Sphenix-tpc-l mailing list

[Bob Azmoun <azmoun AT bnl.gov>]

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
-----------------------------------------------