plasma dynamics lab hibp e ~ 0 v/m in locked discharges average potential ~ 580 v ~ 500-600v less...
TRANSCRIPT
Plasma Dynamics Lab
HIBP
E ~ 0 V/m in Locked Discharges
• Average potential ~ 580 V
~ 500-600V less than in standard rotating plasmas
• Drop in potential possibly due to degradation of ion confinement, reduction in mode velocity or changes in bulk fluid rotation
• Scatter ~ ±100 V; reduced scatter likely due to uniformity of mode velocity ~ 0 km/s, variations in density remain
• Potential profile relatively flat, Er small/zero
Phi vs Radial Sweep (Locked discharge)
y = -0.0189x + 0.5178
0
0.2
0.4
0.6
0.8
1
1.2
1.4
-5 -4 -3 -2 -1 0
Radial Sweep (kV)
Po
ten
tial (
kV
)
0
0.1
0.2
0.3
0.4
0.5
0.6
0.7
0.8
0.3 0.4 0.5 0.6 0.7r/a
Po
ten
tial
(kV
)
Biasing experiment
• 2 electrodes, inserted 8-10cm
• Negative biasing for 10ms with respect to MST wall
• Discharges lock then reaccelerate when biasing is turned off
• Density rises dramatically
• Unlike a standard locked discharge, sawteeth do not cease
E ~ 0 V in Biased Discharges
• The potential is positive, but ~ 200-250V lower than in a standard locked discharge
• The potential profile is flat over the region sampled
• The lower possibly due to:
– higher ne (~20-40%)
– better confinement of e-
0.1
0.2
0.3
0.4
0.5
0.6
-5 -4 -3 -2 -1 0
Radial Sweep (kV)
Po
ten
tia
l (k
V)
0
0.1
0.2
0.3
0.4
0.5
0.6
0 0.1 0.2 0.3 0.4 0.5 0.6 0.7
r/a
Po
ten
tial (
kV)
HIBP Measurements Facilitate Experimental Investigation of
Ion Radial Force Balance• Simplified equilibrium radial force balance for the ion species is given by:
• Quantities:– Er – HIBP – radial electric field– ne – FIR – electron density profile– v – IDS – ion toroidal and poloidal flow velocities (and m=1,n=6 mode
velocity)– P – Rutherford, Thomson scattering – pressure gradient inferred
from ion, electron temp.– B – MSTFit – reconstructed equilibrium field profile– Z – Assumed = 2
rr BvPenZ
E
1 Assumptions
-in equilibrium-incompressible plasma flows-isotropic pressure gradient
Radial Force Balance inLow Current Standard Discharges
• HIBP measured Er is compared to the total computed, and individual RHS terms
• Agreement between measured and computed Er in the range of r = 16-27cm
• Contribution from v x B term 3-6x greater than pressure gradient term in core, 2x greater toward edge
rr BvPenZ
E
1
-2000
-1000
0
1000
2000
3000
4000
0 10 20 30 40
Minor Radius (cm)
V/m
-v x B
1/qn[grad(P)]
-(v x B)+1/qn[grad(P)]
HIBP
The Computed Electric Field Incorporates Mid-Sawtooth Cycle Measured Quantities
• Measured ion and electron temperature profiles are similar in low current discharges
• For r < 23 cm, n ~ 0
• The ratio of toroidal to poloidal flow velocities is ~ 5-7.
• All quantities are from low current discharges, mid-cycle
100
150
200
250
0 20 40Minor Radius (cm)
Tem
per
atu
re (
eV)
T_i T_e
Limited Measurements Contribute to Uncertainty in Er
• Ion flow velocities
– Chord localized (15 cm) rather than profile measurements
– Past experimental measurements indicate that the flow velocity decreases toward the plasma edge (v x B in edge likely smaller than computed)
– A 20% change in the flow velocity is enough to est. agreement between measured and computed Er
• Pressure Gradient
– The uncertainty in the pressure gradient is < 3%
– The uncertainty in the ion-temperature measurements is ~ 20-30%
• Due to lack of spatial resolution
• Uncertainty in Ti translates to an uncertainty of ~ 500 V/m at r~25-33cm
• Measured Er
– Uncertainty in the measurement ~ 700 V/m
Radial Force Balance in Low Current Locked Discharges
• Pressure profiles from standard discharges
• Ion temperature is assumed to be close to the impurity temperature measured mid-cycle; this is based on the similarity of measurements in a standard discharge near a sawtooth crash
• Calculated Er is negative• The ratio of toroidal to poloidal
flow velocities now ~ 1; decrease of the toroidal flow velocity from standard to locked discharges dramatically reduces computed Er
• The use of Timpurity results in a pressure term that is 30% lower than in the standard discharge
-2000
-1500
-1000
-500
0
500
1000
1500
0 10 20 30 40
Major Radius
E_r
(V
/m)
-(vxB)
T_i/n[grad(n)]
-(vxB)+1/qn[grad(P)]
Radial Force Balance in Low Current Biased Discharges
• Suppression of electrostatic fluctuation induced transport has been observed with negative biasing
• The HIBP measurement of Er, while not shown, is close to zero over the range illustrated
• The electron temperature and density profiles were measured in the biased discharges. The ne profile is hollow and the gradient positive in the region investigated.
• The toroidal flow decreases and the ratio of toroidal to poloidal flow ~ 2
-600
-400
-200
0
200
400
600
800
0 5 10 15 20 25 30 35
Minor Radius (cm)
(V/m
)
-(vxB)
1/qn_e[grad(P)]
-(vxB)+1/qn_e[grad(P)]
0
50
100
150
200
250
300
0 10 20 30 40
Minor Radius (cm)
T (
eV)
Low Current Force Balance Summary
• The computed electric field tends to agree with the measured electric field toward the core of the plasma, with greater deviation toward edge
• The v x B term increases with radius and is the dominant term in both standard and biased discharges
• The v x B term is reduced in both locked and biased discharges due to reduction in flow velocities
• The profile of the biased discharge pressure term is partly due to the hollow density profile and positive density gradient (transport barrier)
• Computation of the radial electric field would improve with
– Profile measurements of flow velocities
– Profile measurements of the ion temperature, mid-cycle in locked and biased discharges
Experimental investigation of radial force balance in high current discharges
Experimental Investigation of Radial Force Balance in High Current Discharges
• The radial electric field is computed during two intervals (after (a) and before (b))
• The HIBP measured Er is shown for the same two time intervals
• Both calculated and measured show and increase in Er over the sawtooth cycle
• The n=6 phase velocity tends to be lower in the time window after the crash than the window before. The result, is a smaller contribution from v x B.
0 20 30 40 50
0
-1000
-2000
1000
2000
3000
-3000
2000
0
-2000
-4000
10-6000
0 20 30 40 5010
(a) (b)
Minor Radius (cm) Minor Radius (cm)
0
500
1000
1500
2000
2500
3000
0 10 20 30 40
Minor radius (cm)
V/m After
Before
Particle Drifts in MST Due to Radial Electric Field and Pressure Gradients
• Two drifts are considered: ExB and diamagnetic drift; Er and P are both measured:
• Comparisons to IDS measurements are made (near r=20 cm) : vExB + vP ~ 8.6 km/s; vIDS =
-4.5 km/s (sign error may exist)
: vExB + vP ~ 8.6 km/s; vIDS = 22.5 km/s
– The ExB drift dominates in the core, the diamagnetic toward the edge
22 qnB
PxB
B
BxEv
Perpendicular Drifts
0
2
4
6
8
10
12
14
16
0 10 20 30 40
Minor Radius (cm)
Vel
oci
ty (
km/s
)
V_ExB
V_Diamagnetic
V_Total
Radial Electric Field Predicted by Stochastic Field Theory Does Not Match
Measurements
• Prediction from stochastic field theory (Harvey) is compared with measured Er and ion radial force balance
• This theory examines the relation between particle and heat flux, and the ambipolar electric field
• Ambipolar field is 1-2 orders smaller than either of the others
• Plasma rotation is not taken into account in the theory/eqn.
• Agree only when one considers that measured and predicted fields are both positive.
0 20 30 40 50
0
-1000
-2000
1000
2000
3000
-300010
Minor Radius (cm)
)2
11(
r
T
Tr
n
ne
TE e
e
e
e
e
• Toroidal and Poloidal Flow Velocity Measurements
• Due to higher temperatures C-V emission moves outward to 30 < r < 40 cm. Thus, the measurement region is no longer coincident with the HIBP measurement of Er
• The global m=1,n=6 mode phase velocity is used instead
• Discharge Differences
•HIBP Er measurements are carried out in 383kA discharges, ion pressure gradients and phase velocities from 373 kA discharges
• Time Windows
•1.5 -2.5 ms after a sawtooth crash and 2-3 ms before a crash
0.7
0.9
1.1
1.3
-5 -4 -3 -2 -1 0
Radial Sweep (kV )
Po
ten
tial
(kV
)
n~1.0
n~0.5
• An m=0 perturbation applied by horizontal and vertical field error correction coils at the gap cause the n=6 mode to lock
• Sawteeth cease and local large amplitude density fluctuations decrease
• Confinement is poorer than in rotating plasmas
• The data are from two discharges, one realizationeach discharge (Ip ~ 275 kA)
• The upper trace:
•ne ~ 0.5 x 1013 cm-3
• vn=6 ~ 32.5 km/s
• The lower trace:
•ne ~ 1.0 x 1013 cm-3
•vn=6 ~ 28 km/s
Effect of Plasma Density and Rotation on Potential Measurements
Locked Discharges