European GYrotron Consortium (EGYC)

1. Experimental results and recent developments on the EU 2 MW 170 GHz coaxial cavity gyrotron for ITER S. Kern1, J.-P. Hogge2, S. Alberti2, K. Avramides3, G. Gantenbein1, S. Illy1, J. Jelonnek1, J. Jin1, F. Li2, I. Gr. Pagonakis1, B. Piosczyk1, T. Rzesnicki1, M. K. Thumm1, I. Tigelis4, M. Q. Tran2and 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 2Centre de Recherche en Physique des Plasmas (CRPP), Association Euratom-Confédération Suisse,EPFL, CH-1015 Lausanne, Switzerland 3National Technical University of Athens (part of HELLAS), School of Electrical and Computer Engineering, 9 Iroon Polytechniou st., GR15773 Athens, Greece 4National and Kapodistrian University of Athens (part of HELLAS), Faculty of Physics, 157 84 Athens, Greece. European GYrotron Consortium (EGYC)

2. 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 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

3. 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 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

4. 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 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

5. 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) ! 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

6. 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. 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

7. 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 xdirection only) -> The tube was formally accepted. Green light to proceed with RF tests was given by TED on December 2nd 2011. 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

8. 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. 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

9. RF beam pattern of the prototype Comparison Mode TE 34,19 Mode TE 35,19 ? refurbished prototype Simulation pre-prototype 96 % GC Logbook shot #10410, 2011-12-05 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

10. RF tests of the prototype gyrotron: Summary Negative results • Unexpected, but acceptable body current • Unclear alignment situation • Absorber broken, total loss of tube No long pulses were possible ! 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)-> nth validation of corrugated beam tunnel All design modifications were verified to a high degree !! Unclear observations • Unexpectedly high startingcurrents (~60 A !) • Tube pronetooperation in wrongmoderotation-> results in high strayradiation-> ultimatecauseforbrokeninternalabsorber ! • Body current -> comparetopre-prototype tests These effectscouldberelatedtomisalignment ! Nevertheless, theriskof total lossof a tubedue to a breakingabsorberisinacceptable-> redesignofinternalabsorberscheme 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

11. 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 ycould have been anything between good and bad ! -> Investigations on the influence of tube alignment are needed for evaluation of the observations ! 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

12. Experimental setup of the pre-prototype Experiments withthe KIT pre-prototype arecurrentlyrunning in short pulse in thefollowingconfiguration: • Further improvedlauncher design: smoothedlaunchersurface • Electrongunrefurbishedby TED: • New emitter ring • New cathodeandanodeshape, identicaltoCW prototype-> thisincludes a smallhaloshieldwith ~2mm electron beam clearance • Readyfordepressedcollectoroperation • Gyrotron housingreworkedto fit into 220 mm bore hole-> KIT OI magnet was equippedwithcooled 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

13. Alignment procedure and measurements • Shift of the electron beam position using dipole coils dR/Idipol=0.1mm/A Emission uniformity test and alignment of coaxial insert before the alignment measurement Start / End of the probe Verification of the gyrotron position • 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 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

14. Latest experimental results of the pre-prototype Next steps: • Further conditioning • Preparation for depressed collector test • Tests of the influence of misalignment 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 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

15. 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 to4 % of the RF output powerFormer results:7 % original KIT design5.5 % IAP design burned paper spot window thermal image 85 mm from window thermal image 1000 mm from window 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

16. 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“: • Test other components in short pulse: ‚different launchers, modified electron guns, different beam tunnels • Add CW collector and CVD window -> ~100 ms pulse length • 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) 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

17. Modular gyrotron setup steps • Current step: • First depressed collector tests • Launcher test • CW gun with prototype shape- cathode nose easily exchangeable • 2nd: short pulse component tests • Beam tunnel, launcher, anode shape • 3rd: 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) 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

18. Plans for a hypothetical 2nd CW prototype 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

19. 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 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

20. Acknowledgement 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 ! 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

21. Experimental results with the modular gyrotron • has been observed at the nominal magnetic field parameters 4. Body current • 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 appearsin 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 clearance ~ 2mm • 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 • 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) 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. Experimental results with the modular gyrotron • Body current vs. cathode voltage- nearly linear dependence, it means: no field-emission effect 4. Body current • Body current vs. beam current- strong, linear dependence- body current relatively high, about 3% of the current of the electron beam! 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