F. Peauger – 17/03/10

Modulator and overmoded RF

components status for the 12GHz Test

Stand at CERN

-RF meeting – 17/03/2010

F. Peauger – 17/03/10 2

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

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

The High Voltage Modulator System

HV-DCPSOil Tank

Switch unitsKlystron

F. Peauger – 17/03/10 3

•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.

•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

Two key elements in the Scandinova technology

Pulse transformer

F. Peauger – 17/03/10 4

Modulator procurement follow-up

Weekly conference call with Scandinova proposed

“ 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”

F. Peauger – 17/03/10

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

F. Peauger – 17/03/10 6

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

Esurfmax=52.5MV/m

with 150MW input power at 11.9942GHz

Hsurfmax=162 kA/m

with 150MW input power at 11.9942GHz giving T=5.8° with 250 ns pulse length

Bandwidth of 150 MHz @ -20dB reflection and -0.0618 dB transmission at 11.994 GHz giving 98.6% conversion efficiency in power

=36.27mm

Reflection

90° mode converter

F. Peauger – 17/03/10 7

Esurfmax=37.2 MV/m

with 150MW input power at 11.9942GHz

=36.27mm

Half of the structure is represented Total length = 175 mm

TE10□

TE01O

TE20□

Elliptical profile

WR90

New « bayonet » mode converter

Mechanical design under

progress

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 TE01

o 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 TE01

o mode to the TE10□ mode is -

0.00708 dB (99.8% transmission in power) which is very satisfactory.

F. Peauger – 17/03/10 8

The RF valve has been introduced in the CTF3 30 GHz test stand. It works on the circular mode TE01

o 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

RF Valve

F. Peauger – 17/03/10 9

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

36 mm/50 mm Circular Taper

F. Peauger – 17/03/10 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 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

F. Peauger – 17/03/10 11

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 readyMechanical design in progress

90°circular waveguide bend Under RF design

Double height hybrid Study to be started

RF components status

+ Collaboration with TRIESTE