Modulator and overmoded RF components status for the 12GHz Test Stand at CERN -

1. Modulator and overmoded RF components status for the 12GHz Test Stand at CERN - RF meeting – 17/03/2010

2. The High Voltage Modulator System The modulator is a high voltage pulsed power supply fabricated by Scandinova (Sweden). It delivers high voltage pulses to the XL5 klystron with maximum amplitude of 450 kV – 335 A and a pulse length of 1.5 µs (2.3 µs FWHM) at 50 Hz Switch units Klystron HV-DCPS Oil Tank The modulator includes the main HV-DCPS, the oil tank with a special pulse transformer made by Stangenes, some solid state switches, the three power supplies for the magnet, one DC power supply for klystron filament heating, five ion pump power supplies and a command/control system. It includes also a hydraulic network, a 380 V - 50 Hz 3-phase line electrical distribution network and some racks containing all the elements

3. Twokeyelements in the Scandinovatechnology • Pulse transformer • The particularity is the use of multiple magnetic cores on the primary side. The transformer is connected to an energy storage capacitor. To avoid the droop when discharging the capacitor, RL tuning circuits are mounted near the transformer. There is also a core bias transformer. The ion pump at the klystron gun is connected to an external DC power supply via a separate winding. Pulse transformer • Solid state switch • Each transformer core is driven by low voltage solid states switches called IGBT operating at 1kV-15A and connected in parallel . The IGBTs come Infineon –U-Pack. They are specified to 1.7kV-32A but operate with margin to get more reliability. There are 72 connections on the primary side of the transformer. A Swith Unit (SU) includes 6 IGBT. In case of a default of one IGBT, the system can continue to work at a lower HV. Each switch can dissipate up to 25 kW at 200 Hz and are cooled by a water circuit. Switch Unit including 6 IGBT’s

4. Modulatorprocurementfollow-up • Threereviews in 2009 • Visitat PSI Villigen in Dec 09: • Review of problems met on similar S band Scandinovamodulator • Meeting atScandinova in Jan 10: • X band PSI modulatorshown • Status of the mechanical feature of elements presented • interfaces discussed (SLAC klystron, water cooling, command control…) • Technical report in Feb 10 • mechanical simulation of klystron lifting device • Schedule: • - Factory Acceptance Test => W12 • - Delivery at CERN => W13 • - Installation => W14-W15 • - Start up/System testing => W16-W17 • - Final Acceptance Test at CERN => W18 • Schedule: • - Factory Acceptance Test => W12 => FAT cancelled • - Delivery at CERN => W13 last week • - Installation => W14-W15 • - Start up/System testing => W16-W17 • - Final Acceptance Test at CERN => W18 “ Assembly not yet finished due to late shipping of the DC Power Supply (should be delivered next week)” “ Arcing occurred in the tank during the testing of the PSI modulator… oil insulation under investigation”  Weekly conference call with Scandinova proposed

5. RF components developmentstrategy • RF design by CEA • HFSS • Scaling of existing SLAC components or new design based on SLAC experience and publications • Mechanical design and drawings by CERN • Tolerance of 20 µm • Internal surface roughness of Ra=0.1 • Precise machining of elementary pieces by CEA • Done in french firms • Cleaning, coating, brazing and re-machining by CERN • Vacuum brazing at high temp (>780°C) • Hydrogen treatment to be investigated

6. 90° mode converter The mode converter is made in two parts. The first part is a rectangular waveguide bend on H plane. It converts the TE10□ mode into a TE20□ mode. The second part is a circular waveguide with two posts positioned at 180 ° at a certain distance of the rectangular section. This design is based on an original idea of S. Kazakov. It is compact and relatively easy to fabricate Reflection F=36.27mm Bandwidth of 150 MHz @ -20dB reflection and -0.0618 dB transmission at 11.994 GHz giving 98.6% conversion efficiency in power Hsurfmax=162 kA/m with 150MW input power at 11.9942GHz giving DT=5.8° with 250 ns pulse length Esurfmax=52.5MV/m with 150MW input power at 11.9942GHz

7. New « bayonet » mode converter This second kind of mode converter is also composed of different independent sectors. The first part is a rectangular waveguide on H plane with an enlarged section and shifted transversally compared to the WR90 flange axis. The small side of the waveguide is increased with a smooth taper and the TE10□ mode is transformed into a TE20□ mode. The second part is a regular transition between the rectangular section and an elliptical section first, and then from the elliptical profile to a circular one to obtain a TE01o mode in waveguide diameter of 36.27 mm. The reflection coefficients S11 (mode n°3) and S22 (mode n°1) are respectively -39.5 dB and -40.8 dB and the transmission coefficient S12 from the TE01o mode to the TE10□ mode is -0.00708 dB (99.8% transmission in power) which is very satisfactory. Esurfmax=37.2 MV/m with 150MW input power at 11.9942GHz TE10□ WR90 TE20□ Elliptical profile TE01O F=36.27mm • Mechanical design under progress Half of the structure is represented Total length = 175 mm

8. RF Valve The RF valve has been introduced in the CTF3 30 GHz test stand. It works on the circular mode TE01o mainly to avoid surface electric field and have steps in diameter to “focus” the wave in the center of the guide. Based on the same principle, RF valves working at 11.4 GHz have also been developed at SLAC for accelerating structure testing. The 12 GHz RF valve is a scaled version of the SLAC one. 19 dB Magnetic field on the valve surface at 11.9942 GHz for 1 W injected

9. 36 mm/50 mm Circular Taper A circular taper is needed to connect the existing circular waveguides (50 mm diameter) with the RF valve and the mode converter calculated previously (with a diameter of 36.27 mm). It is composed of different steps of diameters with various lengths optimized for matching on a large bandwidth. It has been scaled from part of the 3 GHz power phase shifter. Electric field at 11.9942 GHz for 1 W injected The reflection is less than -30 dB between 11.82 and 12.18 GHz, which gives a bandwidth of 360 MHz

10. Future workproposed… • Double height 3dB - 90° hybrid power splitter: • - it equally divides the power into two arms while keeping a low RF reflection to the source • the pulse compressor contains a 3 dB hybrid splitter with constant height of 10.16 mm • > a double height splitter may reduce the risk of RF breakdown • Also a second splitter would be needed in the CTF2 test structure area in order to power two structures at the same time • 90° circular waveguide bend: • - it bends efficiently the circular TE01o mode at 90°, which simplify the RF layout

11. RF components status + Collaboration with TRIESTE