Decrease of transport coefficients in the
plasma core after off-axis ECRH switch-off
K.A.Razumova and T-10 team
Usual decrease of Te (normalized) for different radii after ECRH
switch-off.
More rapid decay is seen for r=+11 – +13cm, the surface of
ECRH.
The delayed decrease of Te in the plasma core (inside the
magnetic surface q=1 ) after off-axis ECRH switch-off was
apparently seen last years experiments in the regime:
Bt =2.33T; Ip=180kA; ne=1.5 1019 m-3 ; PECR =400– 420kW.
1.6
#32913
Te-in
TECE (keV)
1.4
Te-out
20ms
r=3cm
1.2
1.0
r=11cm
ECRH
0.8
520
560
600
Time (ms)
640
680
PECR(r) and the
heat diffusivity e
for different times
of the process:
1) OH (eOH);
2) at the end of
ECRH (eEC);
3) during the
transient
improvement of
confinement (e),
calculated by code
COBRA.
Next hypothesis was suggested to explain this
effect:
The enhanced confinement zone appears when dq/dr=0 in the
vicinity of the rational surface. But Te increases during the
local confinement enhancement. This, due to the current
density redistribution, leads to dq/dt increase. So, using
the Ohmic current we always have a positive dq/dr.
ECRH switch-off leads to the current density redistribution in
the ITB region and transient appearance of dq/dr=0, which
means the transport decreases in this region.
4
4
#32913
3
3
q
2
j (MA/m )
t=655
2
2
t=615 t=637
1
1
t=655
0
0.0
0.1
r (m)
0.2
0
0.3
Results of calculation by ASTRA code of j(r) and q(r) for
the end of ECRH and during existence of constant
temperature after ECRH switch-off. Calculations have
been performed with Te(r) and Zeff , taken from the
experimental measurements, and with the neoclassical
resistivity.
The following type of regime was investigated in the last
experimental T-10 campaign.
#35355, Bt =2.31T; Ip =185kA; ne =1.45 1013 cm-3 . Delay of
Te(0) decrease (del=25 ms) always accompanied by ne(r)
peaking.
1400
2.75
Te
2.70
1100
EC
2.60
-3
1200
2.65
2.55
13
nel(0)
2.50
1000
2.45
900
780
800
820
840
t.ms
860
880
900
2.40
920
ne 10 ,cm
Te(0),eV
1300
The density profile always peaks after ECRH switch-off
3.0
860ms
870ms
#35355
2.0
13
ne(r)10 ,cm
-3
2.5
845ms
1.5
1.0
0.5
0.0
-40
-30
-20
-10
0
10
20
30
40
ne(t),a.u.
r,cm
1
ECR
0
760
780
800
820
840
860
t,ms
880
900
920
940
Using Te(r) and ne(r) we can build the plasma pressure
profile, pe (r). The plasma pressure increases inside the
zone of enhancement confinement, which exists after
ECRH switch-off.
1600
1400
#35355
4000
846ms
846ms
820ms
Te(r),eV
1000
874ms
600
400
3000
2500
820ms
2000
1500
860ms
1000
860ms
500
200
0
-30
nTe(r),eV/cm
1200
3
3500
800
874ms
#35355
-20
-10
0
r,cm
10
20
0
-30
-20
-10
0
r,cm
10
20
However, if we increase PECR (4 gyrotrons instead of 2), the
effect of Te(0) conservation has not presented, when only ½
part of PECR was switched-off, but it exists, when the last
part of the power was switched-off.
Te, a.u. normalized
1.0
#35353
0.8
12.5cm
0.6
0cm
0.4
0.2
0.0
9.8cm
4 gyr
EC
2 gyr
700 720 740 760 780 800 820 840 860 880
t,ms
Sawteeth were suppressed in both cases. The difference
was in the ECR power only. For 2 gyrotrons, sawteeth has
been just stabilized and core q was near the unity. In the
case of 4 gyrotrons, q was distinctly higher than unity.
If this is the reason for the confinement difference, then:
1) Under the short operation of 2 from 4 gyrotrons, when j(r)
has not enough time to redistribute and to increase qcore,
the effect of Te(0) delayed decrease has to be exist.
2) We can find another regime, with higher Ip, where this
effect should be seen under 4 gyrotrons heating.
The item 1) was confirmed:
When 2 from 4 gyrotrons were switched-off 40 ms after
start of ECRH, Te(0) continues to increase (del=9ms).
After disconnection of the last two gyrotrons, del was 16
ms, but Te(r) slightly decreased.
1.0
#35352
0 cm
Te ,a.u.
0.8
0.6
12.5 cm
0.4
4gyr
0.2
2 gyr
0.0
620 640 660 680 700 720 740 760 780 800 820
t,ms
When we disconnect 2 from 4 of operating gyrotrons,
then del depends on the duration of their operation:
longer the time of 4 gyrotrons operation, i.e. closer is
the j(r) profile to the stationary one, shorter is del .
However, the del value is not well defined in the cases,
when Te is slightly decreasing.
10
8
del,ms
6
4
2
0
50
100
150
200
250
time till 2 of 4 gyrotrons are switched off, ms
The check-up of the item 2) gave the next result:
To obtain the same delayed decrease of Te(0) under 4
gyrotron heating, we must increase Ip up to the value, when
4 gyrotrons will stabilize sawteeth, and slightly decrease Bt
to deposit the ECR power outside the phase inversion
radius. In the regime: Bt=2.3T; Ip=225kA; ne=1.8 1013cm-3 ,
del=28ms (4 gyrotrons switch-off), but again we see a slow
decrease of Te(0,t) (blue).
1.62
#35510
1.0
1.60
1.58
1.54
Te(11cm)
1.52
0.4
nel
1.50
0.2
1.48
EC, 4gyr
1.46
0.0
760
780
800
820
840
t,ms
860
880
900
-3
0.6
1.56
13
Te(0)
ne10 ,cm
EC,Te , a.u.
0.8
6000
#35510
13
nTe(r)10 ,eV.cm
-3
5000
870ms
4000
850ms
878ms
3000
2000
860ms
1000
0
-30
-20
-10
0
10
20
r,cm
The pressure in the plasma core remains to be constant.
The value of del is very sensitive to the ECR position
in relation to q=1 magnetic surface.
30
35478, 80, 81, 83, 35508, 35509
28
26
st
del, st; ms
24
22
20
del
18
16
14
12
10
8
6
4
-0.6
-0.4
-0.2
0.0
0.2
0.4
Dgor,cm
0.6
0.8
1.0
1.2
We tried to receive the effect in the regime with 4 gyrotons
and with the low current Ip=180kA using preliminary on-axis
heating (the gyrotron with F=130 GHz).
On-axis heating prohibited the sawteeth stabilization, but
we may receive the desirable profile using the high power
off-axis heating by 4 gyrotrons. It turned out, that even ¼ of
the on-axis gyrotron power is too much for the sawteeth
stabilization by 4 gyrotrons with F=140 GHz. Nevertheless,
we succeeded to have del=15ms under not totally
suppressed sawteeth.
This makes clear that the fact of sawteeth stabilization itself
does not important for del existence. It is important to have
well-aligned q(r) profile.
off axis
ECRH
on axis ECRH
500
600
700
800
900
t,ms
Scheme of experiment with off-axis heating (P=0.9MW) of the
preliminary on-axis heated plasma (P=0.5 – 0.12 MW)
#35677; B=2.33T; I=185kA; n=1.4; 4gyr.140GHz against a
background of 1gyr. 130GHz with diminished power
1600
859
#35677
1200
866.6
Te ,eV
1000
863.4
846.6
800
600
1
Te ,a.u., normalized
1400
TECE(0)
TECE(11cm)
TECE(-8cm)
400
854
200
0
0
-20
-10
r,cm
0
10
840
845
850
855
860
865
870
t,ms
In spite of sawteeth existence, one can see del=15 – 16ms
(red).
Why in some experiments Te is constant during del and
sometimes it slowly decreases?
Let us compare two shots with the same initial plasma
parameters (Bt=2.33T; Ip=185kA; ne=1.4 1013cm-3), but with
different ECRH power : 2 and 4 gyrotrons.
1800
35355 __ and 35358---
1400
860ms
0.75
1200
870ms
1000
Te, eV
35358, 4gyr
0.80
849.5ms
800
849ms
875ms
600
882ms
400
IECE(0),au
1600
0.85
0.70
0.65
0.60
0.55
200
0.50
0
0.45
-30
-20
-10
r
0
10
35355, 2gyr
20
switch off
820 830 840 850 860 870 880 890 900
t,ms
The picture looks like the superfluous energy has to be lost
and then the core confinement improves.
0.46
Comparison of regimes
with heating by the total
power of 2 gyrotrons
(black)
and one-half of this power
(red)
#35762
0.44
TECE,a.u.
0.42
0.40
#35763
0.38
0.36
ECRH
end
0.34
0.32
0.30
620
630
640
650
t,ms
660
670
680
690
After 4 gyrotrons switch-off, Te(0) decreases till its value
reach the same value, which was under 2 gyrotron
heating and then remains constant during 15 ms.
In the case of 2 gyrotrons, Te(0) is constant during 25
ms. Note, that not only the Te(0) value, but also Te(r)
profiles are the same in both cases during the period of
Te(0,t)=const.
Experiment shows: more difference between preliminary
ECRH power and the optimal one – more steep Te (0,t)
decrease after the power switch-off.
CONCLUSIONS:
q(r) profile near the rational magnetic surface play the
especially important role in the ITB formation process
and it determines the ITB quality.
The range of dq/dr (near dq/dr=0) exists, in which the
transport is minimized.
The range of q deviation from the rational value exists,
inside which the barrier formation is effective.
Using these rules we can stimulate the ITB formation
in different conditions (at least electron ITB).
2.4
2.0
#33041
#33041
1.5
2.2
Te (r=7cm)
597ms, before dI/dt
Te (keV)
Te (keV)
645ms, after dI/dt
1.0
2.0
Ip2=250kA
OH
0.5
1.8
Ip1=180kA
600
650
700
750
Time (ms)
Profiles of electron
temperature Te before and
after the current ramp-up.
0.0
-30
-20
-10
0
10
20
r (cm)
Profiles of electron
temperature Te before
and after the current
ramp-up.
#29615
650 ms
3
Te (keV)
630 ms
615 ms
2
1
600 ms
0
-20
-10
0
10
r (cm)
20
30
Current ramp up (from 150
to 140kA) leads both to
the plasma core inward
shift and Te core increase
Calculated equilibrium contours of
magnetic surfaces before and after the
current ramp-up. After ramp-up the
plasma column is shifted inward.