Density control in RFX-mod A. Canton , S. Dal Bello Consorzio RFX, Padova, Italy

13th IEA/RFP Workshop A. Canton, S. Dal Bello

Density control in RFX-mod

A. Canton, S. Dal BelloConsorzio RFX, Padova, Italy


Outline

Introduction about RFX-mod

Density behaviour in RFX-mod

DESO: a diagnostic for the wall

Dependence of plasma density on wall condition

How to obtain a discharge with a desired plasma density on RFX-mod?

Conclusions


Introduction

● RFX-mod first wall is entirely made of graphite

● Gas fuelling methods available at RFX-mod:

Pre-pulse gas filling

During pulse gas puffing

Pellet injection

Wall preloading by means of H2 GDC

● Wall treatment procedures:

He GDC (performed on daily base or more)

Baking + GDC cycles (3-5 per year)

Boronization (1-3 per year)

By a system of 8+8 piezoelectric valves installed at 4 toroidal positions

Up to 8 pellet per shot with masses in the range 1.5-5x1020atoms


Density behaviour in RFX-mod - 1

Shot 24250

0

0.4

0.8

1.2

1.6

0 10 20 30 40 50

Pla

sma

curr

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(MA

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1.0

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2.0

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3.0

3.5

0 10 20 30 40 50t (ms)

Cen

tral

den

sity

(10

19m

-3)

Vacuum vessel is filled with gas before the shot.A minimum pressure of about 0.3x10-3mbar allows breakdown. Before reversal particles are not confined and are lost to the wall.

0.E+00

1.E+19

2.E+19

3.E+19

4.E+19

5.E+19

6.E+19

7.E+19

8.E+19

0 0.5 1 1.5 2 2.5

Pre-shot filling pressure (10-3mbar)

Ce

ntr

al n

e:

pe

ak

at

Sta

rtU

p (

m-3

)

Time of reversal


Density behaviour in RFX-mod - 2

RFX-mod first wall is an almost infinite reservoir of particles -> density at flat top (FT) is entirely sustained by particles fluxes from the wall -> it does not depend on the fuelled particles

Shot 24250

0

0.4

0.8

1.2

1.6

0 50 100 150 200 250 300 350 400

Pla

sma

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(MA

)

0.0

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2.0

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tral

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sity

(10

19m

-3)

With a graphite wall density control requires to be able to control the status of the wall

0

5

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20

0 2 4 6 8 10

Fuelled particles (1020 H atoms)

Ele

ctr

on

de

ns

ity

at

FT

(1

019

m-3

)


DESO diagnostic system

We need to carefully control the particles both injected and extracted during the operations with plasma

A new diagnostic tool, named DESO, has been implemented on RFX-mod.

Aims:

-> Improves the reproducibilty of the gas fuelling system a series of dummy openings of the valves (performed some minutes before the shot) stabilize their flow

-> Computes the particles that fuel the discharge, both as filling and as puffing before the shot it executes the same opening of the valves programmed for the discharge and evaluates the number of particles entering the vessel by the measure of pressure

-> Computes the neutral particles outgassed from the wall and extracted from the chamber after the shot


One parameter adopted to describe the status of the wall is the Desorption factor Des%

Extracted particles: -> evaluated by DESO

Total fuelled particles:

- By gas filling/puffing -> evaluated by DESO

- By pellet injection -> pellet masses measured by the injector diagnostic system

Desorption factor Des%

Des% = __________________________________________________* 100Extracted particles after the end of the pulse

Total fuelled particles

Des% expresses, shot by shot, the capability of the wall of absorbing the fuelled particles


Other two parameters:

Particles left in the wall shot by shot: DeltaPart

Particles stored in the wall at each shot:

Total fuelled particles (filling, puffing, pellet injection,H2GDC) – Outgassed particles

Particles stored in the wall by H2 GDC are computed following experimental results found

in literature, see Winter,J. Nucl. Mater.161 (1989), p.265)

Particles removed from the wall by He GDC (computed by mass spectrometer measurements)

Total number of particles stored in the wall since last Baking or Boronization procedure: TotPart

Particles inventory in the wall

DeltaPart = Particles stored in the wall (– Particles removed from the wall by GDC)

TotPart = Σ DeltaPart


Behaviour of TotPart and Des%

0.E+00

1.E+22

2.E+22

3.E+22

4.E+22

5.E+22

6.E+22

7.E+22

8.E+22

24110 24210 24310 24410 24510 24610 24710 24810 24910

To

tPar

t

0

25

50

75

100

125

150

175

200

24110 24210 24310 24410 24510 24610 24710 24810 24910

Shot

De

s%

Baking+GDC

Number of particles to saturate the wall, assuming an implantation depth of the order of tens of nm [Tramontin et al., J. Nucl. Mater. 266-269 (1999), p.709]

We are planning new experiments of samples exposition to the plasma in order to allow a better computation of the wall volume where particles are stored

• Daily He GDCs (20-60 min) remove 1-2x1021 H atoms and allow operation, but they are not effective in preventing particle accumulation in the wall • Des% alone is not an absolute indicator of saturation state for the wall. It depends on plasma parameters (e.g. current), plasma-wall interaction, wall temperature...


Wall and density - 1

0

2E-14

4E-14

6E-14

8E-14

1E-13

1.2E-13

1.4E-13

1.6E-13

1.8E-13

0 50 100 150 200

Des%

I/N (A

m)

Absorbing wall:wide range of density regimes

Not absorbing wall:I/N forced to the value 2x10-14 Am

0

2

4

6

8

10

12

14

0 50 100 150

Vt(0)*I (V*MA)

Cen

tral

ele

ctro

n d

ensi

ty (

101

9m

-3)

Wall condition affects density...• Des% leads I/N• No correlation found between Des% and ne or I alone

...but plasma itself extracts particles from the wall (PWI)• Density depends on Ohmic Power, that regulates particle influxes from the wall

Analysis over a database of about 700 shots. Quantities averaged over 2ms around max plasma current.0.3<I<1.6 MA, 1<ne<14 1019m-3


Wall and density - 2

0.E+00

2.E-14

4.E-14

6.E-14

8.E-14

1.E-13

1.E-13

1.E-13

2.E-13

2.E-13

0.E+00 1.E+22 2.E+22 3.E+22 4.E+22 5.E+22 6.E+22 7.E+22 8.E+22

TotPart

I/N

(A

m)

0.E+00

2.E+19

4.E+19

6.E+19

8.E+19

1.E+20

0.E+00 1.E+22 2.E+22 3.E+22 4.E+22 5.E+22 6.E+22 7.E+22 8.E+22

TotPart

Ce

ntr

al

de

ns

ity

(m-3)

No dependence of density and/or I/N on the total number of particles stored in the wallWhere are the particles stored? Implantation depth, toroidal and poloidal asymmetries can cause that stored particles are not accessible by plasma.


About particles inventory in the wall

-5.0E+20

0.0E+00

5.0E+20

1.0E+21

1.5E+21

2.0E+21

0.0E+00 5.0E+20 1.0E+21 1.5E+21 2.0E+21

Total number of injected atoms (filling+puffing+pellet)

Inje

cted

- o

utg

asse

d a

tom

s

In RFX-mod we outgas a minimum part of the particles that we injectOutgassed particles depend (only?) on the temperature of the wall: limited to <100°CTo avoid particles accumulation in the wall we should always fuel the discharge with the minimum gas to allow breakdown


How to obtain a discharge with a desired plasma density?

- Control particle fluxes from the wall

Control PWI With respect to RFX, the improved control of magnetic boundary (lower error fields and rotating modes) and a new design of the graphite tiles has allowed the operation of RFX-mod up to 1.6 MA

- Control the status of the wall

It means to control both particle content and their spatial distribution in the wallOptimization of He GDC with respect to the plasma to be done

- Effectively fuel the discharge Gas Filling -> useless: between breakdown and reversal the particles are lost

and stored in the wall Gas Puffing -> on RFX-mod proved to be inefficient due to the low flow and

gas velocity of the piezo valves: neutral particles do not penetrate the plasma and accelerate particle accumulation in the wall

Pellet injection -> density can be transiently raised but density control is quickly lost because of the large amount of injected particles that are left on the wall


H2 GDC as a way to obtain reproducible plasma density

Wall pre-loading by means of H2 GDC is under test as a reproducible method to obtain a discharge with desired flat top density.

-> Technique originally developed as a way to condition the wall after heavy treatments like boronisation.

-> The same technique tentatively applied after the morning He GDC. Reproducible correspondence between the number of implanted particles andthe obtained plasma density, even if there is a factor 10 between the two.Optimization in progress

-> At the maximum explored plasma currents I~1.5 MA the effect of pre-load lasts for only one shot, at I<1MA it lasts for several shots (of the order of 10)

The influence on the duration of the pre-load treatment of the voltage applied tothe GDC anodes, that determines the energy and hence the depth of the particles implanted in the wall during the H2 GDC, and of the implant fluencethat influences the depth profile, is under test.


Conclusions

• In RFX-mod density control is strongly influenced by the wall

• A new diagnostic tool, DESO, has been recently implemented. On the one side it is devoted to increase the reproducibility of the gas fueling system, on the other side it allows to precisely compute the particles stored in the wall and to characterise its behaviour

• The relation between plasma density and wall condition has been explored over a large database of pulses. The capability of the wall to absorb particles, described by the desorption factor Des%, was found to influence the value of I/N more than the absolute number of particles stored in the wall

• The strength of plasma wall interaction was found to be the other factor that determines plasma density, confirming the importance of the control of PWI in particular in view of operation at 2 MA in the next future of RFX-mod

• H2 GDC is a promising way to obtain reproducible plasma density but the techinque need to be optimised


Wall and density - extras

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Des%

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00.20.40.60.8

11.21.41.61.8

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Des%

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)

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Vt(0)

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I (MA)

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-3)

Density control in RFX-mod A. Canton , S. Dal Bello Consorzio RFX, Padova, Italy

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Transcript of Density control in RFX-mod A. Canton , S. Dal Bello Consorzio RFX, Padova, Italy