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JE 2.3 : X2 breakdown assist in presence of Etor

Toroidal dynamics is expected to be important for breakdownprocess, especially if ionization avalanche is governed by timeconstants of the order

L / v(20eV) 6x102 m / 2x106 m/s 0.3 ms.

[ Remember: nH/n0 = 1013 ... 1019 = e30 ...e44, BD 40 all observations refer to the last 3 ]

Time constants using X2 pre-ionisation are of this order.

Task: check ECRH-X2-assist applying Etor prior to PECRH

This is the planned ITER-scenario at half-field

Status: Experiments at TJ-II (reported by E. Ascassibar in April)Experiments on AUG

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Reminder : TJ-II Results

A. Cappa, E. Ascasíbar, F. Castejón, J. Romero, A. López-Fraguas and the TJ-II team

last meeting

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Breakdown time, bd: delay between ECRH and H signal

bd

FLAT-TOP0

0.2

0.4

0.6

0.8

1

980 1000 1020 1040 1060 1080

Prefill valve (a. u.)Prefill pressure ()

gyrotron (a. u.)H (a. u. )density (x 1019 m-3)OH current (kA)

time (ms)

#26492

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Breakdown time depends on electric field for perp. injection

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ASDEX Upgrade Results

J. Schweinzer, A.C.C. Sips, O. Gruber, J. Hobirk, J. Stober

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ITER relevant breakdown studies at AUG(BD at low Etor, no OH switch, ECRH assisted)

• Attempts with ECRH-2 did not succeed – ECRH heats up gas pressure gauge -> prefill controll fails

• one repeat of 2009 result #26786

• First attempt with low Uloop > 0 before TS06 #26803

• First attempt with high Uloop > 0 before TS06 #26844 (no BD)

• Bv adjustment to produce magnetic null #26845

• Repeat of #26845 with late ECRH #27020

• In ITER Uloop rises slowly before ECRH provides the „spark“ for Ip buildup

• AUG 2008/9: ECRH makes pre-ionization and then Uloop rise starts

• This AUG campaign: produce Uloop before ECRH is switched on

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ITER-like BD: Uloop prior to PECRH

Ip

ne

PECRH

nmain

Uloop

H

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ITER-like BD: Uloop prior to PECRH – Bv optimisation

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ITER-like BD: Uloop prior to PECRH

Ip

Uloop

Bv adjusted

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ITER-like BD: Uloop prior to PECRH – late ECRH

Bv adjusted

Bv not optimized

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So Uloop is irrelevant for breakdown with ECRH?

A step back: ECRH without Uloop : G. Jackson et al., NF, 2007

Acceleration < 1 s, Final energy depends oninitial energy and harmonic

X1

X2

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So Uloop is irrelevant for breakdown with ECRH?

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Does Etor influence the collisionless heating of single electrons ?

How fast is an electron at rest drawn away from the ECRH beam( < 0.3 m) by an electrical field of E = 0.3 V/m ?

t = sqrt(2 s / a) = sqrt[ 0.6 m / (E e/m)] sqrt(10-11) s 3s

A toroidal field of 0.3 V/m may not influencethe collisionless heating significantly.[Although not orders of magnitude off]

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Electron multiplication

dn/dt = ECRH-term + n vd ( TS - 1/L)] TS = c1p0 exp(-c2p0/E)

Assume a flux tube crossing an ECRH beam at the resonance

A : Area, A*L : Volume, n*vd*A : Flux of ‘slow‘ electronsN : Number of re-usable ‘slow‘ electrons

generated by each ECRH accelerated ‘fast‘ electron#EC : Number of ECRH beams through the flux tube

dn/dt = n vd ( #EC N /L + TS – 1/L)

N 0...100 depends on PECRH, X1/X2, p0, L, Etor ??#EC 1...50 depends on #Gyrotrons and field null

Significant differences to Stellarators

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Electron multiplication : independent from Uloop ?

if #EC N /L >> TS , 1/L and N independent from Etor

crucial point ‘re-usability‘ of generated ‘slow‘ electrons.

re-usable means : electrons must be transported against Etor

during ionisation process.

This may be possible if energy after ECRH-accelleration >> 20 eV.These fast electrons cause several ionisations.During each ionisation process 2 e- with random velocity vectorsare emitted, most of them with vtor > 10eV,sufficient for several toroidal turns against the electrical force.

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Hypothesis

X2-ECRH generated electron density builds upin a flux tube through resonance and beamson the length of several (or a few ten) toroidal turns.

The toroidal Voltage drop along this region of the flux tubemust be significantly smaller than the initial energy of theEC-accelerated electrons, if the breakdown time is independentfrom the loop voltage. The required electron energies in the range of 50 to 100 eV are still compatible with X2 heatingif ECRH power is large enough.

Note: #EC may strongly depend on field null and has a directinfluence on BD

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Conclusion

The experimental results show that there is no general problem using X2-breakdown assist when Uloop is already applied.

The speculations on the reasons on the previous slides wouldneed support by additional experiments closer to the threshold.

Since this is not urgently motivated by ITER, there is no strongsupport for such experiments in AUG.

The results of AUG shall be published in an AUG-paper byJoe Schweinzer.

JE 2.3 could be closed unless a machine volunteers to continue.

In case of a closure, I would write an internal summary.

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Next steps on AUG

AUG microwave diagnostics are disturbed by the injection ofECRH in the empty vessel.

In the next campaign an attempt shall be made to obtain ohmicbreakdown without OH-switch, including sustainment by ECRH.

In such a scheme ECRH stray radiation should not exceedcrucial levels.

Such a scenario would be routinely used to get a better control on the current profile evolution and early divertor transitions.The latter would allow additional heating already at lower Ip.

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ITER-like BD: Uloop prior to PECRH