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Update of Pillbox Grid Testing and Summary of other Pillbox Cavity Studies Alfred Moretti

Update of Pillbox Grid Testing and Summary of other Pillbox Cavity Studies Alfred Moretti 05/30/2014 APC. Outline of Talk. 1. Description of the geometry the grids and reason for study in the LBL Pillbox cavity, and description of the 2 types: 2 grids, and one grid and flat Cu window.

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Update of Pillbox Grid Testing and Summary of other Pillbox Cavity Studies Alfred Moretti

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  1. Update of Pillbox Grid Testing and Summary of other Pillbox Cavity Studies Alfred Moretti 05/30/2014 APC Map Spring Meeting, May 27-31, 2014

  2. Outline of Talk • 1. Description of the geometry the grids and reason for study in the LBL Pillbox cavity, and description of the 2 types: 2 grids, and one grid and flat Cu window. • 2. Other window types tested earlier in the Pillbox cavity studies are described for comparison with current grid results. • 3. HFSS field simulations of the 2 grid configuration are presented as are the other tested configurations and peak field areas highlighted. • 4. pictures of the grids before and after testing. • 5. Plots of the achieved gradient in the 2 grid type cavities tested from 0 to 5T solenoidal fields. Map Spring Meeting, May 27-31, 2014

  3. Purpose for the Grid and their Description • Purpose of the grid design: • It is more resistive to the electromagnetic impulse detuning of the cavity. • Hollow tube can be cooled externally. • It can be made 50 % or more transparent to the beam. • Thin wall tubes can have less mass/sq intercepting the beam • It can be made more economical the thin high purity be windows. 8 cm radius window Tube Dia 1 cm Tube spacing 3.2 cm Material electropolished Aluminum Un-Coated Window 2 Un-Coated Window TiN Coated Window Material electropolished Flat Cu window Pillbox with 2 Grids windows: gap=8.6-1cm dia-Grid Pillbox with 1 Grid and 1 Flat Cu window: gap=8.4-0.5 rad-grid Map Spring Meeting, May 27-31, 2014

  4. The Other Pillbox testing Configurations Symbolic double Curved Be window: 254 um thick B & E LBL Pillbox cavity First RF Commissioning Test arrangement: Tested with flat Cu and with flat and curved Be windows; Later after Refurbishing at JLAB: re-polished the surfaces and put radius 0.3125 cm On coupling iris: gap=8.2 cm. Note that the electromagnetic impulse detuning was stopped while testing flat 254 um Be windows by Eddy curent brake provide by the non-uniform Solenoidal magnetic field. LBL Pillbox cavity with smaller single button and curved Be Window Testing arrangement. This was the first button Testing arrangement: gap=7.8 cm LBL Pillbox cavity with 2 larger buttons Testing arrangement: gap=5.7 cm Map Spring Meeting, May 27-31, 2014

  5. Simulation of the Pillbox cavity E fields for the Configurations The HFSS E field simulation for the 2 grid configuration shows the peak electric field is on the central grids. It is a factor 2 large than the average accelerating field on axis and shows no enhancement on the coupling iris. The HFSS E field simulation for the 1 grid and 1 flat Cu window configuration shows the peak electric field is on the central grids. It is a factor 2 large than the average accelerating field on axis and shows no enhancement on the coupling iris. Map Spring Meeting, May 27-31, 2014

  6. Simulation of the LBL Pillbox cavity without buttons Gradient pattern for LBL Pillbox cavity with flat windows (the first RF commissioning test arrangement) with no radius on coupling aperture. The peak gradient is on the RF coupling aperture and not on axis and is 3 times greater than the on axis field. There is a smaller enhancement on the cavity iris. A B • Ratio of the peal electric gradients on the cavity surfaces with Flat windows after 0.3125 radius on coupling aperture. • The ratio of A/C = 1.47; • where C is the gradient on the axis of the cavity and A is the gradient on the coupling iris • The ration of B/C =1.25; • where C is the gradient on the axis of the cavity and B is the gradient on the coupling window iris. • Also note that the peak gradient is shifted about 1 cm toward the coupling aperture instead of being on axis. • The ratio’s will change when we add buttons: • With the palmer style (larger) button • The ratio of A/C becomes equal to 0.396 • for the smaller first test button equal to 0.675. C Map Spring Meeting, May 27-31, 2014

  7. Electric Gradient Field Patterns for the 3 Test Arrangements Gradient pattern for LBL Pillbox cavity with flat windows (the first RF commissioning test arrangement). The peak gradient is on the RF coupling aperture and not on axis and is 3 times greater than the on axis field. There is a smaller enhancement on the cavity iris. Gradient pattern for LBL Pillbox cavity with smaller single button and curved Be Window testing arrangement (this was the first button testing arrangement). The gradient pattern has changed the peak is on the button and smaller enhancement on the coupling iris. Gradient Pattern for the LBL Pillbox cavity with 2 smaller but deeper penetrating buttons (Final Testing arrangement). Greater gradient enhancement on the button tips and much less on the RF coupling aperture. Note not shown: the RF magnetic field is peaked on the coupling iris for all Config Map Spring Meeting, May 27-31, 2014

  8. Pictures of the Grids at installation and after Testing The first picture shows the grids at installation just before the SS vacuum cover plate is attached. Notice the sparking pits at the bottom of the picture. These are sparks that have accumulated after running the Flat Cu windows test and 2 button pillbox cavity test and after the cavity was refurbished at JLAL. The second bottom picture shows the results of testing of the 2 grid Pillbox cavity after over 20 E6 pulses and 200 accumulated sparks. Notice that there are no spark pits on the grid. The sparks must be located outside of the field of view or on the cavity Cu iris or on the RF coupling aperture. Map Spring Meeting, May 27-31, 2014

  9. Gridded Cavity Configuration results Peak Surface and on Axis Note that the Gradient is flat or nearly flat with the solenoidal Magnetic field for 0.25 to 5 T. This was not the case for the other configuration that will show next. There are 2 levels shown one for the peak suface gradient on the grid and the other the average integrated gradient on the accelerating axis of the cavity. 1/2/2020 Map Spring Meeting, May 27-31, 2014 9

  10. Peak Surface E field on button and average line fit and below in green average accelerating gradient on axis and line fit. Map Spring Meeting, May 27-31, 2014

  11. Table of Achieved Gradients at 1.5 T and 3 T with sparking rate less than 1/100,000 pulses 2 button Pillbox cavity Test The table is shown: putting more the plots on pages 8 or 9 Would be too confusing. Map Spring Meeting, May 27-31, 2014

  12. Comments on the next slide that show the relationship between Magnetic field, Vacuum and Radiation The slide show as the magnetic field decreases in time the radiation directly in front of the cavity increases and has to resonances at low field values at 0.16T and 0.11T. The vacuum also has double resonance 0.022 T and 0.11 T. All cavities tested showed this behavior including the 201. For this to occur you also need a high E gradient on the cavity 10 to 20 MV/m. Another interesting fact is that for the gridded cavities we also saw some sparking. Map Spring Meeting, May 27-31, 2014

  13. Plots of Mag. Current, cavity vacuum and radiation levels vs. Time for Pillbox with Flat Cu Windows Yellow Mag I eq. 0.391T/Unit Red – radiation mRem/hr Green – vacuum In Torr. E-7 The X axis is time in sec. 1/2/2020 Map Spring Meeting, May 27-31, 2014 13

  14. Typical Radiation pattern for most cavities tested except for the gridded cavity configuration. One sees a mark difference in the patterns. For the typical case the radiation is much higher with gradient than without gradient. By looking at the asymptotic slop of the curves one can almost predict the gradient limit in each case. For the gridded cavity case there is little or no separation of radiation with gradient. One sees increases in radiation and increases in vacuum, they both decrease with a time constant of about 11hours. Increasing the gradient start the process all over. In some cases decreasing the gradient Starts the process over again. In some cases increasing the gradient 2 % step up or down leads to a spark. I am speculating that this is due to low level multipactor because the grids are made of Aluminum which has very high secondary emission coefficient. This has been seen in all copper structure with the windows coated with TiN (a material with a low high secondary emission coefficient). 3T 0T Map Spring Meeting, May 27-31, 2014

  15. Summary and Conclusions • The gridded cavity gradient with magnetic field was flat from 0T to 5 T. This in contract with the typical behavior of the other window versions tested in the Pillbox cavity. • The radiation pattern with magnetic field for the gridded cavity does not vary with gradient in contract • to the behavior of the other window types tested. • The gridded cavity is more sensitive to sudden • increases or decreases in gradient which can lead to • sparking. Possibly due to the high secondary • emission coefficient of the aluminum grids. • This leads to the conclusion that low level • multipactor may lead to cavity sparking. • In place coating all parts of the cavity with TiN may • prevent breakdown. This has been done successfully • in aluminum cavity in FermilabsPbar source. Map Spring Meeting, May 27-31, 2014

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