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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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slide1

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
Outline
  • Current parameters of Gas Dynamic Trap (GDT)
  • Why upgrade?
    • to increase electron temperature and hot ion energy content
    • to optimize magnetic field
    • to improve MHD stability
  • Possibilities
    • proceed from 5 to 20 ms neutral beam injection
    • adjust the present coil system
    • add coils to enhance magnetic field from 3.3 to 4.5 kGs
gdt layout
GDT layout

Length:7 m

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

Warm plasma:

(2-3)1013 cm-3, 200 eV

Hot ions (H+, D+):

up to 5·1013cm-3, <E>≈10 keV

typical experimental scenario
Typical experimental scenario
  • Cusp and expander are not used
  • MHD stability is provided by a biased limiter
  • Gas puffing maintains warm plasma density

NB injection

Gas puffing

Plasma source

0.5

3.5

8.5

t, ms

incident nbi power
Incident NBI power

Injection energy Einj = 2225 keV

d 0 injection into d plasma
D0 injection into D plasma

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 51013cm3andTe  180 eV

 for deuterons ei  4 ms

No steady state yet

slide7

Electron temperature at the axis

Te , eV

No steady state yet

t, ms

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

slide8

Search for steady state

Zero-order (space-averaged) numerical model includes

  • kinetic equation for hot ion distribution function fhi(e)
  • particle balance equations for warm ions and electrons nwi, ne
  • energy balance equations for electrons and warm ions Twi, Te
  • NB injection, gas puffing, and axial gas-dynamic plasma losses

The model was adjusted to reproduce Te(t) and Wf(t) for 5 ms injection in the current experiments.

slide9

Numerical simulation for 5 ms injection

Calculation: ne = 1014 cm3, Pinj = 4 MW

Experiment

Te , eV

t, ms

slide10

Increase of injection pulse length

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.

outline1
Outline
  • Current parameters of Gas Dynamic Trap (GDT)
  • Why upgrade?
    • to increase electron temperature and hot ion energy content
    • to optimize magnetic field
    • to improve MHD stability
  • Possibilities
    • proceed from 5 to 20 ms neutral beam injection
    • adjust the present coil system
    • add coils to enhance magnetic field from 3.3 to 4.5 kGs
slide12

Plasma  near the turning point

Estimation from magnetic field depression: max  0.6

Hot ion density estimation near the turning point

< e > = 10keV n51013 cm3

Value of  is close to the ballooning instability limitin GDT (crit ~ 0.70.8) 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.

slide13

Length of hot-ion turning region

Let us change angle by and calculate the shift of the turning point

  • Hot-ion pressure near the turning region can be reduced by increasing the volume of this region, i.e. its length.
  • Either angular spread of hot-ion D.F. must be increased or magnetic field gradient must be reduced near the turning point.
  • Angular spread cannot be increased much,
  • Magnetic field gradient dB/dz(zs) can be increased by correction of currents in the coils or their positions near the turning point.
slide14

Hot-ion population in GDT

For n~51013 cm3, Te~200 eV, Ei ~ 20 keV

ion energy loss

ms for H+ and 4.8 ms for D+

ion scattering

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

 angular width

slide15

Hot-ion density and pressure distributions

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

slide16

Reduction of pressure in the turning region

Multiplier for the coil current

1.7

1.22

0.8

0.48

b(z)

b(z)

corrected

z, cm

z, cm

turning point

now

p(z)

corrected

r/rB2

z, cm

slide17

Effect of coil current correction

  • limit in the hot-ion turning region can be significantly improved by reducing the peak plasma pressure ~1.5 times using correction of the coil currents.

It should increase the hot-ion energy content Wf possible for the given magnetic field strength.

outline2
Outline
  • Current parameters of Gas Dynamic Trap (GDT)
  • Why upgrade?
    • to increase electron temperature and hot ion energy content
    • to optimize magnetic field
    • to improve MHD stability
  • Possibilities
    • proceed from 5 to 20 ms neutral beam injection
    • adjust the present coil system
    • add coils to enhance magnetic field from 3.3 to 4.5 kGs
mhd flute stability criterion
MHD flute stability criterion

Assumptions:=8p/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

Advantages:

 simplicity, clearness

Disadvantages:

 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

slide20

Optimal B(z) profile for GDT with p(z)=const

For p(z)=const, |W| is minimal for[Bushkova, Mirnov, Ryutov, 1986]

r, cm

b(z)10

z, cm

Magnetic field was originally optimized for p=const

slide21

More realistic p(z) profile

Now pressure is strongly anisotropic due to sloshing ions

p

pr\'\'/rB2

r\'\'/rB2

unfavorable curvature, r\'\'<0

z,cm

R=2, turning point for ions injected at 45

Magnetic field should be corrected to reduce unfavorable curvature. It will improve MHD stability.

slide22

Corrected coil positions in GDT

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

slide23

Corrected coil positions in GDT

pr\'\'/rB2

pGDT

corrected

GDT

z,cm

Relatively simple adjustment of coils can improve MHD stability

outline3
Outline
  • Current parameters of Gas Dynamic Trap (GDT)
  • Why upgrade?
    • to increase electron temperature and hot ion energy content
    • to optimize magnetic field
    • to improve MHD stability
  • Possibilities
    • proceed from 5 to 20 ms neutral beam injection
    • adjust the present coil system
    • add coils to enhance magnetic field from 3.3 to 4.5 kGs
increase of magnetic field
Increase of magnetic field

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

conclusions
Conclusions
  • 20 ms NBI together with magnetic field increase should provide steady state with significantly enhanced Wfast and Te
  • Proposed experiment with lengthening of hot-ion turning region may give additional information about  limit and increase Te
  • Adjustment of the present coil system may significantly improve MHD stability
  • Increase of central cell magnetic field by a factor of 1.36 is possible with available additional coils and capacitor storage

A.A.Ivanov

“Perspectives of development of magnetic mirror traps in Novosibirsk”

Friday, July 9

12:10

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