up to 4.5 kGs. These coils can be fed by available capacitor storage of the GOL device.
Increase of B will allow accumulation of hot-ion population with greater energy content Wf and further increase of Te
“Perspectives of development of magnetic mirror traps in Novosibirsk”
Friday, July 9