Problem with Igun regulation

[Probeman]

Andreas posted this question on the Cameca Listserver:

Dear SX100 users,
Our probe has a problem with the regulation of the gun current (Igun, or bias). When we heat the filament and set HV to e.g. 15 kV the default Igun value of 80 µA shows up in the window. We can then set any beam current (e.g. 10 nA), no problem. However, when loading a setup (or even after storing and re-loading a working setup) the error message "setup failed" quickly appears. Apparently it fails to set the 80 µA Igun value.

Looking at the measured values for the gun parameters reveals the problem. Heat and HV are fine, but measured Igun is something around 200...230 instead of 80 µA. This far beyond any reasonable uncertainty of the measurement. Moreover, changing the desired Igun value to e.g. 70 or 120 µA doesn't affect the measured value. On the other hand, increasing / decreasing HV results in increasing / decreasing measured Igun values. In some cases, however, Igun doesn't follow the HV change immediately but
changes gradually over a minute or so, e.g. when changing from 15 to 20 kV.

Also, increasing the heat value strongly increases the measured Igun value - is this normal? In fact, a plot of heat (from 190 to 220) versus measured Igun shows a positive slope with a kink at the saturation value. Although this relation between heat and emitted current is principally expected, I had thought that the Igun value (e.g. 80 µA) is regulated and should be constant.

Based on an earlier e-mail by Peter Collett in this list (3 June 2016) I believe that the problem is in the Igun control loop, namely, within the HV tank (photoresistors). I have an old spare SX100 and can therefore exchange the HV tank (I have done this before), but before embarking on this again I want to be sure that the problem is not located somewhere else. Can someone confirm that the most likely cause for the trouble is the HV tank?

Thanks in advance and best regards,
Andreas
--------------------------------------------------------------
Dr. Andreas Klügel, PD
Universität Bremen
Fachbereich 5 - Geowissenschaften
Postal: Postfach 330440, 28334 Bremen, Germany
Delivery: Klagenfurter Str. 2-4, GEO2/5360, 28359 Bremen, Germany
Tel: +49-421-218-65402
Fax: +49-421-218-65429

And Philipp Poeml responded:

Hi Andreas,
From my experience that indeed sounds like the photoresistors. I have changed those (two) 3-4 times in my career. It is not very difficult if you have some procedure to follow. They are a bit expensive, I think EUR 1000 last time.

I don't think I would change the whole HV tank in your case, I would rather change the photoresistors.

There is this test by unplugging the HV cable and measuring the resistance between the pins. Did you do that? To check if the photoresistors are ok or not.

SWITCH OFF HV TANK !!!! Then remove the big connector.

You have 3 pins there, pin 1, pin 2, and the pin that goes to the Wehnelt.

Between Filament Pin 1 and Wehnelt, should find at least 35 MOhms

Between Filament Pin 2 and Wehnelt, should find at least 35 MOhms

Between Filament Pin 1 and Filament Pin 2, should find about 780 Ohms

Hope it helps
Philipp

[Probeman]

Philipp provides a procedure also...

I thought this procedure is of general interest.
It applies to our SX100R: SX902r.

Disclaimer: Use at your own risk! 🙂

Regards
Philipp

Hi,

as the last info on this:
It is possible to change the LDRs on those two plastic tubes. AFAIK, the type should be NSL-6110.

Regards
Philipp

[sem-geologist]

Ok, I was silently following this, but got triggered by this

Hi,

as the last info on this:
It is possible to change the LDRs on those two plastic tubes. AFAIK, the type should be NSL-6110.

Regards
Philipp

NSL-6110 looks wrong for the function. its voltage  rating is only 100V; where by specification bias voltage should get as high as 3500V. If Cameca is encapsulating these photoresistors in these tubes, then it is no surprise that they burn out eventually and needs periodic replacement. Or they should at least warn users to not experiment with setting Igun anything below 80µA, and clearly state the derate value for higher HV voltage, as increasing HV voltage also increases bias voltage. As two LDRs are used in series, the bias within rating range will be 200V (100V x2) . That looks reasonable bias voltage for 15kV HV 80uA beam... but 25kV, 30kV, and >30kV where bias for 80uA will need to go into thousands of Volts, I have serious doubts if this NSL-6110 would not breakdown. Even working at 20kV would probably go easily over 200V.

Another question is replacing it in plastic package, and we are not talking about some plastic, but transparent plastic. I wonder if Philipp had tried to do that. Its a bit too late for us as we had completely changed the HV power supply to different vendor (there were some additional considerations and chain of reasons) on our SX100. But for more recent SX100 HV tanks, where glow discharge suppression is not jeopardised, repairing LRD seems to me attractive cost efficient solution.

And even if it would be that model, I cant find it anywhere on stock. oh something changed: it is available on mouser

[camebax]

In fact there are 7 LDRs in each tube, for a total of 14 in series.

[sem-geologist]

In fact there are 7 LDRs in each tube, for a total of 14 in series.

Thanks, camebax, for this crucial info.


So this draws a clear picture:

14 LDRs with 100V rating in series gives theoretical 1400V as maximum safe bias voltage. BTW max rated value is not some magical value, where below it there is no degradation and above it there is guarantied degradation. Often in electronics datasheet stated values are some threshold points in gradual slope of applied voltage vs fault probability, working close to rated voltage always increase the risk of fault. For comparison, the original Tank design had 0-3500V range for bias (at least that is written in those papers attached to every original TANK (these are  kept in pocket from other side of cover of regulation electronics)). I think that was easily achievable by much better single component PbS LDR's in a single package per Tube (Probably, I lack of that exact information), as PbS was widely available decades ago (I think design is inherited from SX50, thus initial design is from 50 years ago).

I just want to point out to the fact that 7 LDR's in series in single package won't divide voltage evenly - that means that less illuminated unit could have larger voltage across compared to mean voltage of all LDR's - and previously stated sum bias voltage of 1400V is clearly not maximum safe voltage.  Also datasheet of these LDR's clearly writes "100Vpk" - that is a peak rating, not continuous load rating! So value should be practically further de-rated closer to 1000V. Also if one of IR Diodes would get darker (due to its age) or completely burn out - it will put whole corresponding LDR tube (with 7 LDR's) under higher stress than other tube (they will have higher voltages across compared to LDR's in the other tube). Which means that actually de-rating only o 1000V is probably overoptimistic, and safe value would be even further below (maybe 700V - that is half of 1400V for situation where  one of IR diode would completely burnout). 700V vs 3500V that is 1:5 derating ratio!

This leads to further considerations:
It is clear that full 50kV range of SX line EPMA's, which it was designed for, with this new type of LDR's installed is no more safe for LDR's, or when setting any >25kV EHT it needs to be compensated with setting much larger I gun current (which lowers bias voltage and so it protects LDR's). Setting anything above ~20kV increases a risk of LDR degradation if keeping the same I_gun current as used for 15kV EHT (i.e. 80µA). In case of a need of setting higher voltage than 20kV I strongly advice to increase I_gun current. (The settable range on original OEM HV tank is up to 500µA - using larger values, compared to those used at 15kV EHT, are safer for LDR, where keeping same or lower values are more risky).

The another note - do not experiment with I_gun current by setting it low (i.e. do not try scanning down to 0 µA to find a lowest limit of I_gun ) – that will increase the bias voltage and you easily will damage LDRs even while at 15kV EHT!