[A1n_d2n] To do items from today's meeting

[Brad Sawatzky]

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