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. Two key elements in the Scandinova technology
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.  Weekly conference call with Scandinova proposed
Modulator procurement follow-up
Three reviews in 2009
Visit at PSI Villigen in Dec 09:
Review of problems met on similar S band Scandinova modulator
Meeting at Scandinova in Jan 10:
X band PSI modulator shown
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 => W 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 development strategy
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 Reflection
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 dB reflection and dB transmission at GHz giving 98.6% conversion efficiency in power
Hsurfmax=162 kA/m with 150MW input power at GHz giving DT=5.8° with 250 ns pulse length
Esurfmax=52.5MV/m with 150MW input power at GHz

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 mm.
The reflection coefficients S11 (mode n°3) and S22 (mode n°1) are respectively dB and dB and the transmission coefficient S12 from the TE01o mode to the TE10□ mode is dB (99.8% transmission in power) which is very satisfactory.
Esurfmax=37.2 MV/m with 150MW input power at GHz
TE10□
WR90
TE20□
Elliptical profile
TE01O
F=36.27mm
Mechanical design under progress
Half of the structure is represented
Total length = 175 mm

8. Magnetic field on the valve surface at 11.9942 GHz for 1 W injected
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 GHz for 1 W injected

9. Electric field at 11.9942 GHz for 1 W injected
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 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 GHz for 1 W injected
The reflection is less than -30 dB between and GHz, which gives a bandwidth of 360 MHz

10. Future work proposed… 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 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 Components Status
90° mode converter
2 ordered + 2 to be launched
RF Valve
2 ordered
36/50 circular taper
4 ordered
Bayonet mode converter
RF design ready
Mechanical design in progress
90°circular waveguide bend
Under RF design
Double height hybrid
Study to be started
+ Collaboration with TRIESTE