1. KIT – University of the State of Baden-Wuerttemberg and National Research Center of the Helmholtz Association KARLSRUHE INSTITUTE OF TECHNOLOGY, Institute for Pulsed Power and Microwave Technology (IHM) www.kit.edu Experimental results and recent developments on the EU 2 MW 170 GHz coaxial cavity gyrotron for ITER European GYrotron Consortium (EGYC) S. Kern 1, J.-P. Hogge 2, S. Alberti 2, K. Avramides 3, G. Gantenbein 1, S. Illy 1, J. Jelonnek 1, J. Jin 1, F. Li 2, I. Gr. Pagonakis 1, B. Piosczyk 1, T. Rzesnicki 1, M. K. Thumm 1, I. Tigelis 4, M. Q. Tran 2 and the whole EU home team at EGYC 1 Karlsruhe Institute of Technology (KIT), Institute for Pulsed Power and Microwave Technology (IHM), Association EURATOM-KIT, D-76131 Karlsruhe, Germany 2 Centre de Recherche en Physique des Plasmas (CRPP), Association Euratom-Confédération Suisse,EPFL, CH-1015 Lausanne, Switzerland 3 National Technical University of Athens (part of HELLAS), School of Electrical and Computer Engineering, 9 Iroon Polytechniou st., GR15773 Athens, Greece 4 National and Kapodistrian University of Athens (part of HELLAS), Faculty of Physics, 157 84 Athens, Greece.

2. Institute for Pulsed Power and Microwave Technology 208.05.2012S. Kern et al., Experimental results and recent developments on the EU 2 MW 170 GHz coaxial cavity gyrotron for ITER, EC17 Workshop, 07-11 May 2012, Deurne, The Netherlands Overview Introduction Summary and consequences of former experiments Design modifications to the industrial CW prototype Tests of the refurbished CW prototype SAT and RF test Post-test evaluations Experiments with the short pulse pre-prototype Future plans

3. Institute for Pulsed Power and Microwave Technology 308.05.2012S. Kern et al., Experimental results and recent developments on the EU 2 MW 170 GHz coaxial cavity gyrotron for ITER, EC17 Workshop, 07-11 May 2012, Deurne, The Netherlands Introduction The EGYC consortium develops under F4E contract 170 GHz gyrotrons in support of EU’s contribution to ITER’s ECRH system. - EGYC is currently CRPP, KIT, HELLAS, IFP-CNR. The industrial partner is Thales Electron Devices (TED). Until now, the goal was a 2 MW coaxial cavity gyrotron. Three CW prototypes at increasing pulse length goals (1s/60s/3600s) foreseen, only one build so far. A 1 MW conventional tube development as fallback was started 2008. Due to essential delays, switch to the fallback solution is probable. This presentation reports on latest results and future plans for the coaxial cavity project

4. Institute for Pulsed Power and Microwave Technology 408.05.2012S. Kern et al., Experimental results and recent developments on the EU 2 MW 170 GHz coaxial cavity gyrotron for ITER, EC17 Workshop, 07-11 May 2012, Deurne, The Netherlands Summary of former experiments In 2008 first experiments with CW prototype at CRPP: Serious problems with the electron gun: low frequency oscillations (~100 MHz region) low voltage standoff with magnetic field applied -> Analysis shows the existence of potential traps in the gun RF output beam pattern insufficient (77% Gaussian content) -> Application of new launcher design methods RF power limited to 1.4 MW in short pulse due to mentioned problems Power capability of the collector successfully tested (2.2 MW/10s) Pre-prototype short pulse tests at KIT until end 2008: Power limitation by limited magnetic field (6.7 T instead of 6.87 T) -> Application of additional normal conducting (NC) coil Indications of beam-tunnel oscillations (159 GHz) -> Application of corrugated beam tunnel Same RF beam as in prototype

5. Institute for Pulsed Power and Microwave Technology 508.05.2012S. Kern et al., Experimental results and recent developments on the EU 2 MW 170 GHz coaxial cavity gyrotron for ITER, EC17 Workshop, 07-11 May 2012, Deurne, The Netherlands Resulting modifications to the CW prototype The CW prototype was refurbished with design improvements: New gun design following improved design rules New launcher design Corrugated beam tunnel Additional modifications in mechanical construction: Ion getter pumps moved to lower magnetic field Large ceramic isolator mounting revised for lower force during bakeout Validation tests with the short pulse pre-prototype at KIT in 2009: Corrugated beam tunnel applied -> No parasitic oscillations New launcher design as above -> 96 % Gaussian content Achieving 6.87 T with NC coil -> operation at nominal parameters -> nominal operation: 2.2 MW @ 30 % efficiency (w/o depr. collector) !

6. Institute for Pulsed Power and Microwave Technology 608.05.2012S. Kern et al., Experimental results and recent developments on the EU 2 MW 170 GHz coaxial cavity gyrotron for ITER, EC17 Workshop, 07-11 May 2012, Deurne, The Netherlands Mechanical issues with refurbished CW prototype Refurbishment and modification started in 2009 Manufacturing problems caused massive delays - some clearly attributed to the attempt to refurbish a large device. The refurbishment of the mirror box was considered critical by the manufacturer, after brazings became untight at bakeout. In particular, one RF absorber had to be removed. Delivery date moved from summer 2010 to finally end September 2011. The tube was untight on delivery. It could be sealed and still showed good vacuum properties after pumping. Achievable pulse length was unclear then. After four days of successful RF operation, another RF absorber broke and flooded the tube with water, terminating any further experiment. Reasons still have to be investigated in detail, ultimate cause is accidental operation in wrong mode rotation. After experiment, additional problems of alignment were found.

7. Institute for Pulsed Power and Microwave Technology 708.05.2012S. Kern et al., Experimental results and recent developments on the EU 2 MW 170 GHz coaxial cavity gyrotron for ITER, EC17 Workshop, 07-11 May 2012, Deurne, The Netherlands SAT test of the prototype gyrotron Site acceptance test (SAT) was successful: Excellent HV stand-off without and with magnetic field Cooling tests OK Beam extraction tests OK (58 kV / 75 A after 2 days of conditioning) Coaxial insert.vs. electron beam alignment OK Body current higher than expected, but acceptable Body.vs. electron beam alignment ~OK (coarse measurement, made in x direction only) -> The tube was formally accepted. Green light to proceed with RF tests was given by TED on December 2 nd 2011.

8. Institute for Pulsed Power and Microwave Technology 808.05.2012S. Kern et al., Experimental results and recent developments on the EU 2 MW 170 GHz coaxial cavity gyrotron for ITER, EC17 Workshop, 07-11 May 2012, Deurne, The Netherlands RF tests of the prototype gyrotron (Dec. 5.-9.) After 4-5 days of RF conditioning, the tube delivered 2 MW / 170 GHz (short pulse ~1ms) with an efficiency of 45 % at 75 A (nominal value) and 90.5 kV (60 kV + 30.5 kV), (nominal: 90kV, 55kV + 35kV) No particular optimization, no evident sign of saturation, not finally conditioned.

9. Institute for Pulsed Power and Microwave Technology 908.05.2012 Simulation pre-prototype 96 % GC refurbished prototype Comparison S. Kern et al., Experimental results and recent developments on the EU 2 MW 170 GHz coaxial cavity gyrotron for ITER, EC17 Workshop, 07-11 May 2012, Deurne, The Netherlands RF beam pattern of the prototype Mode TE 34,19 Mode TE 35,19 ? Logbook shot #10410, 2011-12-05

10. Institute for Pulsed Power and Microwave Technology 1008.05.2012 Positive results 2 MW / 170GHz short pulse efficiency of 45 % (using depressed collector), non-optimized, at nominal beam parameters 75 A, 90.5 kV, depression voltage 30.5 kV (nominal: 35 kV) Very good RF beam pattern -> second validation of launcher redesign No low frequency oscillations Excellent voltage standoff -> validation of gun redesign / design principles No evidence for parasitical RF (160GHz range) -> n th validation of corrugated beam tunnel All design modifications were verified to a high degree !! S. Kern et al., Experimental results and recent developments on the EU 2 MW 170 GHz coaxial cavity gyrotron for ITER, EC17 Workshop, 07-11 May 2012, Deurne, The Netherlands RF tests of the prototype gyrotron: Summary Unclear observations Unexpectedly high starting currents (~60 A !) Tube prone to operation in wrong mode rotation -> results in high stray radiation -> ultimate cause for broken internal absorber ! Body current -> compare to pre-prototype tests These effects could be related to misalignment ! Nevertheless, the risk of total loss of a tube due to a breaking absorber is inacceptable -> redesign of internal absorber scheme Negative results Unexpected, but acceptable body current Unclear alignment situation Absorber broken, total loss of tube No long pulses were possible !

11. Institute for Pulsed Power and Microwave Technology 1108.05.2012S. Kern et al., Experimental results and recent developments on the EU 2 MW 170 GHz coaxial cavity gyrotron for ITER, EC17 Workshop, 07-11 May 2012, Deurne, The Netherlands Post-test evaluations: Alignment The magnet alignment (ASG magnet at CRPP) could only be checked after the experiment. Results of subsequent check: Magnetic axis is shifted by 0.7 mm in y-direction (check with tube indicating good alignment done in x-direction only !). Additional result: Due to some loose stabilisation rods, the tube can be easily bend by millimeters, resulting in basically undefined alignment ! ->The relative body-beam alignment along y could have been anything between good and bad ! -> Investigations on the influence of tube alignment are needed for evaluation of the observations !

12. Institute for Pulsed Power and Microwave Technology 1208.05.2012 Experiments with the KIT pre-prototype are currently running in short pulse in the following configuration: Further improved launcher design: smoothed launcher surface Electron gun refurbished by TED: New emitter ring New cathode and anode shape, identical to CW prototype -> this includes a small halo shield with ~2mm electron beam clearance Ready for depressed collector operation Gyrotron housing reworked to fit into 220 mm bore hole -> KIT OI magnet was equipped with cooled CW NC coil S. Kern et al., Experimental results and recent developments on the EU 2 MW 170 GHz coaxial cavity gyrotron for ITER, EC17 Workshop, 07-11 May 2012, Deurne, The Netherlands Experimental setup of the pre-prototype

13. Institute for Pulsed Power and Microwave Technology 1308.05.2012 Start / End of the probe measurement Alignment procedure and measurements Emission uniformity test and alignment of coaxial insert before the alignment dR/I dipol =0.1mm/A Shift of the electron beam position using dipole coils Verification of the gyrotron position S. Kern et al., Experimental results and recent developments on the EU 2 MW 170 GHz coaxial cavity gyrotron for ITER, EC17 Workshop, 07-11 May 2012, Deurne, The Netherlands Concentricity of the coaxial insert with respect to the electron beam: δR ~ 0.04mm Concentricity of the electron beam with respect to cavity wall: δR ≤ 0.1 mm -> excellent alignment conditions

14. Institute for Pulsed Power and Microwave Technology 1408.05.2012 Comparison to CW prototype tests: Low starting currents (10A) No unusual danger of operating in wrong rotation Good voltage standoff But: strong LF oscillations during startup – due to gun rear part or small remaining potential traps? Body current at design parameters -> can be avoided by parameter settings -> but calls for investigations: reasons are unclear, halo shield design ? - electron beam radius appears 1.8 mm larger than expected ! Current results of the ongoing short-pulse tests are: RF power:1.9 MW @ 28% efficiency (w/o depr. collector), not optimized Reduced stray radiation: 4% instead of 7% Good beam pattern Next steps: Further conditioning Preparation for depressed collector test Tests of the influence of misalignment S. Kern et al., Experimental results and recent developments on the EU 2 MW 170 GHz coaxial cavity gyrotron for ITER, EC17 Workshop, 07-11 May 2012, Deurne, The Netherlands Latest experimental results of the pre-prototype

15. Institute for Pulsed Power and Microwave Technology 1508.05.2012S. Kern et al., Experimental results and recent developments on the EU 2 MW 170 GHz coaxial cavity gyrotron for ITER, EC17 Workshop, 07-11 May 2012, Deurne, The Netherlands New Launcher / q.o. system measurements The smoothed launcher shows an equally good RF beam pattern as its predecessor. Measured stray radiation is essentially reduced to 4 % of the RF output power Former results: 7 % original KIT design 5.5 % IAP design window thermal image 85 mm from window thermal image 1000 mm from window burned paper spot

16. Institute for Pulsed Power and Microwave Technology 1608.05.2012S. Kern et al., Experimental results and recent developments on the EU 2 MW 170 GHz coaxial cavity gyrotron for ITER, EC17 Workshop, 07-11 May 2012, Deurne, The Netherlands Future plans 2nd Industrial CW prototype: formal decision pending, 1MW decision probable Agreement on necessary modifications on scientific side achieved In view of their future relevance, KIT will continue experiments with coaxial cavity gyrotrons – if necessary on stretched time scales. The KIT short-pulse pre-prototype gyrotron will be sequentially extended for longer pulse lengths. This is made possible through a modular approach which enables an easy exchange of components. Next experimental steps with this “Modular Gyrotron Concept“: (1)Test other components in short pulse: ‚ different launchers, modified electron guns, different beam tunnels (2)Add CW collector and CVD window -> ~100 ms pulse length (3)Replace remaining short pulse components (cavity,q.o. system, beam tunnel, gyrotron housing) by cooled CW parts -> 10 s pulse length In parallel: broad band operation tests (around 140 GHz /1.8MW already done)

17. Institute for Pulsed Power and Microwave Technology 1708.05.2012S. Kern et al., Experimental results and recent developments on the EU 2 MW 170 GHz coaxial cavity gyrotron for ITER, EC17 Workshop, 07-11 May 2012, Deurne, The Netherlands Modular gyrotron setup steps Current step: First depressed collector tests Launcher test CW gun with prototype shape - cathode nose easily exchangeable 2 nd : short pulse component tests Beam tunnel, launcher, anode shape 3 rd : CW collector and window Increase of pulse length to ~ 100 ms 4th: fully CW compatible CW cooling added, new RF absorber scheme New redesigned electron gun Increase of pulse length up to 10s (KIT power supply limit)

18. Institute for Pulsed Power and Microwave Technology 1808.05.2012S. Kern et al., Experimental results and recent developments on the EU 2 MW 170 GHz coaxial cavity gyrotron for ITER, EC17 Workshop, 07-11 May 2012, Deurne, The Netherlands Plans for a hypothetical 2 nd CW prototype Component, topic, observation Internal absorbers, mirror box Launcher, Q.o. system Halo shield Collector Beam tunnel, parasitic modes Tube alignment Modes with wrong rotation High starting currents High body current Better cooling of shaft Any other tube compo- nent Summary Changes Remove and discuss replacement: Preferably two relief windows with or without stainless steel pipes Replace the launcher by the new version, presently under test (lower stray radiation already proven) Increase halo shield radius by 0.5-2mm To be short term optimized No change Revised by TED; possibility for controlled alignment tests added No change No change No change Next step activities cooled stainless steel pipes coated/uncoate d/inside carbon tubes - relief windows - mirror vessel partly or totally coated (CrO, CR 2 O 3 ) with external cooling Consider tube inspection

19. Institute for Pulsed Power and Microwave Technology 1908.05.2012S. Kern et al., Experimental results and recent developments on the EU 2 MW 170 GHz coaxial cavity gyrotron for ITER, EC17 Workshop, 07-11 May 2012, Deurne, The Netherlands Summary The latest test results with EU 2 MW 170 GHz coaxial cavity tubes were reported. CW prototype results: 2 MW / 170 GHz / 45 % non-optimized in very short time All design modifications validated No long pulse validation of RF performance or cooling (except collector) Tube damaged, internal RF absorber scheme needs redesign Short pulse pre-prototype results: 1.9 MW 28 % (without depressed collector, not finally conditioned) Smoothed launcher validated: stray radiation reduced to 4 % Depressed collector operation under preparation Future plans: 2 MW or 1 MW ITER development: F4E decision pending Coaxial experiments towards long pulse continue at KIT

20. Institute for Pulsed Power and Microwave Technology 2008.05.2012S. Kern et al., Experimental results and recent developments on the EU 2 MW 170 GHz coaxial cavity gyrotron for ITER, EC17 Workshop, 07-11 May 2012, Deurne, The Netherlands The authors acknowledge gratefully the continuing support of TED and F4E staff in these projects, in particular F. Albajar, F. Cismondi and T. Bonicelli at F4E as well as R. Marchesin, C. Lievin, F. Legrande and P. Benin at TED. This work was supported by Fusion for Energy under Grant F4E-2009-GRT-049 (PMS-H.CD)-01 and within the European Gyrotron Consortium (EGYC). The views and opinions expressed herein do not necessarily reflect those of the European Commission. EGYC is a collaboration among CRPP, Switzerland; KIT, Germany; HELLAS, Greece; IFP-CNR, Italy. Thank you for your attention ! Acknowledgement

21. Institute for Pulsed Power and Microwave Technology 2108.05.2012 clearance ~ 2mm 4. Body current has been observed at the nominal magnetic field parameters increasing of the magnetic field compression (= reduction of the beam radius ~0.2 mm only) allows to reduce the body current to 0 the investigation of the several magnetic field profiles shows that the problem appears in the region of anode, at the halo-shield (and not at the launcher cut) however, the calculated clearance between the halo-shield is ~2mm, - it indicates that some additional “electron flow” exists outside the regular beam flow / or the thickness of the beam is higher than expected possible reasons: - field emission at the edges in the rear part of the gun - additional electron emission from the regions around the emitter (i.e. due to bad thermal isolation) to avoid the body current flow, a different magnetic field configuration has been proposed - it was necessary to energize of the bucking-coil separately from the main coil Experimental results with the modular gyrotron S. Kern et al., Experimental results and recent developments on the EU 2 MW 170 GHz coaxial cavity gyrotron for ITER, EC17 Workshop, 07-11 May 2012, Deurne, The Netherlands

22. Institute for Pulsed Power and Microwave Technology 2208.05.2012 4. Body current Body current vs. beam current - strong, linear dependence - body current relatively high, about 3% of the current of the electron beam! Body current vs. cathode voltage - nearly linear dependence, it means: no field-emission effect Experimental results with the modular gyrotron S. Kern et al., Experimental results and recent developments on the EU 2 MW 170 GHz coaxial cavity gyrotron for ITER, EC17 Workshop, 07-11 May 2012, Deurne, The Netherlands