[Tgun] GTS Photogun Beam Structure

[Riad Suleiman]

For the Harp: it is both; we want high peak current to get a large 
signal. Since the readout is sampled at 60 Hz, there is no need for beam 
when not reading the Harp. This will reduce the heat load on the Harp 
wire (and on the fork if beam is mis-steered, which is usually the case 
when trying a Hap scan). We can run CW on Harp, usually with same limit 
on average current as peak current, but it is too risky especially if 
there is a chance of hitting the fork.

For the YAG, it is the average current. When the issue is heat damage, 
then it is the average that is relevant. The time response of the YAG is 
very fast (~80 ns); the camera has to capture the light at the right 
time - not sure how these cameras work (?). There should be enough light 
and at the right time for the camera. We will find out when we run nC 
bunches on viewers.

Yours,
Riad.


On 06/22/2018 09:50 AM, Fay Hannon wrote:
> Just so I understand is it average or peak current on the diagnostics which
> drives the duty factor? Or is it both?
>
> Thanks
> Fay
>
> -----Original Message-----
> From: Tgun<tgun-bounces at jlab.org>  On Behalf Of Riad Suleiman
> Sent: Friday, June 22, 2018 9:36 AM
> To:tgun at jlab.org
> Subject: Re: [Tgun] GTS Photogun Beam Structure
>
> For the micro structure, we started at 374.25 MHz but will change to 499 MHz
> later this summer to test a TE011 cavity. The micro structure repetition
> rate can vary - but must match any RF components on beamline.
>
> When we switch to the high power laser to generate nC bunches, we will run
> at 1-20 Hz with a single micro bunch of about 50 ps. At this time, we have
> to resolve any issues related to diagnostics.
>
> Riad.
>
>
> On 06/22/2018 09:25 AM, Matt Poelker wrote:
>> But you don't mind that the tgun will pulse at a different rep rate,
>> right?
>>
>>
>> On 6/22/2018 9:22 AM, Riad Suleiman wrote:
>>> Hello,
>>>
>>> These are the GTS beam structures used with the photogun:
>>>
>>> 1. CW Beam for high current runs: Micro structure of 374.25 MHz, 50 ps.
>>>
>>> 2. Tune Beam for Harp: Macro structure of 60 Hz, 250 us. The nominal
>>> average beam current during a harp scan is 100 nA. This gives a peak
>>> current during the 250 us of about 0.1*1/(60*0.000250) = 6.7 uA.
>>>
>>> 3. Viewer Limited Beam for Viewers (YAG): Macro structure of 60 Hz, 4
>>> us. The nominal average beam current on viewer is 2 nA. This gives a
>>> peak current during the 4 us of about 0.002*1/(60*0.000004) = 8.3 uA.
>>>
>>> Notes:
>>> 1. We use a Beam Sync signal that goes to the laser table to generate
>>> the beam macro structure. This signal also goes to the harp to
>>> trigger the readout. Another copy goes to the camera of the third
>>> viewer. Of the three viewers, only the third one has a camera with an
>>> external trigger. We are planning to run at 10 Hz this summer and
>>> will find out if there is any issues related to running at
>>> frequencies other than 60 Hz.
>>>
>>> 2. The Harp ADC records only one sample at approximately the middle
>>> point of the 250 us beam pulse. Thus, reducing the width of the Tune
>>> Beam may require other changes to Harp readout.
>>>
>>> Yours,
>>> Riad.
>>>
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