Development in Russia of Megawatt Power Gyrotrons for Fusion ITR/1-4Ra Development in Russia of Megawatt Power Gyrotrons for Fusion A.G.Litvak1, G.G.Denisov1, V.E.Myasnikov2, E.M.Tai2,E.V. Sokolov, V.I.Ilin3 1Institute of Applied Physics, Nizhny Novgorod, Russia 2GYCOM Ltd., Nizhny Novgorod, Russia 3Tokamak Physics Institute, Kurchatov Center, Moscow, Russia Recent results in development of RF gyrotron for ITER Development of multi-frequency gyrotrons Recent results in development of JAEA gyrotron for ITER Summary (ITR/1-4Ra and FTP-1/3Rb)

The main specifications of the gyrotrons for ITER are described below: Item Specification Nominal output power  0.96 MW at MOU output Nominal frequency 170±0.3 GHz (TBD) including initial transient phase Pulse length 3600 sec (TBD) RF power generation efficiency  50 % (with collector potential depression) Gaussian content > 95 % at output waveguide (63.5 mm) of MOU Power modulation 1 kHz (cathode); 5 kHz (anode) For more details see Technical specifications (https://user.iter.org/?uid=4GV66L)

Russian 170 GHz ITER Gyrotron. Design features. Gyrotron cavity designed for TE25.10.1 mode with specific wall loading 2kW/cm2 at 1MW Diode type electron gun forms the electron beam with optimal size, up to 50A / 70-80kV Synthesized Built-in quasi-optical converter .Gaussian mode content over 95% at stray radiation less than 5% Main output window is based on CVD diamond disk of 106-mm diameter with 88-mm clear aperture. Relief window uses ceramics (BN or AlN) disk of 123-mm diameter to transmit 40 kW. Depressed-collector with longitudinal beam sweeping is capable to withstand 1-MW electron beam. Enhances gyrotron efficiency over 50%. DC break insulator upon cryomagnet top flange. C8F20 fluorocarbon as a coolant . LHe-free cryomagnet has a bore diameter of 160 mm. Gyrotron inner surfaces are fabricated from copper with water cooling for CW operation. Gyrotron total height is 2.7m. Gyrotron weight is about 300+ kg.

Achievement of ITER relevant parameters with RF gyrotron In the last five years four gyrotron prototypes were fabricated and tested. Gyrotrons V-10, V-11 were tested in 2010 and 2011 respectively with CRYOMAGNETICS LHe –free magnet. It is important to note that two last gyrotrons (V-10 and V-11) demonstrate very similar output parameters (see Table below). Gyrotron V-11 at the test stand in Kurchatov instituite

** Demonstrated serial pulses Summary of test results attained with V-10 and V-11 gyrotrons Gyrotron Beam voltage kV Beam current A Retarding voltage Power * kW Efficiency % Pulse duration sec V-10 V-11 71 70 34 39.5 30.5 30 ~750 ~850 ~54 ~53 1000 600 ** 45 31.5 ~960 ~55 400 (serial pulses) 70.5 * Power measured at MOU output and it is approximately 5% less than gyrotron output power ** Demonstrated serial pulses

ITER-relevant repetitive operation As for a reliability test, 0.8MW/600s shots were requested in every 50 minutes for 3 days in 12-14 of April 2011. Only 3 pulses of 30 were interrupted by some reasons. Ten pulses were made in presence of IO representatives. Ub, kV Urec, Ib, A I_m, А t_req, s I_g.c t_pulse, s Date Regime Pulse stop 70,9 30 34 82,65 600 1,5 13.04.2011 Operating_800kW Regular 71 510 Cut-off 70.8 14.04.2011 1.5

0.8MW/34A/600s serial pulse of V-10 gyrotron

terminal load collector cavity Calorimetry in the 1MW/1000s pulse for gyrotron V-11: blue curve- terminal load, pink - collector, green – cavity.

Hence the cooling system allows power increase. Hard structure and intense cooling of a gyrotron cavity provide good stability of operating frequency. Total frequency drift for 1 MW operation slightly exceeds 100MHz. After ~1.5s frequency is practically stable with very small deviations of about 10MHz. Hence the cooling system allows power increase. 170 GHz gyrotron, TE25.10 100 MHz

1.2 MW/ 100 s operation of V-11, 170 GHz gyrotron, TE25.10 (1.2 MW at MOU output). Cavity and collector survived in long pulses. Electron beam (43.4 + 29.6 ) kV 53 A 2300 kW Calorimeters, kW: Main load -1150 Pre-load - 25 MOU - 35+15 ( refl.) Relief wind. - 40 Collector – 1080  2345

TE28.12 mode gyrotron model. 100-µs/5-10Hz tests. OLD & NEW QO CONV. 1.5 MW CW compatible Long pulse tests planned for Oct.-Nov. 2012

indices at each radial index IAP RAS GYCOM Frequency tuning/switching in MW gyrotrons One mode Magnetic field or voltage tuning Azimuth index change Magnetic field and voltage optimization Series of azimuth indices at each radial index Typical frequency tuning ~ 0.1% Typical frequency step 2-3%, 3-4 steps About 10 steps in 30% frequency range FADIS Multi-purpose ECW systems

Two- and multi-frequency gyrotrons High-eff. mode converter IAP RAS GYCOM IAP/GYCOM experience in MW power level multi-frequency gyrotrons Frequency, GHz Power Pulse, sec Note Two- and multi-frequency gyrotrons 105 / 140 0.7 - 0.9 MW 10 4 tubes delivered to ASDEX-Up 147 / 170 0.7/1.0 MW 0.1 Tested with BN window 100-150 1.2-1.5 MW 10-4 Short –pulse mock-up, 6 frequencies, high-eff. converter 0.7-0.8 Short-pulse mock-up, 11 frequencies, BN Brewster window 105 -140 0.7 – 0.9 MW 0.1 (10) 4 frequencies High-eff. mode converter 71.5 / 74.8 / 78.1 0.8 MW 3 frequencies, 56% eff.

Design requirements for the multi-frequency gyrotron for ASDEX Upgrade Example: Design requirements for the multi-frequency gyrotron for ASDEX Upgrade Frequency range (at least 4 frequencies) 105 – 140 GHz Output power: at frequency 140 GHz 1 MW at frequency 105 GHz 0.8 MW at intermediate frequencies 0.8 MW Pulse duration 10 s Efficiency with energy recovery 40-50% Output radiation Gaussian beam 7 years of common (with FZK, IPP, IPF) development

+ ? Multi-frequency gyrotron. Main problems. IAP RAS GYCOM Multi-frequency gyrotron. Main problems. Effective gyrotron operation at different modes Effective conversion of all modes into Gaussian beam Tuneable or broadband window + ?

One-disc CVD diamond windows for gyrotrons Small dielectric loss e-o thermal conductivity Widely used for conventional single/double frequency gyrotrons; 1.8 mm thick => 4 half-wavelength at 140 GHz and 3 h-w at 105 GHz

POSSIBLE WINDOW TYPES FOR A MULTI-FREQUENCY GYROTRON Advantage Drawback Double- disc Clear concept Two discs Disturbed gyrotron operation due to narrow band of low reflection Brewster, circular Wide instant band High field near the disc Vacuum duct Thicker disc Brewster, elliptical Simple scheme Poor transmission characteristics Corrugated matched surface Broad instant band Expensive fabrication Worse mechanical stability Travelling wave resonator Zero reflection Easy tuning

Novel window concept - travelling wave resonator. IAP RAS GYCOM Novel window concept - travelling wave resonator. (Recollected experiment in 1991 & recent FADIS experiments) θ θ At the resonance frequency kL = 2πq (q = 1, 2, 3…) the reflecting power from the cavity is equal zero: |R|2 = 0, |T|2 = 1 , |A|2 = 1/t2 Mirror cavity with a traveling wave. W1 and W2 are CVD diamond disks. T, R and A are amplitudes of transmitted, reflected and traveling wave beams.

IAP RAS GYCOM Quality factors of the ring resonator: ~ 10 000 for FADIS (QFADIS >> Q gyr cavity ) < 500 for the window Possible trajectories for the window ring cavity and 3D drawing for the second option

IAP RAS GYCOM Relief window Calorimetric load Drawing for the MF gyrotron assembly with window. Standard 106 mm disc.

105 GHz (a), 117 GHz (b), 127GHz (c) and 140 GHz(d) gyrotron operation IAP RAS GYCOM 1 MW/10 sec MF gyrotron for ADEX-Upgrade has been fabricated 2-frequency tests in June, 2012 – 0.9 MW/140 GHz; 0.8MW/105 GHz 4 -frequency tests (initial in June 2012 (beam measurements), final in early 2013) Field structures (wave beam amplitude and phase spatial distributions) for 105 GHz (a), 117 GHz (b), 127GHz (c) and 140 GHz(d) gyrotron operation

170GHz Gyrotron Development in JAEA TE31,8 mode gyrotron • 1MW/800s • 0.8MW/1hr operation • Max. efficiency: ~60% • Total output energy: >250GJ • For power increase, higher mode oscillation has been challenging TE31,11 mode gyrotron (J7) (Multi-frequency operation, >1.3MW operation ) • 5kHz power modulation is improved using novel anode switching 170GHz gyrotron

Initial experimental results 170GHz / 137GHz (TE31,11/TE25,9) 170GHz high power test 0 1 2 3 4 5 Time [s] Pressure Vbody =28.5kV Vanode=-6.8kV Vmain =-46kVkV Ibeam =51.6A RF(170GHz) ~1.1MW (45.4%) Pulse duration : 0.5msec 1.3MW output was demonstrated at both frequencies Stable 1.1MW / 5s oscillation was obtained.

Multi- Frequency Gyrotron Beam profiles at output window Experimental Result Calculation 203GHz 170GHz 137GHz 104GHz (optional) (optional) RF beams of three frequencies pass through the center of the window.

Long Pulse Experiment (170GHz/137GHz) 0 25 50 75 Time[s] 0 10 20 Time[s] 170GHz / 31,11 0.91MW (75s) 137GHz / TE25,9 0.54 MW (20s) Long pulse operation was demonstrated at both frequencies (under conditioning for further performance)

5 kHz power modulation (Single anode switch) Vbody Vanode Vcathode 60sec Ibeam ITER target 5kHz/50s (0.5MW-1MW) mod. Attained 5kHz/60s Full 1.1MW mod. 50.5A RF (1.1MW) 60s 1ms

5kHz Power modulation using double anode switch Single switch 0 20 30 40 50 60 70 80 Time[msec] Single switch Double switch Improved Vanode-cathode RF Lower frequency excitation at start-up phase was improved by introducing the double anode switch. Double switch

Summary RF The main ITER requirements to gyrotron performance have been demonstrated: 1MW power, 1000 seconds pulse duration, 53% efficiency. Reliability tests were performed. The gyrotron operation regime of 1.2 MW was found for 100 second pulses. The gyrotron prototype for enhanced power of 1.5 MW was designed, fabricated and successfully tested in short pulses. Testing of the multi-frequency gyrotron for ASDEX Upgrade with a new tunable window is in progress. Short pulse (0.1 sec) tests showed good operation at chosen four frequencies. Gaussian mode content in the output wave beam is high. In 3 second pulses the gyrotron showed 0.9 MW at 140 GHz and 0.8 MW at 105 GHz. The gyrotron was delivered to ASDEX Upgrade in July 2012 and first tests at the site are planned for October 2012. JA The dual frequency 170 GHz/137 GHz gyrotron with triode electron gun is successfully designed and tested. In the short pulse experiments (<1 ms), more than 1.3 MW is generated for both frequencies. The long pulse experiments show 905 kW/45%/75 s for 170 GHz and 540 kW/42%/20 s for 137 GHz. The 5 kHz modulation achieves the 1.16MW with 48% electrical efficiency by using the short-circuited switch between the anode and the cathode. In order to improve the rising time of the cathode-anode voltage, the double switch conjuration is introduced. The fast frequency change by using the SCM with the fast sweeping coil inside the bore for quick magnetic field change is successfully demonstrated. A 3 GHz frequency change is succeeded within 3.5 s.