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johnf

Fri May 31 2013, 07:42AM

Registered Member #230 Joined: Tue Feb 21 2006, 08:01PM
Location: Gracefield lower Hutt
Posts: 284

I did this a few years back to switch C12,C13,C14 beams in our accelerator for a 90 degree wein type filter all of this to protect the stripper foils in the terminal from the huge C12 signal during carbon 14 dating
C12 beam for a few uS C13 beam for a few mS and the rest of the 2 second period for the C14 beam.
From memory c13 @ 0 volts C12 @ -10kv and c14 @ +14kV

I usued Belke fast switches and by matching the source impedance to 200 ohms and making my own 200ohm impedance high voltage coax.
Voltage settled to within 0.1% in 4uS after each switching event

These switches are not cheap but very good
the switches were setup in a totem pole configuration and capacitive load was around 1000pf
making for around 15 amps flowing during switching.
Even RG213 makes noise when hit this hard and ceramic caps I used during testing sang very loudly.

Linas

Fri May 31 2013, 09:48AM

Registered Member #1143 Joined: Sun Nov 25 2007, 04:55PM
Location: Vilnius, Lithuania
Posts: 721

I don't exactly need a switch, i need to be able to control output voltage to 5pF load.

Maybe some one know higher voltage BJT/FET/IGBT with faster than 84ns total transition time ?

Ash Small

Fri May 31 2013, 12:22PM

Registered Member #3414 Joined: Sun Nov 14 2010, 05:05PM
Location: UK
Posts: 4245

Linas wrote ...

Maybe some one know higher voltage BJT/FET/IGBT with faster than 84ns total transition time ?

I think these are fast enough. I think these are only 1200V, but higher voltages are available:

turn on delay of 8.8nS, a fall time of 21nS, a turn off delay of 38nS and a rise time of 34nS

They are ~Â£20 from Farnell, probably cheaper elsewhere.

Steve Conner

Fri May 31 2013, 01:47PM

Registered Member #30 Joined: Fri Feb 03 2006, 10:52AM
Location: Glasgow, Scotland
Posts: 6706

They are lower voltage than the SiC JFETs Linas already found.

The SRPP circuit

was designed for vacuum tubes, but it might work with SiC JFETs.

...

Fri May 31 2013, 01:49PM

Registered Member #56 Joined: Thu Feb 09 2006, 05:02AM
Location: Southern Califorina, USA
Posts: 2445

This application screams vacuum tube, I made something similar with an 811a powered by a simple mazzilli driver a few years back.

As a side note, its worth mentioning that in an application like this people usually use an AOM, they are much faster (1MHz is no problem for a modern AOM), using the transmitted (not diffracted, as is commonly used) beam you can get lower insertion loss than an pockels to boot.

Linas

Fri May 31 2013, 02:25PM

Registered Member #1143 Joined: Sun Nov 25 2007, 04:55PM
Location: Vilnius, Lithuania
Posts: 721

Steve Conner wrote ...

They are lower voltage than the SiC JFETs Linas already found.

Already has

... wrote ...

As a side note, its worth mentioning that in an application like this people usually use an AOM, they are much faster (1MHz is no problem for a modern AOM), using the transmitted (not diffracted, as is commonly used) beam you can get lower insertion loss than an pockels to boot.

My first approach to my problem was by AOM, but it has lot of problem, since i have ultra-short (fs range) laser pulse ( with high spectral width) and doe to diffraction my pulse will spread by frequencies ,and i will not gonna be able to assemble that pulse. Pockels cell is very good choice, but it changes my polarization, i don't know how much, but it can kill my SHG and my PID controller will stop working, resulting fatal fail.

Ash Small

Fri May 31 2013, 02:28PM

Registered Member #3414 Joined: Sun Nov 14 2010, 05:05PM
Location: UK
Posts: 4245

Steve Conner wrote ...

They are lower voltage than the SiC JFETs Linas already found.

Yes, and there isn't much difference in switching times. I'd also suggest using valves (tubes).

Proud Mary

Fri May 31 2013, 09:55PM

Registered Member #543 Joined: Tue Feb 20 2007, 04:26PM
Location: UK
Posts: 4992

E. S. Fulkerson. D. C. Norman. R. Booth Driving Pockels Cells Using Avalanche Transistor Pulsers.

Proceedings of the 16th International Workshop on Laser Ranging. Fast Switching Pockels Cell Driver for SLR Laser System. J. KÃ¶lbla, M. FrÃ¶schla

A Solid-State, Inductive-Adder, 10-kV Pulse Generator for Driving Large-Aperture Pockels Cells

Nanoseconds Switching for High Voltage Circuit Using Avalanche Transistors

johnf

Sat Jun 01 2013, 02:10AM

Registered Member #230 Joined: Tue Feb 21 2006, 08:01PM
Location: Gracefield lower Hutt
Posts: 284

Linas
The two supplies in what I did were controlled so that the voltage was variable. but there is no supply on earth that can shift the voltage that quickly that is why i used switches. In my application I had to hold the voltage within a volt for the velocity filter to pass the required beam.
I have used lesser variants of this system for pockel cells and Q switches

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