rfx – mod: what does the present device allow to do? r. piovan

R. Piovan “RFX-mod: what does ...” RFX 2009 Programme WorkshopPadova, 20-22 Jan 2009

1

RFX – mod:what does the present device allow to do?

R. Piovan


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Outline

RFX design

Main machine limits

What has been done up to now

What can be done?

Open issues

Conclusions


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RFX design

Major radius R 2 m

Minor radius a 0.46 m

Flux swing (from Im to 0) 15 V s

Toroidal field 0.6 T (old 0.7)

Loop voltage 700 V

First wall graphite tiles

Shell time constant 70 ms (old 450 ms)

Target Plasma current 2 MAFlat top 250 ms @ 18 V


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Winding performances and limits

Winding Max current[kA]

Max I2t per shot [MA2s]

Note

Magnetizing 50 3.500

Equilibrium 6.25 20 4°C @ 0.5 s

Toroidal 18.3 300

Magnetizing 15 Vs with 50 kASplitted into 4 sections

Equilibrium 5.2 kA (average) with 2 MA plasma currentSplitted into 8 sections

Toroidal 0.7 T with 18.3 kASplitted into 12 sectors


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adiabatic tiles uniform temperature vs pulse durationat different thermal power load

0

200

400

600

800

1000

1200

1400

0,10 0,15 0,20 0,25 0,30 0,35 0,40 0,45 0,50

pulse flat top duration [s]

Tem

per

atu

re

[°C

]

T for P=5 [MW/m2]

T for P=10 [MW/m2]

T for P=40 [MW/m2]

T ini = 20°C

Limit in the max overtemperature is related to the maximum stress in the probes between tiles and vessel

Tmax = 200 °C

Machine limits: first wall


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1.6 MA

# 24533

Machine limits: first wall


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Present performances

Vacuum shot with 50 kA magnetizing current 15 Vs


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Present performances

• Toroidal circuit tested up to 12 kA (0.46 T).• Commissioning to 16 kA in the next future.• Very fast current inversion.


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Flux consumption & rise time (#23800-#25672)Flux consumption & rise time (#23800-#25672)

800 1000 1200 1400 1600 18000

0.02

0.04

0.06

0.08

0.1

Iplasma [kA]

Ris

e ti

me

[s]

800 1000 1200 1400 1600 18004

5

6

7

8

9

10

Iplasma [kA]

Flux

con

sum

ptio

n [V

s]

Rt = 0.584 Rt = 0.420

Rt = 0.467 Rt = 1.011

Plasma current & volt seconds


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Ptd

WdIVIV m

p

(theta_w = 1.4, constant)

pIV IVtd

Wd m

P

“Plasma” flux consumption


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Plasma current & volt seconds

Very simple plasma with truncated Bessel function model

a plasma minor radiusrw 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 mrw = 0.459 mtheta_w = 1.4

Plasma side


12

td

Ida

td

dV po

2

pIJ

J

R

aI

1

0

2

1

02

1

20 ln

11

2 pwo

m Ia

r

J

J

J

JRW

td

I

PI

J

J

R

a

a

r

J

J

J

JRdtV

pp

wo

1

02

2

1

02

1

20 1

2

1ln

11

tdI

PILdtV

ppeq HLeq 3

Ptd

WdIVIV m

p

assumedisondistributicurrentuniformifHR

L oTeq

63.04_

“Plasma” flux consumption


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Fluxes in the machine

B

IM

Ip

IF

LeqLstay

ST L+ R

BB

KR


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Simplified circuit

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

IM IFIRIconv


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Varying the transfer resistorFixed magnetizing current: 40.4 kA 12.1 Vs

ST = 15 (IMo – IMres)/50 - rw (currents in kA)

L + R = rw

Shot RT tmax Ip IMres rw ST L R

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

Lstray = ST/Ip ~ 1.4 H

From experiments:

Stray inductance

* In case of no amper-turn compensation Lstray ~ 4 H


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Varying the magnetizing current - Fixed transfer resistor: 0.42 ohm

KR ~ 2.1 @ RT = 0.42

From experiments:R scale about with Ip and depends on RT

R ~ KR Ip

If Ip in MA:

“Resistive” flux consumption

Shot IM0 tmax Ip IMres rw ST L R

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

KR ~ 1.6 @ RT = 0.58


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What can be done?

= M0 - MF = ST + L + R

= 6 Ip (Ip in MA) @ RT = 0.58

= 6.5 Ip (Ip in MA) @ RT = 0.42

IM IFIRIconv

IF = 10.4 Ip/2 [kA] (Ip in MA)

RTIR = 50 Vp-p / RT

(Vp-p is the plasma loop voltage during the flat top)

IMF = IF + IR - IconvF


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What we have done

Case 1 – Rise with RT and flat-top with converters

RT = 0.42 Vp-p = 20 V VR = 50 Vp-p = 1000 VIconv = 15 kA

Ip = 1.77 MA

Flat-top: 20 V & 220 ms 30 V & 150 ms


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What can be done?

Case 2 – Rise with RT and flat-top with converters


Ip = 1.92 MA

Flat-top: 20 V & 220 ms 30 V & 150 ms


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What can be done?

Case 3 – Flat top converters with series configuration (8 kA & 60 V voltage loop) used to rise plasma current (R probably underestimated)


Ip = 2.1 MA

Flat-top: 20 V & 50 ms


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Open issues

Can the plasma current further increased with the present machine?

Decreasing the resistive flux consumptionR ~ 3.2 Vs @ 2 MA

With different setting-up from the constant (matched mode)L = ~ 6 Vs @ 2 MA and w=1.4

1 V s Ip = 0.17 MA


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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


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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 (R probably underestimated)This case requires power supply improvements and other verifications on peak power from HV grid


Ip = 2.38 MA

Flat-top: 20 V & 50 ms


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RUN 1401


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Shots with higher currents


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0

5.0

10.0

15.0

20.0

25.0

30.0

13

24

31

32

44

13

24

51

32

46

13

24

81

32

49

13

25

01

32

51

13

25

21

32

54

13

25

51

32

56

13

25

71

32

58

13

26

01

32

61

Maximum (locking)

Uniform

Po

wer

den

sity

[M

W/m

2 ]

Pulse number

0

1.0

2.0

3.0

4.0

5.0

6.0

7.0

8.0

13

24

31

32

44

13

24

51

32

46

13

24

81

32

49

13

25

01

32

51

13

25

21

32

54

13

25

51

32

56

13

25

71

32

58

13

26

01

32

61

E_lock.E_unif.

En

erg

y [

MJ]

Pulse number

RFX - 1 MA campaign


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68.5

69

69.5

70

70.5

71

71.5

0 50 100 150 200

Kcont=250 W/m^2 KKcont=70 W/m^2 KAdiabatic conditionIR Camera measurement

Tem

per

atur

e [°

C]

Time [sec]

RFX - 1 MA campaign

0

2040

6080

100120

140160180

0 1 2 3 4 5 6 7 8

Tem

per

atur

e [°

C]

Time [h]

16/12/'994/06/'99


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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

rfx – mod: what does the present device allow to do? r. piovan

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