Sphenix-tpc-l mailing list

[Bob Azmoun <azmoun AT bnl.gov>]

Hi Dan,

Thanks for going through the TracePro simulations for the smooth ellipsoidal mirror. I guess we are just asking too much to do beam shaping and steering in one shot. It's too bad since this was such an elegant solution. In any case, regarding your two motor beam steering configuration, I think this is a "have your cake and eat it too" design too since it not only provides a continuum of angles, but also gives us the flexibility of placing the beam at virtually any point on the light pipe for a given angle, which is a real benefit in terms of performance. Another huge benefit is that this is an "actively focusing" style of beam steering (as Tom pointed out), not the "lever-arm" style that I showed last week, where the final beam position very strongly depends on the angle of the beam delivered to the final steering mirror. Just to make sure I didn't miss something during the meeting today (since I stepped out a few times), if you can fit the two motors needed for the two rotating mirrors, is there any reason not to go with this design instead the mirror array design? The two motor design seems like a win-win since we bypass having to build the mirror array and get better performance. Yes, there are two rotating mirrors instead of one, but if we use the CCD camera as feedback, the additional uncertainty can be accounted for, right? Anyway this seems a little too good to be true, which is why I'm asking :).  

Thanks,

Bob

On Tue, May 5, 2020 at 11:00 PM Bob Azmoun <azmoun AT bnl.gov> wrote:

Yes, agreed. The only advantage of having the lens near the laser is if you use a single focusing element and wanted a more parallel beam. This way, the beam spot at the light pipe is smaller than it would be if you place a stronger focusing lens near the beam shaping. This of course comes at the big cost of having a larger w0. Turned out the beam is ever so slightly smaller at the light pipe with the lens near the laser, so it was a bad compromise.

On Mon, May 4, 2020 at 1:25 PM Thomas K Hemmick <tkhemmick AT gmail.com> wrote:

Hi guys

The easiest way to hit a target is to stand close to it.  The target in this case is (if I understand the Email) is to get a primary focus of the laser between the rotating mirror and the (now smooth) ellipse.  Let me put in a few numbers:

1/o + 1/i = 1/f 

I think that we already have and "f" of about 1 cm.  The "i" should be about 50 cm.  In this case we get an "o" of 1.0204 cm.  Here is a plot of "image" distance vs "object" for a lens of 1 cm focal length:

The ideal image location is something like 500 mm away.  So...we need something like +200 -100 micron stability on the location of the focus.  If we put beam shaping optics near the laser...unlikely.  We would probably need a stronger lens as close as possible (possibly inside) the egg.  If, for example, we had a lens of focal length ~3 cm located ~2cm before the rotating mirror then we need stability of this focus to 1%...much easier than of the lens is 30 cm back.

Tom

On Mon, May 4, 2020 at 9:54 AM Bob Azmoun <azmoun AT bnl.gov> wrote:

Yes, maybe if we focus the reflected light to the focus of a secondary ellipsoidal mirror located in a convenient, nearby location and place the CCD sensor at or close to the second focus, then we should be able to decode the beam angle from the size/shape of the measured beam spot, right? After all, we should be able to measure the beam width to very high resolution. (Hopefully we can account for variations in the beam width using the beam energy monitor of the laser.)

On Mon, May 4, 2020 at 9:29 AM Thomas K Hemmick <tkhemmick AT gmail.com> wrote:

This is the equivalent to a "zero" tag on a shaft encoder, but even better since it says where the LIGHT goes (rather than where the motor is placed).

Tom

PS--Optimistic on a Monday rather than complaining that this mixes motor position and laser inclination...

On Mon, May 4, 2020 at 1:08 AM Dan Cacace <dannycacace AT gmail.com> wrote:

Bob,

Agreed. That’s a clever idea to use the beams reflection. I’d say we can probably get away with one well placed CCD and capture the reflection at all angles. However, the relative angle would obviously affect the beam diameter and shape, though we should be able to account for that.

-Dan

On Sun, May 3, 2020 at 11:29 PM Bob Azmoun <azmoun AT bnl.gov> wrote:

Hi Dan,

Yes, due to this kind of sensitivity, I'm pretty sure we'd need a feedback mechanism in place as well, in addition to the ability for high precision (very neat linear motor, btw!). Since the light pipe reflects ~4% or more of the incident light off of the entrance facet, I wonder if we could organize a linear array of small CCD sensors in place of the 2nd arc (now that we don't need more mirrors) and get an exact image of the beam x-sect. This would of course also take care of providing an absolute beam position, in real time. CCD cameras can be pretty cheap: we bought a 3.1MP camera recently with 3.1um pixel size (6.5mm x 4.9mm area) for about $140 and imaged a UV PiLas laser with it. A quick google search revealed many small sensors that would probably fit in your enclosure. We'd probably have to stagger them on top of one another to get full coverage. Not sure at all how practical this all is of course; we'd have to see the sensor and what services it needs, etc.

Cheers,

Bob   

On Sun, May 3, 2020 at 11:31 AM Dan Cacace <dannycacace AT gmail.com> wrote:

Bob,

Agreed, just buying *the* mirror is a major advantage. As for the sensitivity of the position of the focal point, as you may imagine, it is largely dependent on the parameters of the beam (diameter, half angle, etc.) when it hits the ellipsoidal mirror. For example a beam with a half angle of 0.25 deg, diameter of about 0.15 mm and a +/-1 mm move of the focal point corresponds to the beam diameter changing 0.15 mm to 2 mm, 2ft away. I would say it is quite sensitive, but the same company that makes the rotational mirrors, also makes linear motors with a resolution of <0.05 µm. Or maybe we can buy a pre-made solution, like this: https://www.youtube.com/watch?v=X9AXI8czA7s.

-Dan

On Fri, May 1, 2020 at 5:52 PM Bob Azmoun <azmoun AT bnl.gov> wrote:

Hi Dan,

Aha!, thanks for checking this. This is one less thing to have to worry about regarding this beam steering option. I have to say, of all the very interesting (and pretty clever) options you showed us today, I think I like this one the most. Like you said, if we can just buy *the* mirror and only have to deal with the beam shaping (ie, the variable focal length shaping), I think this would be a big win. Not to mention, we get a continuum of angles in a *focused* beam steering configuration, the way Tom likes it :). One immediate issue I see is that the focusing of the beam in the center of the TPC may be a bit more challenging since we'd have to tune how much we favor the focusing element over the de-focusing one. In this sense, it would  be good to investigate this configuration some more in TracePro before the next meeting (also to see how sensitive the position of the focal point is on the beam divergence, etc), where we should probably try to narrow down the beam steering choices.

Cheers,

Bob

On Fri, May 1, 2020 at 4:16 PM Dan Cacace <dannycacace AT gmail.com> wrote:

Hi Bob,

One the last things I brought up and we discussed during today's meeting was using a true ellipsoidal mirror and adding a motor to adjust a focal lens and thus the focal point. One concern you had was if the focusing effect of the ellipsoidal mirror would be the same in both axes, so I quickly through it in TracePro assuming everything was "perfect". Looks like the focusing effect is the same, see attached.

-Dan

On Thu, Apr 30, 2020 at 7:43 PM Bob Azmoun <azmoun AT bnl.gov> wrote:

Dear All,

Given the need to hone in on a final design for the beam steering mechanism for the straight line laser source, it was decided to meet again this week, which is a departure from our normal bi-weekly schedule. We will have the meeting tomorrow at 9am. 

A link to the Indico page is here: https://indico.bnl.gov/event/8430/

and the bluejeans link is here: https://bluejeans.com/881710315

I put together a tentative agenda, but please feel free to modify it and upload any material you wish to present. 

Talk to you tomorrow!

Cheers,

Bob

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B. Azmoun
Brookhaven National Laboratory
Physics Dept. Bldg. 510 C
Upton, NY 11973
P: (631) 344-4082
F: (631) 344-3253
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-----------------------------------------------
B. Azmoun
Brookhaven National Laboratory
Physics Dept. Bldg. 510 C
Upton, NY 11973
P: (631) 344-4082
F: (631) 344-3253
-----------------------------------------------

--

-----------------------------------------------
B. Azmoun
Brookhaven National Laboratory
Physics Dept. Bldg. 510 C
Upton, NY 11973
P: (631) 344-4082
F: (631) 344-3253
-----------------------------------------------

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-Dan

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-----------------------------------------------
B. Azmoun
Brookhaven National Laboratory
Physics Dept. Bldg. 510 C
Upton, NY 11973
P: (631) 344-4082
F: (631) 344-3253
-----------------------------------------------

--

-----------------------------------------------
B. Azmoun
Brookhaven National Laboratory
Physics Dept. Bldg. 510 C
Upton, NY 11973
P: (631) 344-4082
F: (631) 344-3253
-----------------------------------------------

--

-----------------------------------------------
B. Azmoun
Brookhaven National Laboratory
Physics Dept. Bldg. 510 C
Upton, NY 11973
P: (631) 344-4082
F: (631) 344-3253
-----------------------------------------------