Difference between revisions of "18" Photogun ala GTS"

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'''Synopsis'''
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*The gun high voltage chamber is 18 " diameter compared to CEBAF's 14 " diameter.
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*The vacuum chamber accommodates an R30 inverted insulator with spherical electrode and shed.
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*There were two vacuum chambers made: one by MDC currently installed in the GTS which produced 300 kV 4 mA un-polarized beam, and 200 kV 28 mA magnetized beam, both with CsKSb cathodes.
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**The GTS gun was installed in 2016 with a Pierce geometry focusing cathode front end and 6 inch diameter anode 9 cm anode-cathode gap.
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**Later in 2021 the cathode front face was replaced with a flat (non-focusing) en piece, flat anode, and the anode-cathode closed to 5 cm to increase the photocathode gradient from 3 to 8 MV/m [https://logbooks.jlab.org/entry/3879145]
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**The new anode was designed with the aperture shifted y=-1.6 mm to compensate for beam kick, which was effectively demonstrated with beam [https://logbooks.jlab.org/entry/3985315]
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*The second 18" diameter vacuum chamber was made by Kurt J. Lesker and has a thicker downstream endplate.
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**This is the vacuum chamber that is being used for the CEBAF gun chamber with R30 insulator+spherical electrode to be installed in March 2023 in the tunnel.
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**This chamber IS NEG coated. See
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***[https://wiki.jlab.org/ciswiki/images/0/04/NEG_coating_chambers_for_extreme_high_vacuum.pdf "JVST Paper"]
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***[[Media:18" gun chamber from KJL NEG coating performance tests_matt edits.pptx]]
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**The electrode and anode are the same as CEBAF with Pierce focusing geometry
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**Anode-cathode gap remains unchanged from CEBAF 14" gun at 6 cm
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**The anode has a new fixture tube that allows for 2 deg tilt to compensate for beam kick, more slots for improved vacuum, and 10 kV SHV feedthrough for anode (not tube) biasing.
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**The links document the process to accomplish this task, and include links to the history of this vacuum chamber, background, and status as of October 4 2022.
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**The models following the links represent the gun with 18" KJ Lesker NEG coated chamber + white R30 insulator spherical electrode, CEBAF style anode with 6 cm anode-cathode gap AND 2 degree titled anode for beam kick correction.
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**The plan is to install this gun in UITF in November 2022. Demonstrate 200 keV beam without field emission, demonstrate beam kick has been corrected by anode tilt, and install in CEBAF in March 2023.
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* <span style="font-size:100%"> '''[[18" gun build status as of October 04 2022]]''' </span>
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* <span style="font-size:100%"> '''[[18" high voltage chamber history]]''' </span>
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* <span style="font-size:100%"> '''[[Background and status as of July 1 2021]]''' </span>
 
* <span style="font-size:100%"> '''[[Background and status as of July 1 2021]]''' </span>
  
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This photo-gun was intended for the UITF to operate at 200 kV. It is based on the GTS 18" diameter vacuum chamber, doped R30 insulator and spherical electrode. The vacuum chamber is NEG coated, and the anode is
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* <span style="font-size:100%"> '''[[Must, Should  and Likes]]''' </span>
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the CEBAF style, with 6 cm anode-cathode gap. The GTS 18" photo-gun does not have a NEG coated chamber, and the anode is larger than the CEBAF type, with holes for laser in/out.
 
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It was first high voltage conditioned in the UITF [https://logbooks.jlab.org/entry/3478173], but processing was very slow with unprecedented gas load with voltage increase. On July 28 2017 the insulator arced over at 227 kV: [https://logbooks.jlab.org/entry/3480185].
 
  
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The photo-gun was refurbished in August-September 2018 but this time with a pure alumina (white) R30 insulator, as there were eno more doped R30s [https://logbooks.jlab.org/entry/3483589]. The gun was installed, baked but the cable was not changed. Although a visible black (carbon likely) track was on the cable plug, it cleaned off easily. On October 20 1017, the insulator arced over at only 164 kV [https://logbooks.jlab.org/entry/3489049], likely due to the cable plug being damaged from the first breakdown.  Not changing the cable was a mistake on our part. The low voltage at which the insulator suffered electrical breakdown is surprising, considering that this type of pure alumina insulator with a shed demonstrated 375 kV in the GTS using a dummy ball (2016, see our IEEE paper).
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* <span style="font-size:100%"> '''[[Tasks]]''' </span>
  
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Upon the second insulator failure, the 18" photo-gun was removed from the UITF beam line, parked in our TL1137 lab, and replaced with the 14" photo-gun and R28 insulator.
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* <span style="font-size:100%"> '''[[Drawings]]''' </span>
  
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The spherical electrode was disassembled and stored in the TL1137 clean room cabinet. The shed has been polished and clean. The electrode is ready for assembly.
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* <span style="font-size:100%"> '''[[Electrostatic  and beam dynamics simulations]]''' </span>
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A new pure alumina insulator has been welded to a 10" CF that has holes for S&A fixtures, and that has been vacuum-degassed.
 
  
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The anode was removed from the vacuum chamber and is now in the clean room at TL11237. The anode has scratches and some of the ceramic rings and washers are broken. This anode features the capability fo independently biasing the anode or the anode drift tube.
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* <span style="font-size:100%"> '''[[Update meetings]]''' </span>
  
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The WP-1250 cartridges are still installed in the vacuum chamber, but the woven wire shielding screen has been removed. The NEG cartridges have an electrical short and would not be activated running current through them.
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* <span style="font-size:100%"> '''[[Presentations]]''' </span>
  
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The vacuum chamber is dusty, and has pieces of nitrile glove, bits and pieces of metal like washers, wire, and screws. There is no longer an extractor gauge attached to the vacuum chamber.
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*[[File:Iso view distance from z=0 to anode spider US plane is 200 mm.png]]
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*[[File:Cross section elevation distance from z=0 to anode spider US plane is 200 mm.png]]
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*[[File:Tilted anode elevation model.png]]

Latest revision as of 10:21, 5 October 2022

Synopsis

  • The gun high voltage chamber is 18 " diameter compared to CEBAF's 14 " diameter.
  • The vacuum chamber accommodates an R30 inverted insulator with spherical electrode and shed.
  • There were two vacuum chambers made: one by MDC currently installed in the GTS which produced 300 kV 4 mA un-polarized beam, and 200 kV 28 mA magnetized beam, both with CsKSb cathodes.
    • The GTS gun was installed in 2016 with a Pierce geometry focusing cathode front end and 6 inch diameter anode 9 cm anode-cathode gap.
    • Later in 2021 the cathode front face was replaced with a flat (non-focusing) en piece, flat anode, and the anode-cathode closed to 5 cm to increase the photocathode gradient from 3 to 8 MV/m [1]
    • The new anode was designed with the aperture shifted y=-1.6 mm to compensate for beam kick, which was effectively demonstrated with beam [2]
  • The second 18" diameter vacuum chamber was made by Kurt J. Lesker and has a thicker downstream endplate.
    • This is the vacuum chamber that is being used for the CEBAF gun chamber with R30 insulator+spherical electrode to be installed in March 2023 in the tunnel.
    • This chamber IS NEG coated. See
    • The electrode and anode are the same as CEBAF with Pierce focusing geometry
    • Anode-cathode gap remains unchanged from CEBAF 14" gun at 6 cm
    • The anode has a new fixture tube that allows for 2 deg tilt to compensate for beam kick, more slots for improved vacuum, and 10 kV SHV feedthrough for anode (not tube) biasing.
    • The links document the process to accomplish this task, and include links to the history of this vacuum chamber, background, and status as of October 4 2022.
    • The models following the links represent the gun with 18" KJ Lesker NEG coated chamber + white R30 insulator spherical electrode, CEBAF style anode with 6 cm anode-cathode gap AND 2 degree titled anode for beam kick correction.
    • The plan is to install this gun in UITF in November 2022. Demonstrate 200 keV beam without field emission, demonstrate beam kick has been corrected by anode tilt, and install in CEBAF in March 2023.
  • Iso view distance from z=0 to anode spider US plane is 200 mm.png
  • Cross section elevation distance from z=0 to anode spider US plane is 200 mm.png
  • Tilted anode elevation model.png
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