Ex/4-3: On the challenge of plasma heating in the JET Metallic Wall Dr. Marie-Line Mayoral 24th IAEA Fusion Energy Conference, 8-13 October, San Diego, USA

Ex-4-3: On the challenge of plasma heating in a the JET Metallic Wall Marie-Line Mayoral AND V. Bobkov, A. Czarnecka, I. Day, A. Ekedahl, P. Jacquet, M. Goniche, R. King, K. Kirov, E. Lerche, J. Mailloux, D. Van Eester, O. Asunta, C. Challis, D. Ciric, J.W. Coenen, L. Colas, M. C. Giroud, M. Graham, I. Jenkins, E. Joffrin, T. Jones, D. King, V. Kiptily, C.C. Klepper, C. Maggi, F. Marcotte, G. Matthews, D. Milanesio, I. Monakhov, M. Nightingale, R. Neu, J. Ongena, T. Pütterich, V. Riccardo, F. Rimini, J. Strachan, E. Surrey, V. Thompson, G. Van Rooij and JET EFDA contributors. Ex-4-3: On the challenge of plasma heating in a the JET Metallic Wall

Outline Ion Cyclotron Resonance Frequency heating ICRF Lower hybrid Current Drive LHCD Neutral Beam Injection heating NBI Ion Cyclotron Resonance Frequency heating ICRF Neutral Beam Injection heating NBI Lower hybrid Current Drive LHCD All the things to watch for before or when applying high heating power in a metallic machine based on the JET experience For each heating systems System layout and capabilities Modifications to the wall or the systems for safe operation in the JET ITER-like wall (ILW) Operation of the systems during the 1st experimental campaigns with the ITER-like wall (ILW)  power achievements, heats loads issues & increase in plasma impurities content

JET NBI system  2 neutral injectors boxes, recently upgraded Two neutral beam injector boxes Each equipped with 8 Positive Ion Neutral Injectors: PINIs  grouped into tangential and normal banks [Ciric et al., FED, 86, 2011] In parallel with ITER-like wall installation  NBI system upgraded 1st goal: Increase NBI power from 24MW to 34MW  PINIs converted to 125kV PINIs with modified ion source, accelerator configuration & refurbished power supplies 2nd goal: Increase NBI pulse duration at full power from 10s to 20s  beam duct changed to actively cooled beam duct View of the JET vessel from inside the new beam “duct”

NBI-related modifications prior to ILW Main chamber W-coated on shine-through areas Neutral Beam test bed measurements Shine through power increased by a factor of 2 for the upgraded PINIs Peak power densities without plasma ~ 48 MW/m2 on inner wall & 24MW/m2 on outer wall W-coated CFC tiles instead of Be tiles on inner & outer wall for areas at risk Real-time protection system  switch PINIs –off in case predicted tile temperature become too high JET inner wall

NBI-related modifications prior to ILW W-coated tiles on beam re-ionisation areas Another main chamber area made of W-coated tiles  Beam “re-ionisation” tiles Near poloidal limiter clockwise to beam ducts Take power loads from neutrals ionised in plasma edge that drift & impinge on outer wall Maximum power densities estimated to be 10th MW/m2

Operation of NBI in the ILW Record NBI power achieved Some delays at the beginning but finally new power supplies & control system successfully commissioned Record power 25.7MW (14 PINIs at 92 to 117kV) For the 1st time NBI power coupled for 15s from single PINIs Good behavior of actively cooled beam ducts designed to operated at max. temperature ~200OC Steady state temperatures achieved for 15s pulse 150 oC reached for 15MW injected from one beam duct

Operation of NBI in the ILW No issues but observation re-ionisation tiles heating No experimental observation of W increase due to fast D sputtering of W-coated CFC shine through tiles (inner wall) No issues with NBI fast ions losses (outer wall) Heating of the re-ionisation tiles observed for the 1st time Monitored by the protection of ILW (PIW) viewing system So far temperature reached was at max 870 oC  OK !

Outline Ion Cyclotron Resonance Frequency heating ICRF Neutral Beam Injection heating NBI Lower hybrid Current Drive LHCD For each heating systems System layout and capabilities Modifications to the wall or the systems for safe operation in the ITER-like wall (ILW) Operation of the systems during the 1st experimental campaigns with the ITER-like wall (ILW)  power achievements, heats loads issues & increase in plasma impurities content

JET ICRF system  4 antennas with new Be private limiters 4 antennas (A, B, C & D) called the A2s antennas Frequency range is 23 to 55 MHz Wave launched with symmetric spectra (“dipole” phasing) or asymmetric spectra (“± 90o” phasing) ICRF system is ELM tolerant (able to couple steady power on ELMs) ITER-like ICRF antenna not used during last campaign ITER-like wall related change : All private limiters changed from CFC to Be tiles [GRAHAM M., et al., PPCF (2012)]

Infra-Red view during operation of the antenna A & B ICRF operation in the ILW  No arcing issues, 4MW on type-I ELMs, heat-load within limits 1st possible issue  signs of arcing on new antenna Be private limiters or change in antenna behaviour No problem Up to usual maximum voltage (~30kV) reached 4MW on type I ELMy H-mode (with ¾ of the system) 2nd possible issue  heat loads due to ions accelerated in RF sheath rectified voltages A Maximum power load (estimated from IR thermography and a thermal model for the ILW Be tiles) was ~ 4.5 MW/m2 As design limit for the Be tiles is 6MW/m2 for 10s  we are safe but monitoring by viewing protection system is still needed. B [JACQUET et al., PSI2012] Infra-Red view during operation of the antenna A & B

ICRF / NBI pulse comparison ICRF operation in the ILW Good heating / Higher bulk radiation (W & Ni ) then with NBI ICRF / NBI pulse comparison Efficient electron heating As expected from H minority ICRF heating Long sawtooth periods characteristic of centrally peaked fast H ions pressure Bulk radiation with ICRF higher then for NBI and due to Higher W content Higher Ni content with Ni contributing to 20% of the bulk radiation Bulk radiation during ICRF minimized for higher edge density symmetric wave spectra (compared to asymmetric) by  H concentration up to 20% [BOBKOV et al., PSI 2012], [VAN EESTER et al., EPS 2012], [CZARNECKA et al. EPS 2012], [KLEPPER et al. PSI 2012]

JET Outer wall (simulated) & E// pattern (TOPICA) ICRF operation in the ILW No obvious sign of enhanced W emission from divertor entrance 1st possible W source to explain high W during ICRF  sputtering of divertor & its entrance by ions accelerated in RF sheaths rectified voltages Likelihood of effect emphasised by modelling  region of high E// connected to tile 8, B & C JET Outer wall (simulated) & E// pattern (TOPICA) The first effect that we have suspected is some interaction with the divertor and its entrance due to sputtering by ions accelerated in RF sheaths rectified voltages. The likelihood of this effect was confirmed by the electrical modelling of the A2s antennas that shown that the parallel electric filed responsible for the RF sheath extend on even in the poloidal limiter., which mean that the magnetic connection between RF sheaths definitely exist … nevertheless, when we looked at the W emission from the divertor and tile 8 and B there was no obvious difference with the NBI! Even if we have to keep in mind that tile C is not properly diagnose we started to look at interaction with the main chamber BUT …no obvious experimental observation W emission from divertor, tile 8 & B slightly higher during NBI ! Possible that we miss emission from tiles C not seen by diagnostics ? [BOBKOV, PSI 2012]

W and Ni source in recessed areas Responsible effect unclear? ICRF operation in the ILW W and Ni source from main chamber recessed areas Next possible W and Ni source in the main chamber  evidence seems by comparing ICRF and NBI heated limiter pulses and Be evaporation Comparison of the reference pulse before a light Be evaporation with a pulse just after a Be evaporation Reduction of the bulk radiation by ~ 45% Strong decrease of Ni & W emission Comparison of the reference pulse before with a pulse 11 ELMy H-mode after the Be evaporation Radiation still lower Ni & W emission still lower W and Ni source in recessed areas Responsible effect unclear?

Outline Ion Cyclotron Resonance Frequency heating ICRF Neutral Beam Injection heating NBI Lower hybrid Current Drive LHCD For each heating systems System layout and capabilities Modifications to the wall or the systems for safe operation in the ITER-like wall (ILW) Operation of the systems during the 1st experimental campaigns with the ITER-like wall (ILW)  power achievements, heats loads issues & increase in plasma impurities content

JET LHCD system  New Be frame, dedicated viewing system Lower hybrid current drive (LHCD) launcher 48 multi-junctions modules fed by 24 klystrons 3.7GHz ITER-like wall related change : Launcher frame in Be instead of CFC New LH viewing systems (IR, visible & pyrometers) to: Monitor possible damaging heat flux due to generation of fast electrons in front of the launcher Develop new arc detection systems based on visible light

LHCD operation with the ILW  2. 5 MW coupled trouble-free (max LHCD operation with the ILW  2.5 MW coupled trouble-free (max. until viewing system ready) Power density limited to 15MW/m2 (180 kW per klystrons) until dedicated viewing system on-line First image from IR camera viewing LH system only at the end of the campaign Visible camera available from mid-campaign 1st  wavelength filtering to minimize light from source others than arc Example: unstopped arc propagating on launcher Additional to the existing arc detection system, a real-time protection based on bright –spot detection being implemented 2.5 MW coupled trouble-free  no impurities, no heat loads issues [KIROV et al., EPS 2012]

Lot more to say….Poster Ex/4-3 tomorrow morning Summary Major upgrade of the NBI system Slow start but record power and power length were obtained We are now confident that NBI upgraded targets: 34 MW, 20s can be reach SAFELY in the next JET campaigns Good heating performance  started to use central electron heating to prevent W peaking in high performance scenarios [Pütterich Ex/P3-15 ] Higher bulk radiation  W & Ni  possible source of W from divertor entrance (not fully confirmed experimentally) & source of W & Ni from main chamber recessed areas In ITER  No main chamber recessed area made of W and Ni ! Interaction with divertor likely to be minimised (ratio antenna size/divertor surface, antenna position, design effort…) ICRF Lot of work involved for safe application of the heating power in the JET ILW Working group had been created to deal with issues of heating systems and ILW protection 3 years before any power application NBI LHCD Run so far at only at low power density because the viewing systems came on line only at the end of the campaign More to come next year… Lot more to say….Poster Ex/4-3 tomorrow morning