1 JE 2.3 : X2 breakdown assist in presence of E tor Toroidal dynamics is expected to be important for breakdown process, especially if ionization avalanche is governed by time constants of the order   L / v(20eV)  6x10 2 m / 2x10 6 m/s  0.3 ms. [ Remember: n H  /n 0 = = e 30...e 44,  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 E tor prior to P ECRH This is the planned ITER-scenario at half-field Status: Experiments at TJ-II (reported by E. Ascassibar in April) Experiments on AUG

2 Reminder : TJ-II Results A. Cappa, E. Ascasíbar, F. Castejón, J. Romero, A. López-Fraguas and the TJ-II team last meeting

3 Breakdown time,  bd : delay between ECRH and H  signal  bd

4 Breakdown time depends on electric field for perp. injection

5 ASDEX Upgrade Results J. Schweinzer, A.C.C. Sips, O. Gruber, J. Hobirk, J. Stober

6 ITER relevant breakdown studies at AUG (BD at low E tor, 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 U loop > 0 before TS06#26803 First attempt with high U loop > 0 before TS06#26844 (no BD) Bv adjustment to produce magnetic null#26845 Repeat of #26845 with late ECRH#27020 In ITER U loop rises slowly before ECRH provides the „spark“ for Ip buildup AUG 2008/9: ECRH makes pre-ionization and then U loop rise starts This AUG campaign: produce U loop before ECRH is switched on

7 ITER-like BD: U loop prior to P ECRH IpIp nene P ECRH n main U loop HH

8 ITER-like BD: U loop prior to P ECRH – B v optimisation

9 ITER-like BD: U loop prior to P ECRH IpIp U loop B v adjusted

10 ITER-like BD: U loop prior to P ECRH – late ECRH B v adjusted B v not optimized

11 So U loop is irrelevant for breakdown with ECRH? A step back: ECRH without U loop : G. Jackson et al., NF, 2007 Acceleration < 1  s, Final energy depends on initial energy and harmonic X1 X2

12 So U loop is irrelevant for breakdown with ECRH?

13 Does E tor 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( ) s  3  s A toroidal field of 0.3 V/m may not influence the collisionless heating significantly. [Although not orders of magnitude off]

14 Electron multiplication dn/dt = ECRH-term + n v d (  TS - 1/L)]  TS = c 1 p 0 exp(-c 2 p 0 /E) Assume a flux tube crossing an ECRH beam at the resonance A : Area, A*L : Volume, n*v d *A : Flux of ‘slow‘ electrons N : 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 v d ( # EC N /L +  TS – 1/L) N depends on P ECRH, X1/X2, p 0, L, E tor ?? # EC depends on #Gyrotrons and field null Significant differences to Stellarators

15 Electron multiplication : independent from U loop ? if # EC N /L >>  TS, 1/L andN independent from E tor crucial point ‘re-usability‘ of generated ‘slow‘ electrons. re-usable means : electrons must be transported against E tor 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 vectors are emitted, most of them with v tor > 10eV, sufficient for several toroidal turns against the electrical force.

16 Hypothesis X2-ECRH generated electron density builds up in a flux tube through resonance and beams on the length of several (or a few ten) toroidal turns. The toroidal Voltage drop along this region of the flux tube must be significantly smaller than the initial energy of the EC-accelerated electrons, if the breakdown time is independent from the loop voltage. The required electron energies in the range of 50 to 100 eV are still compatible with X2 heating if ECRH power is large enough. Note: # EC may strongly depend on field null and has a direct influence on  BD

17 Conclusion The experimental results show that there is no general problem using X2-breakdown assist when U loop is already applied. The speculations on the reasons on the previous slides would need support by additional experiments closer to the threshold. Since this is not urgently motivated by ITER, there is no strong support for such experiments in AUG. The results of AUG shall be published in an AUG-paper by Joe Schweinzer. JE 2.3 could be closed unless a machine volunteers to continue. In case of a closure, I would write an internal summary.

18 Next steps on AUG AUG microwave diagnostics are disturbed by the injection of ECRH in the empty vessel. In the next campaign an attempt shall be made to obtain ohmic breakdown without OH-switch, including sustainment by ECRH. In such a scheme ECRH stray radiation should not exceed crucial 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 I p.

19 ITER-like BD: U loop prior to P ECRH