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4hv.org :: Forums :: High Voltage
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nanosecond pulses

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Microwatt
Wed Feb 16 2011, 05:30PM
Microwatt Registered Member #3282 Joined: Wed Oct 06 2010, 05:01PM
Location:
Posts: 224
Do you think that using a parabolic axis for the blumlien might make for even sharper pulses?
1297877404 3282 FT108659 Godard1
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cedric
Wed Feb 16 2011, 05:54PM
cedric Registered Member #2941 Joined: Fri Jun 25 2010, 08:08AM
Location:
Posts: 143
Microwatt wrote ...

Do you think that using a parabolic axis for the blumlien might make for even sharper pulses?
1297877404 3282 FT108659 Godard1

it's not really related ,this specific blumen was design to have an population inversion preceding the laser pulse ,if you want a sharp pulse ,you need a sharp switch...
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vircator
Thu Feb 17 2011, 07:45AM
vircator Registered Member #3217 Joined: Mon Sept 20 2010, 06:14AM
Location: Switzerland
Posts: 15
cedric wrote ...
it's not really related ,this specific blumen was design to have an population inversion preceding the laser pulse ,if you want a sharp pulse ,you need a sharp switch...
Or something forming your pulse into a sharp one... for example a Blumlein (thats how it's spelled) pulse forming network wink
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cedric
Thu Feb 17 2011, 09:51AM
cedric Registered Member #2941 Joined: Fri Jun 25 2010, 08:08AM
Location:
Posts: 143
vircator wrote ...

cedric wrote ...
it's not really related ,this specific blumen was design to have an population inversion preceding the laser pulse ,if you want a sharp pulse ,you need a sharp switch...
Or something forming your pulse into a sharp one... for example a Blumlein (thats how it's spelled) pulse forming network wink

sorry for the spelling..
,using a solid states witch in blumlein configuration don't provide a better rise time than the one you get from a (large enough) capacitor discharge in a low impedance load,to my understanding the blumlein is made from two identical transmission line and the use of it is to provide a square pulse,in effect this square pulse is always a trapeze as no switch switch on instantaneously.

the picture presented before can be seen as a blumlein made of two strip-line,if both side would be equal ,the load would be match and the switch would be perfect it would deliver a square pulse.
there is other technique to sharpen a pulse namely:magnetic pulse compression,nonlinear capacitor,sos diode,sharpening gap and probably some other...
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cedric
Thu Feb 17 2011, 12:48PM
cedric Registered Member #2941 Joined: Fri Jun 25 2010, 08:08AM
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Posts: 143
iJim wrote ...

If you want a something fast and highly repetitive then you could try a 'corona stabilised switch', these are like spark-gaps but are capable of switching at rep rates approaching 10kHz. They were developed several years ago at Strathclyde Univ and have received renewed interest recently; pretty much because they are relatively simple to design and use. I've worked with them in the past switching around 30kV 2kA @ 2kHz... they work, but I'm a little sceptical about the proposed mechanism of 'corona stabilisation'.

A reference:
J A Harrower et al 1999 J. Phys. D: Appl. Phys. 32 790

Abstract
Self-closing corona-stabilized switches have been developed to operate at pulse repetition frequencies (PRFs) which extend well into the kilohertz regime. The performance of these devices illustrates that corona-stabilized plasma closing switches are an effective alternative to conventional uniform or quasi-uniform field switches which utilize gas-flow techniques, high gas pressures or both to achieve high-PRF operation.

Corona stabilization takes place in electronegative gases such as air and and requires the presence of a highly divergent electrical field. Under dc, or slowly rising charging voltage conditions, a space charge develops around the highly stressed electrode which prevents premature breakdown and allows full voltage recovery to take place. This means that PRFs extending into the kilohertz regime can readily be achieved. Self-firing corona-stabilized switches have been demonstrated to provide PRFs of up to 5 kHz and triggered corona switches have been shown to be capable of operation at 10 kHz. The lifetime of corona-stabilized switches has been demonstrated to approach about 10^8 shots.

The paper describes the design of a corona-stabilized switch, including detailed electrostatic field analysis of the electrode geometry. This analysis was undertaken to establish the effect of the electrode geometry on the field distribution within the switch. The switch design was flexible and allowed a study of the effect of certain parameters on the switching performance. These parameters include the gap spacing, the gas pressure and the profile of the electrodes. The switch was operated under single-shot conditions to establish the breakdown voltage against pressure (V-p) characteristics and in a continuous repetitive mode to assess the PRF operation. The results show that the best repetitive performance is produced using electrodes which generate a high field over a region sufficient to generate a stabilizing corona.

There's many more papers availiable if you do a quick google search.

Cheers,

iJim

that's sound interesting,I know that hydrogen operated spark gap have a very fast recovery and can operate in the khz region ,is it something similar?
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iJim
Thu Feb 17 2011, 04:40PM
iJim Registered Member #2662 Joined: Fri Jan 29 2010, 10:14AM
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Posts: 36
Well, all the switch really consists of is multiple sharp pins suspended above a plate, each pin acts like an individual spark gap, a high pressure gas is used but it doesn’t need to be as exotic as hydrogen; air would probably do. The theory is that considerable corona forms around each pin tip during charging; as the gas is electronegative a cloud of negative ions forms and has an E-field shielding effect. This somehow balances things and all gaps breakdown simultaneously, which is great as you can switch higher currents (and has impedance/geometry related benefits). To me, this doesn’t explain how the repetition rate can be so high! Also, as far as I can tell the only evidence for this simultaneous behaviour is that the plate below the pins shows clear ‘burn’ marks under each pin, this only indicates that each pin has fired at some point, it doesn’t prove simultaneous operation. To be honest though, as far as this application goes who cares if they do fire simultaneously, just as long as you can get kHz rep rates....

iJim.
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Microwatt
Thu Feb 17 2011, 06:33PM
Microwatt Registered Member #3282 Joined: Wed Oct 06 2010, 05:01PM
Location:
Posts: 224
How to design such a mulptile spark system? Do you need gas flowing?

Can you use a radio active material to help with ionizaton?
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