Experimental results and recent developments on the EU 2 MW 170 GHz coaxial cavity gyrotron for ITER

Transcription

1 Experimental results and recent developments on the EU 2 MW 170 GHz coaxial cavity gyrotron for ITER 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 KARLSRUHE INSTITUTE OF TECHNOLOGY, (IHM) 1 Karlsruhe Institute of Technology (KIT), (IHM), Association EURATOM-KIT, D 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, Athens, Greece. European GYrotron Consortium (EGYC) KIT University of the State of Baden-Wuerttemberg and National Research Center of the Helmholtz Association

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

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 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

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

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: % efficiency (w/o depr. collector)! S. Kern et al., Experimental results and recent developments on the EU 2 MW 170 GHz coaxial

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 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

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 x direction only) -> The tube was formally accepted. Green light to proceed with RF tests was given by TED on December 2 nd S. Kern et al., Experimental results and recent developments on the EU 2 MW 170 GHz coaxial

8 RF tests of the prototype gyrotron (Dec ) 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 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

9 RF beam pattern of the prototype Mode TE 34,19 Mode TE 35,19? Logbook shot #10410, S. Kern et al., Experimental results and recent developments on the EU 2 MW 170 GHz coaxial

10 RF beam pattern of the prototype Mode TE 34,19 Comparison Simulation pre-prototype 96 % GC refurbished prototype S. Kern et al., Experimental results and recent developments on the EU 2 MW 170 GHz coaxial

11 RF tests of the prototype gyrotron: Summary Positive results Negative 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 Unexpected, but acceptable body current Unclear alignment situation Absorber broken, total loss of tube No long pulses were possible! Unclear observations 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!! 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 S. Kern et al., Experimental results and recent developments on the EU 2 MW 170 GHz coaxial

12 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! S. Kern et al., Experimental results and recent developments on the EU 2 MW 170 GHz coaxial cavity gyrotron for ITER, EC17 Workshop, May 2012, Deurne, The Netherlands

13 Experimental setup of the pre-prototype 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

14 I Xdipol / A before the alignment Alignment procedure and measurements Shift of the electron beam position using dipole coils Emission uniformity test and alignment of coaxial insert measurement 10 5 dr/idipol=0.1mm/a Verification of the gyrotron position Start / End of the probe Iy I Ydipol / A 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

15 P RF / MW efficiency / % Latest experimental results of the pre-prototype Current results of the ongoing short-pulse tests are: RF power:1.9 28% efficiency (w/o depr. collector), not optimized Reduced stray radiation: 4% instead of 7% Good beam pattern 50 2,0 45 1,8 40 1,6 35 1,4 30 1,2 1,0 0,8 0, U C / kv S. Kern et al., Experimental results and recent developments on the EU 2 MW 170 GHz coaxial

16 Latest experimental results of the pre-prototype 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! S. Kern et al., Experimental results and recent developments on the EU 2 MW 170 GHz coaxial

17 Latest experimental results of the pre-prototype Next steps: Further conditioning Preparation for depressed collector test Tests of the influence of mislalignment S. Kern et al., Experimental results and recent developments on the EU 2 MW 170 GHz coaxial

18 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 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

19 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) S. Kern et al., Experimental results and recent developments on the EU 2 MW 170 GHz coaxial

20 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) S. Kern et al., Experimental results and recent developments on the EU 2 MW 170 GHz coaxial

21 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 component 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 No change Revised by TED; possibility for controlled alignment tests added No change No change 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 S. Kern et al., Experimental results and recent developments on the EU 2 MW 170 GHz coaxial

22 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

23 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