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RFX – mod:
what does the present device allow to do?
R. Piovan
R. Piovan “RFX-mod: what does ...”
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RFX 2009 Programme Workshop
Padova, 20-22 Jan 2009
Outline
 RFX design
 Main machine limits
 What has been done up to now
 What can be done?
 Open issues
 Conclusions
R. Piovan “RFX-mod: what does ...”
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RFX 2009 Programme Workshop
Padova, 20-22 Jan 2009
RFX design
Major radius R
2m
Minor radius a
0.46 m
Flux swing (from Im to 0)
15 V s
Toroidal field
0.6 T
Loop voltage
700 V
First wall
graphite tiles
Shell time constant
Target
R. Piovan “RFX-mod: what does ...”
(old 0.7)
70 ms (old 450 ms)
Plasma current 2 MA
Flat top 250 ms @ 18 V
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RFX 2009 Programme Workshop
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Winding performances and limits
Magnetizing
15 Vs with 50 kA
Splitted into 4 sections
Equilibrium
5.2 kA (average) with 2 MA plasma current
Splitted into 8 sections
Toroidal
0.7 T with 18.3 kA
Splitted into 12 sectors
Winding
Max current
[kA]
Max I2t per
shot [MA2s]
Magnetizing
50
3.500
Equilibrium
6.25
20
Toroidal
18.3
300
R. Piovan “RFX-mod: what does ...”
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Note
4°C @ 0.5 s
RFX 2009 Programme Workshop
Padova, 20-22 Jan 2009
Machine limits: first wall
adiabatic tiles uniform temperature vs pulse duration
at different thermal power load
1400
T ini = 20°C
Temperature [°C]
1200
1000
T for P=5 [MW/m2]
T for P=10 [MW/m2]
800
T for P=40 [MW/m2]
600
400
200
0
0,10
0,15
0,20
0,25
0,30
0,35
0,40
0,45
0,50
pulse flat top duration [s]
Tmax = 200 °C
Limit in the max overtemperature is related to the maximum
stress in the probes between tiles and vessel
R. Piovan “RFX-mod: what does ...”
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RFX 2009 Programme Workshop
Padova, 20-22 Jan 2009
Machine limits: first wall
1.6 MA
# 24533
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RFX 2009 Programme Workshop
Padova, 20-22 Jan 2009
Present performances
Vacuum shot with 50 kA magnetizing current
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15 Vs
RFX 2009 Programme Workshop
Padova, 20-22 Jan 2009
Present performances
• Toroidal circuit tested up to 12 kA (0.46 T).
• Commissioning to 16 kA in the next future.
• Very fast current inversion.
R. Piovan “RFX-mod: what does ...”
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RFX 2009 Programme Workshop
Padova, 20-22 Jan 2009
Plasma current & volt seconds
Flux consumption & rise time (#23800-#25672)
Rt = 0.584 Rt = 0.420 
Rt = 0.467  Rt = 1.011 
0.1
9
0.08
8
Rise time [s]
Flux consumption [Vs]
10
7
6
0.04
0.02
5
4
800
0.06
0
800
1000 1200 1400 1600 1800
Iplasma [kA]
R. Piovan “RFX-mod: what does ...”
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1000 1200 1400 1600 1800
Iplasma [kA]
RFX 2009 Programme Workshop
Padova, 20-22 Jan 2009
“Plasma” flux consumption
d Wm
V I p  V I 
 P
dt
V I p
d Wm
dt
V I
P
(theta_w = 1.4, constant)
R. Piovan “RFX-mod: what does ...”
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RFX 2009 Programme Workshop
Padova, 20-22 Jan 2009
Plasma current & volt seconds
Plasma side
Very simple plasma with truncated Bessel function model
a
rw
plasma minor radius
internal vessel minor radius
Further hypothesis: plasma current rise with reversed toroidal
field (RFP) and constant theta
Values assumed in the model:
a = 0.42 m
rw = 0.459 m
theta_w = 1.4
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RFX 2009 Programme Workshop
Padova, 20-22 Jan 2009
“Plasma” flux consumption
V I p  V I 
o a d I p
d
V 

dt
2 dt
Wm 
 o R  J 02
2
d Wm
 P
dt
I 
1 J0
 rw 
 2 1
 ln  
J

J
a
1
 1
a J0
Ip
R J1
 2
 Ip


2
 J 02
1 J0
 rw  1 a 1 J 0 
 V dt  o R  J12  1   J1  ln  a   2 R2  J1  I p 
 V dt  Leq I p 
Leq _ T 
o R
4
R. Piovan “RFX-mod: what does ...”
P
 Ip d t
 0.63 H
P
 Ip d t
Leq  3 H
if uniform current distribution is assumed
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RFX 2009 Programme Workshop
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Fluxes in the machine
IF
IM
B
Ip
B
B
YST YL+ YR
Lstay Leq KR
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RFX 2009 Programme Workshop
Padova, 20-22 Jan 2009
Simplified circuit
Iconv
IR
IM
IF
Before the converter start:
IF = 10.4 Ip / 2 [kA] (Ip in MA)
IMF =
IF + IR
=
10.4 Ip / 2 + VR/RT
at the plasma current peak significant
magnetizing current and transformer residual flux
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RFX 2009 Programme Workshop
Padova, 20-22 Jan 2009
Stray inductance
Varying the transfer resistor
Fixed magnetizing current: 40.4 kA
12.1 Vs
RT
tmax
Ip
IMres
Yrw
YST
YL
YR
ohm
ms
MA
kA
Wb
Wb
Wb
Wb
25091
0.584
57
1.482
10.1
6.92
2.17
4.45
2.47
25326
0.467
75
1.420
9.5
7.21
2.06
4.26
2.95
25329
0.42
82
1.373
9.5
7.28
1.99
4.12
3.16
Shot
YST = 15 (IMo – IMres)/50 - Yrw
(currents in kA)
YL + YR = Yrw
From experiments:
Lstray = YST/Ip ~ 1.4 H
* In case of no amper-turn compensation Lstray ~ 4 H
R. Piovan “RFX-mod: what does ...”
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RFX 2009 Programme Workshop
Padova, 20-22 Jan 2009
“Resistive” flux consumption
Varying the magnetizing current - Fixed transfer resistor: 0.42 ohm
IM0
tmax
Ip
IMres
Yrw
YST
YL
YR
kA
ms
MA
kA
Wb
Wb
Wb
Wb
25360
38.2
75
1.294
9.3
6.69
1.98
3.88
2.81
25330
43.2
76
1.482
10.8
7.65
2.07
4.45
3.20
25334
46.1
70
1.606
11.8
8.06
2.23
4.82
3.24
25366
48.3
76
1.691
11.9
8.67
2.25
5.07
3.60
25367
50.0
78
1.770
12.0
8.98
2.42
5.31
3.67
Shot
From experiments:
YR scale about with Ip and depends on RT
YR ~ KR Ip
If Ip in MA:
KR ~ 2.1 @ RT = 0.42 
R. Piovan “RFX-mod: what does ...”
KR ~ 1.6 @ RT = 0.58 
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RFX 2009 Programme Workshop
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What can be done?
DY = YM0 - YMF = YST + YL + YR
DY = 6 Ip
DY = 6.5 Ip
(Ip in MA) @ RT = 0.58
(Ip in MA) @ RT = 0.42
IF = 10.4 Ip/2 [kA] (Ip in MA)
IR = 50 Vp-p / RT
RT
(Vp-p is the plasma loop
voltage during the flat top)
Iconv
IR
R. Piovan “RFX-mod: what does ...”
IM
IF
IMF = IF + IR - IconvF
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RFX 2009 Programme Workshop
Padova, 20-22 Jan 2009
What we have done
Case 1 – Rise with RT and flat-top with converters
RT = 0.42 
Vp-p = 20 V
Iconv = 15 kA
VR = 50 Vp-p = 1000 V
Ip = 1.77 MA
Flat-top:
R. Piovan “RFX-mod: what does ...”
20 V & 220 ms
30 V & 150 ms
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RFX 2009 Programme Workshop
Padova, 20-22 Jan 2009
What can be done?
Case 2 – Rise with RT and flat-top with converters
RT = 0.58 
Vp-p = 20 V
Iconv = 15 kA
VR = 50 Vp-p = 1000 V
Ip = 1.92 MA
Flat-top:
R. Piovan “RFX-mod: what does ...”
20 V & 220 ms
30 V & 150 ms
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RFX 2009 Programme Workshop
Padova, 20-22 Jan 2009
What can be done?
Case 3 – Flat top converters with series configuration
(8 kA & 60 V voltage loop) used to rise plasma current
(YR probably underestimated)
RT = 0.58 
Vp-p = 60 V
Iconv = 8 kA
VR = 50 Vp-p = 3000 V
Ip = 2.1 MA
Flat-top:
R. Piovan “RFX-mod: what does ...”
20 V & 50 ms
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RFX 2009 Programme Workshop
Padova, 20-22 Jan 2009
Open issues
Can the plasma current further increased with the
present machine?
Decreasing the resistive flux consumption
YR ~ 3.2 Vs @ 2 MA
With different setting-up from the constant 
(matched mode)
YL = ~ 6 Vs @ 2 MA and w=1.4
DIp = 0.17 MA
1Vs
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RFX 2009 Programme Workshop
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Conclusions
 RFX performances agree completely with design assumptions
done almost 30 years ago
 2 MA plasma current, according to the initial specification, can
be reached
 Volt-second needed for plasma current rise and sustainment
experimentally derived from experimental data
 Further
current
increasing
saving
volt-second
with
the
optimization of plasma setting-up
R. Piovan “RFX-mod: what does ...”
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RFX 2009 Programme Workshop
Padova, 20-22 Jan 2009
What can be done?
Case 4 – Doubling the flat top converters with series
configuration (15 kA & 60 V voltage loop) used to rise
plasma current (YR probably underestimated)
This case requires power supply improvements and
other verifications on peak power from HV grid
RT = 0.58 
Vp-p = 60 V
Iconv = 15 kA
VR = 50 Vp-p = 3000 V
Ip = 2.38 MA
Flat-top:
R. Piovan “RFX-mod: what does ...”
20 V & 50 ms
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RFX 2009 Programme Workshop
Padova, 20-22 Jan 2009
RUN 1401
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RFX 2009 Programme Workshop
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Shots with higher
currents
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RFX 2009 Programme Workshop
Padova, 20-22 Jan 2009
M aximum (locking)
25.0
Uniform
20.0
15.0
10.0
5.0
8.0
Pulse number
RFX - 1 MA campaign
7.0
6.0
Energy [MJ]
0
13243
13244
13245
13246
13248
13249
13250
13251
13252
13254
13255
13256
13257
13258
13260
13261
Power density [MW/m 2]
30.0
E_lock.
E_unif.
5.0
4.0
3.0
2.0
0
13243
13244
13245
13246
13248
13249
13250
13251
13252
13254
13255
13256
13257
13258
13260
13261
1.0
Pulse number
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RFX 2009 Programme Workshop
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Temperature [°C]
180
160
140
120
100
80
60
40
20
0
4 /06 /'99
1 6/1 2/'9 9
0
1
2
3
4
5
Time [h]
6
7
8
71.5
Kcon t=2 50 W/m^2 K
Kcon t=7 0 W/m^2 K
Adiabatic co ndition
IR Camera measuremen t
RFX - 1 MA campaign
Temperature [°C]
71
70.5
70
69.5
69
68.5
R. Piovan “RFX-mod: what does ...”
0
27
50
100
150
200
RFX 2009 Programme Workshop
Time [sec]
Padova, 20-22 Jan 2009
RFX initial scientific objectives
1.
Extent the investigations to higher currents to study the confinement
properties of RFP type so that comparison with properties of large
stellarators and tokamaks can be made
2.
To study the temperature, beta and confinement time scale with
minor radius and current over an extended range
3.
To study the setting-up of stable RFP configurations to minimize
energy losses and optimize the configuration. This includes studying
the effects of density control using gas injection, the first wall
condition and impurities including the use of limiters, the importance
of field error, the role of wall stabilization and, at a later stage, of
operating without a shell
4.
To study the sustainment phase and investigate the density/curren
behavior
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RFX 2009 Programme Workshop
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