JET RF Physics Programme Overview C20-C25

1. JET RF Physics Programme OverviewC20-C25 J.Ongena (acting TF H leader)

2. RF work at JET highly focussed on ITER • ICRH antenna and LH launcher on ITER should: • Deliver continuously high power levels (high power densities at antenna) • At large distance between plasma and antenna strap/launcher • Needs development of load resilient systems • Needs development of techniques to couple power in the presence of large (15-20cm) gaps between plasma and power launching component

3. ITER requirements for ICRF and LH ICRF LH • ICRF in ITER 20 MW in ELMy H-Mode with antenna-separatrix distance of 14cm • LH planned for ITER in second phase • Under discussion for first phase : possibly as a downsized prototype (5MW) • Full LH system in ITER 20MW in ELMy H-Mode with launcher-separatrix distance of 14cm JET is ideal for such studies

4. Coupling of LH power to ITER plasmas • Density at grill mouth needs to be above cut-off density ne,cut-off for LH waves to couple. • JET LH protection system not so sensitive to ELMs: LH can couple during ELMs without trips, provided ne in front of the grill remains > ne,cut-off • Cut-off density could be several cms from the launcher, depending upon shape of ITER density profile. Problem more severe for 5GHz operation(Possible option for ITER to avoid power absorption on alpha particles). • Tool: D2 Gas puffing SOL density in ITER From A Kukushkin and A Loarte Launcher Separatrix

5. ITER Like Antenna (ILA) Objectives Demonstrate High Power Density ICRF Launcher in Relevant Plasmas and Heating Scenarios • 30 - 55MHz • Power density 8MW/m2 • 7.2MW • 1.3m x 0.7m (0.91m2) • Resilient to ELMs See (Next) Talk by F.Durodié

6. Externally matched Conjugate T (ECT) antenna system External Conjugate T with trombones as variable element See (Next) Talk by I.Monakhov

7. Main programmatic elements in TFH • Establish efficient power coupling for different ELM-tolerant systems • Establish gas injection requirements for optimal ICRH (LH) coupling while maintaining good plasma confinement and up to ITER-relevant antenna (launcher)-separatrix distances • Improvement of LH&ICRF coupling for general JET operation • Monitor localised heat loads & material erosion (especially due to fast particles, sheath effects and LH-accelerated electrons) • ILA commissioning

8. ILA commissioningKey goals • Goal 1: Matching • Two algorithms to be tested: • Feedback control on capacitors value to control ZT • Feedback control on capacitors value to control voltage on the antenna straps • Goal 2: ELM resilience • During ELMs: maintain low reflection levels  carefully chosen impedance at T-junction • Goal 3: Arc detection (see also talk by Ph. Jacquet Thursday morning) • Voltage Standing Wave Ratio (VSWR) • Principle: Reflection in transmission line when arc • Easy to use (used on A2s) but low voltage arcs can go undetected • Sub-harmonic arc detection (SHAD) • Principle: Detection of 5-20 MHz enhanced RF signals in the line • Works well in L-mode, further results required to qualify if it is also reliable in H-mode • Scattering Matrix arc detection (SMAD) • Principle: Consistency between RF measurements and model assuming no arcs (S matrix representation) • Promising but still to be tested !

9. ILA commissioningKey goals • Goal 4: High power density • Demonstrate high power density with ILA • Novelty : short & closely – spaced straps • Show that such a system can be matched even in the presence of high mutual coupling • Highly relevant for ITER ICRF antenna design • Deliver high power levels to plasma • In L-mode, H-mode with fuelled Type I ELMs, H-mode with Type I ELMs • Short pulse (<5s) , long pulse (>5s) • Small ROG (~ 4 cm), ITER-relevant ROG (up to 12 cm) • Goal 5: JET operational and physics issues • Study of thermal capabilities in long pulses • Compare heating efficiency of ILA and A2 antennas • Investigate interaction between A2 antennas / ILA antenna / LH launcher • Investigate impact on plasma performance of ~ 7 MW from ITER-Like antenna on top of 10 MW from A2s

10. Efficient power coupling for different ELM-tolerant systems ECT • 3 different systems at JET • External Conjugate T (ECT) • 3dB (hybrid) couplers • ITER like Antenna (ILA) • Compare efficiency of RF power delivery in ELMy-H mode plasmas of the 3 systems • Assess implications on ICRF antenna performance and plasma characteristics 3dB ILA

11. ICRH coupling & Gas injection / ITER distance / Good confinement • Optimisation of ICRF coupling in ITER relevant conditions • Study peculiarities of coupling of A2 ICRH antennae to ELMy plasmas with a large distance between antenna and plasma separatrix • Pursue development of method of coupling improvement using different locations for gas injection • Develop understanding of edge density modifications during gas puff and ELMs • Real-time optimisation of ICRF antenna coupling using feedback on gas puffing • Demonstrate the feasibility of the real-time control of the coupling resistance by gas injection (during ROG sweep, shape modifications by XSC…) • Optimise amount of gas injected while maintaining good plasma confinement • Maximise ICRH power delivered to the plasma in ELMy H mode and at ITER relevant antenna-plasma distances • Validation of RCP measurements with ICRH • Establish conditions/regimes where the probe data is valid during ICRH

12. LH coupling & Gas injection / ITER distance / Good confinement • Gas injection optimisation for LH coupling • To determine whether LH power can be efficiently coupled using gas injection from the top of the machine, magnetically coupled to the launcher (as foreseen on ITER), at large plasma-launcher distance and during large ELMs.  with type I ELMs at H~1 • To assess possible confinement changes during gas puff • Real-time optimisation of LHCD antenna coupling using feedback on gas puffing • Demonstrate the feasibility of the real-time control of the reflection coefficient by gas injection (during ROG sweep, shape modifications by XSC with type I ELMs at H~1 • Optimise amount of gas injected while maintaining good plasma confinement • Maximise LHCD power delivered to the plasma in ELMy H mode and ITER relevant antenna-plasma distances • Effect of gas injection & wide SOL on LC current drive efficiency • Assess possible changes in current drive efficiency in experiments with a large distance between antenna and plasma separatrix and gas puffing to increase coupling. in L-Mode

13. Improvement of LH&ICRF coupling for general Task Force work • Push LH power toward ITER requirements • Push LH power above usual restart milestone (i.e. > 4 MW equivalent with 16 MW/m2) [Get better conditioning, prepare high power experiments in general TF work] in L-mode • Demonstrate feasibility of ITER power density measurements • Modulation LHCD power to quantify power deposition and current drive up to high densities • Use LH power modulation to quantify LH wave penetration up to 5 1019 m-3 H mode with small ELMs • Assess wave absorption at densities above which codes failed to model the absorption due to non-linear effects • Push LH and ICRH powers on ELMs • Increase LH power on large ELMs : assess if presence of large ELMs influences LH coupling + LH protection systems • Increase ICRH power on ELMs using 3dB couplers (ICRH generators A&B) • Increase LH/ A&B ICRH power simultaneously

14. Study of localised heat loads • Characterisation of heat loads during ICRH and NBI • Identify/characterise localised heating mechanisms (in preparation of ILW) • Measure surface temperature as a function of ICRH power, plasma density, NBI power • Develop/refine modelling to deduce heat-flux from IR measurements • Edge parasitic absorption during ICRF heated plasmas • Use low single pass absorption scenarios to enhance edge parasitic losses • Investigate parasitic absorption due to RF-sheath effects and parasitic mode conversion on the HFS • LH power losses in the SOL • Quantify fraction of LH power lost in the SOL and heat-flux as function of the launched power density, L/H mode, SOL density, gas puff • Measure surface temperature using IR cameras • Develop modelling to deduce heat-flux from IR measurements • Impurity production associated with high ICRF power • Assessment of impurity release associated with ILA/A2 antennae • Determination of specific sources of impurities during RF and elaboration of methods to minimize their effects • SOL density modifications by sheath effects induced by ICRH (for A2 and ILA) • Map density modifications due to ICRF sheath effects both for ILA and A antenna (in R,Z space using RCP) • Input for testing of coupling predictions by TOPICA

15. ILA commissioning plansC20A

16. ILA commissioning plansC20B

17. ILA commissioning plansC21

18. ILA commissioning plansC22

19. ILA commissioning plansC23

20. ILA commissioning plansC24

21. There is plenty of work to be done Hope this summary will help you to identify period to come at JET and participate to the 2008 JET work-programme ! … the more we are the better ! Any questions contact Jef or myself