[A1n_d2n] To do items from today's meeting

[Xiaochao Zheng]

Hi Brad:

Thanks for your reply.  

1) optics:

So  we need 20 PAC hours with 1-pass beam for an extensive optics study in the fall 2019 run.

What about Spring 2020?  We will need to take elastic and Delta with 1-pass beam again. Can we do a reduced optics study with 1-pass for that run period?

3) target protection:

the empty target position now has +/-0.522" or +/-1.325cm of space both above and below. This is the full distance to the solid target holder and to the reference cell wall.  Sounds like a 2.5cm OD Al tube is too big to fit in.  Jianping probably can comment on this.

Xiaochao


----- Original Message -----
> From: "Brad Sawatzky" <brads at jlab.org>
> To: "Xiaochao Zheng" <xiaochao at jlab.org>
> Cc: "A1n d2n" <a1n_d2n at jlab.org>, "Mark Jones" <jones at jlab.org>
> Sent: Thursday, 17 May, 2018 00:12:24
> Subject: Re: [A1n_d2n] To do items from today's meeting

> On Wed, 16 May 2018, Xiaochao Zheng wrote:
> 
>> 1) How much beam time do we need for the optics calibration at 1-pass,
>> for the fall 2019 and the spring 2020 run periods, respectively?
>> (Brad, Mark, Dave, JP?)
>> 
>> - we think a total of 2 days is needed: 1 day (8+4 PAC hours for
>>   elastic and Delta), and 1 day for careful optics study. The Sp2020
>>   optics can be shorter.
> 
> ** Note the units below are PAC hours.  Multiply by 2'ish to get
>   calendar hours/days.
> 
> My notes suggest something on the order of 20 PAC hours at *1 pass*
>  Optics                                                [2 PAC hours]
>  H(e,e'p) elastics, C, 3He elastic, QE, Delta, etc    [15 PAC hours]
>  BCM, BPM calibrations                                 [2 PAC hours]
> 
> I'd want to plan to take 1--2 PAC hours of optics data at each energy,
> angle setting.  (Most of this time is at 5 pass, of course.)
> 
> We may want to 'front load' a group of optics runs at several angles
> and momentum settings so the experts can chew on the data and tune the
> online optics matrices.  Once we're in reasonably good shape for
> online work, then we can just take the optics data at each kinematic
> setting (as needed) for post-production cross checks and fine-tuning.
> 
>> 2) carbon foil max current: 30 or 60 uA?
> 
> I'm sure we can get by with 30 uA, but the maximum current should
> probably be determined/backed by some modeling work by Silviu to
> ensure we know when the target ladder material might get soft.
> I would suggest a phrase something like this: "minimum 30 uA up to
> a maximum (no greater than 60 uA) to be determined by simulation."
> 
>> 3) how do we protect the target from possible large beam halo and
>> mis-steering?
>>
>> option 1: have two carbon hole targets. But current target design does
>> not have enough space to have both the central long carbon with hole
>> and two additional carbon holes at +/-z edges; So we will need to
>> sacrifice the central single foil.  Do we need single-foil optics?
> 
> The single foil 'point target' optics are nice for C xsec sanity
> checks...  My preference would be to keep it..
> 
>> Also if we do have double carbon holes, steering the beam into both
>> may require a shift of tuning. On the other hand this needs to be done
>> only occasionally.
> 
> The single carbon-hole gives us the following:
>  - an empirical measure of the beam-on-target position at z=0
>  - an absolute measurement of the raster diameter/shape at the target
> 
> Maybe a crazy idea?
>  - What is a minimum beam clearance requirement for a 'No target'
>    position?  Would a 2cm diameter hole be 'enough' to steer an
>    unrastered tune beam through?
>  - If so, then perhaps we could mount a short Al pipe, coaxial with the
>    beam, beneath the solid target holder in the 'No target' region of
>    the existing ladder.  It would serve as a safe(?) position for MCC
>    to tune unrastered beam though, and would allow us to run a few
>    sanity checks on beam alignment and quality.
>    - I am imagining a stubby Al pipe with 2.5cm OD, 2.0cm ID, ~5cm long.
>      This is similar to the so-called "Raster Tube" target we've used
>      in Hall A in the past.
>  - The upstream edge of this 'aperture target' would be a nominal 15cm
>    downstream of the target center.  We would want that edge to be
>    in the acceptance of the SHMS at (basically) all angles.
>  - MCC could still steer up (tune beam, raster off) through that fairly
>    large aperture.
>    - If they steered into the cylinder (raster off), the ion chambers
>      would trip the beam as usual, and probably for the best.
>  - When MCC hands control to us, we would stay on the 'aperture target'
>    position and request the raster be ramped to production size while
>    we watch the spectrometer rates.  If there is significant halo, or
>    the beam is badly cockeyed it would scrape and we will see a clear
>    response in rates.  If no change, then we're clearing it all the way
>    through.
> 
> So, if we know we are centered at z=0 and know the raster size from the
> C-hole, then clearing this 'aperture target' would give us two more
> pieces of useful info:
>  - It bounds any off-axis mis-steering (ie. we can say the rastered
>    cylindrical beam profile is centered at z = 0 and at z = 15--20cm,
>  - and tells us that the beam halo is minimal.
> 
> NB: There was talk of putting a collimator at the upstream beam window
>    position, so if that was plausible from a steer-up perspective, this
>    is better.  I think the fact that this 'aperture target' can be
>    aligned against, and moves with, the actual target ladder is a real
>    plus.
> 
> -- Brad
> 
> 
> --
> Brad Sawatzky, PhD <brads at jlab.org>  -<>-  Jefferson Lab / Hall C / C111
> Ph: 757-269-5947  -<>-  Fax: 757-269-5235  -<>- Pager: brads-page at jlab.org
> The most exciting phrase to hear in science, the one that heralds new
>   discoveries, is not "Eureka!" but "That's funny..."   -- Isaac Asimov