need help (netherlands) spectra-physics 168-35 whitelight

[LesioQ]

Obviously at 20A the voltage would be a little higher... (Remember, plasma has negative resistance!)

I tend to disagree here; there shouldn't be negative resistance here, unless You're really low on gas.
Steve ?

Piotr.K

[mixedgas]

[QUOTE=LesioQ;138582]I tend to disagree here; there shouldn't be negative resistance here, unless You're really low on gas.
Steve ?


Inverted resistance depends on the operating point the designer picked for the tube. If the tube were NOT current limited, by the power supply, yes, the plasma would "RUN AWAY" till the tube destructs or the power line resistance or the pole transformer limits it. At that point the tube would melting down, but usually something else, like a fuse or passbank, melts first!

It can have a inverted slope once stable, depending on where you are in the pressure curve. It will "run away" if not limited, ie the plasma conductance can approach infinity. But once limited things are different. On a SP like that, Its "Apparent" negative resistance if the tube pressure is too low.

If the tube voltage goes negative with increasing current your fill pressure is too low on a long positive column tube like a SP. Its called a inversion point, AND designers prefer to be on the positive side of it, The voltage vs pressure curve for a real tube will have a "V" shaped curve (well, more like a U) when plotted with the crossover point at 0,0 on the graph..

Sometimes I'll underfill a tube and center it on the inversion point and the slope will slip backwards. I have my reasons. BUT NOT FOR LASER SHOW! HGM medical tubes are usually right on the line, for more power. That is part of why they are pulsed.

but for lexel, sp, most coherent:

For LEXEL 75.
Older tube (lower pressure):
Current (A) Voltage (V)
---------------------------
6 115.6
7 115.7
8 116.9
9 117.6
10 118.5
11 120.4
12 121.8
13 124.0
13.7 121.5 (unstable plasma)
Excellent new tube with only 20 or so hours on it: Current (A) Voltage (V)
---------------------------
6 114.2
7 115.8
8 117.7
9 119.4
10 120.9
11 122.5
12 124.2
13 125.7
13.7 127.0

for a 88

Current (A) Voltage (V)
---------------------------
15.5 154.8
16 156.9
16.5 157.7
17 158.4
18 159.4
19 160.3
20 161.1
first of all one would never run a 88 at these high currents for very long, not at least much longer then to measure the voltage. Tube curves are never flat anyways. from (http://pagesperso-orange.fr/redlum.x...r/lexel88.html)


[IMG]file:///C:/Users/Steve/AppData/Local/Temp/moz-screenshot-7.png[/IMG][IMG]file:///C:/Users/Steve/AppData/Local/Temp/moz-screenshot-8.png[/IMG]
Note the reverse slope starting around 12 amps. 4 Amps below that , not graphed, it gets NASTY and starts to go reversed. At a bit less then 6 amps it gets unstable, inverts, and goes out!

steve

[Stoney3K]

[QUOTE=mixedgas;138584]

I tend to disagree here; there shouldn't be negative resistance here, unless You're really low on gas.
Steve ?


Inverted resistance depends on the operating point the designer picked for the tube. If the tube were NOT current limited, by the power supply, yes, the plasma would "RUN AWAY" till the tube destructs or the power line resistance or the pole transformer limits it. At that point the tube would melting down, but usually something else, like a fuse or passbank, melts first!

It can have a inverted slope once stable, depending on where you are in the pressure curve. It will "run away" if not limited, ie the plasma conductance can approach infinity. But once limited things are different. On a SP like that, Its "Apparent" negative resistance if the tube pressure is too low.

I just quoted the whole negative resistance-thing because, at least, that's what I've always been thought from my EE physics book. Plasma likes current! That's the whole point of having a current-source power supply in the first place, as you said, if it were powered by a non-limited power supply (ideal voltage source), you can squeeze a hell of a lot of current through that stuff.

(Hey, now that you mention it, I recall that being used somewhere... wait. Next Generation Technical Manual? Those guys were way ahead of their time! )

BTW, doing the math, if you run at 30A and 234V of tube voltage, some guy called Ohm would tell you that you're converting about 30 * 234 = 7kW of hasty electrons into heat. Is the other 5kW being magically generated somewhere, or can the tube just handle more power? I can't imagine 5kW of heat being generated by the cathode filament... At least not for long!

[mixedgas]

cathodes are 75-110 watts, give or take, depending on laser maker and model. You need additional heating current due to work function losses, ie as you pump electrons out of the cathode's charge cloud, you cool it, so you have to pump more energy in to maintain emission. The work function losses, monitored with a amp clamp and a voltmeter during startup, give me a means of monitoring the health of the tube. Low pressure tubes show no delta T or a inverse delta T at startup. No Delta t means your at the inversion point. neg delta T means your very low pressure, or the emission layer is depleated on the cathode.

Since your a EE student, google "dispenser cathode" or "Philips cathode"

One thing you never want to do is run a cathode too cold, that causes sagging. Metal migration also causes sagging, but that is another lesson.

4-8 amps into the magnet, adjustable on that laser, is where a lot of power goes.

Steve

[Stoney3K]

OK, so we're holding off on adjusting/aligning the unit for the moment since Laserwinkel and I haven't had the time to get together again yet. We'll probably do another run soon, and give aligning this thing a shot.

I am a bit worried about the aligning process. The mirrors are completely out of whack, so if we want to align from scratch, we need to at least remove the OC mirror to get the plasma glow centered down the bore.

I don't have a HeNe (just a pile of cheap diodes and DPSS), so there's probably no way of knowing we're EXACTLY right on the alignment. I've been putting this off since I don't want to rush disassembling the optics, possibly snapping a Brewster stem and rendering the tube useless.

So, asking the experts here, what's the safest way to tackle this? The plasma tube itself is running fine at 30 amps, max field, so there won't be any problems there.

[Laserman532]

because it is a 168 there is no aperture.

all you can do is walk the beam to the center of the mirrors

then adjust the plasma tube for maximum output powers

about a 10 minute job (including unlocking the PT and locking the PT)

about 30 minutes for a complete novice, it is not hard especially a 168.

you guys can do it! I have posted several threads helping gooeygus out, look those up and they will help you.

[Stoney3K]

because it is a 168 there is no aperture.

all you can do is walk the beam to the center of the mirrors

then adjust the plasma tube for maximum output powers

about a 10 minute job (including unlocking the PT and locking the PT)

about 30 minutes for a complete novice, it is not hard especially a 168.

you guys can do it! I have posted several threads helping gooeygus out, look those up and they will help you.

I've read up on how to do it (including the GooeyGus thread), but since we don't have any manuals for the SP168 or SP265 here, I can only do adjustments on the fly.

Needless to say, slightly educated guesswork and a precision optical instrument are two things that are hardly ever a good match. The HR isn't aligned by far, so I need to at least take the Brewster cover off the front, to get the plasma glow back in the middle (center with a piece of paper). And since I'm currently fairly clueless as to which bolts to untighten and pieces to remove to take the cover off, or to move the plasma tube (if that's even neccessary, which I don't suspect), I'm not touching the head with a 6-foot pole unless I know what I'm doing.

Sam's Laser FAQ has some good textual descriptions on how to do it, but no images. As I said, my main worry is having to do something around the Brewster stems/windows, and I need to know what NOT to do, so I can keep the tube undamaged. Otherwise we'll have an angry Laserwinkel with a $4K piece of glass that's up for air...

The manual for the 265 would also help, if we had some schematics here, we can get the interlock fixed and get the main control switch working again. Power cycling the laser with the breaker every time isn't good for the cathode, by far...

[Laserman532]

the manual wont help you.

you never answered my question earlier...is it lasing in ANY way???

[Stoney3K]

the manual wont help you.

you never answered my question earlier...is it lasing in ANY way???

The plasma tube is working, but no, it has not been lasing since we fired it up.

So walking the beam isn't really going to happen. Need to get a beam to walk it first.

[mixedgas]

The manual for the 265 would also help, if we had some schematics here, we can get the interlock fixed and get the main control switch working again. Power cycling the laser with the breaker every time isn't good for the cathode, by far...[/QUOTE]


You have a schematic, I sent the link to him to download and the password for the zip. its on the FTP.

Steve