ECLOUD Calculations of Field Gradients During Bunch Passage Jim Crittenden Cornell Laboratory for Accelerator-Based Sciences and Education Electron Cloud.

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Presentation transcript:

ECLOUD Calculations of Field Gradients During Bunch Passage Jim Crittenden Cornell Laboratory for Accelerator-Based Sciences and Education Electron Cloud Simulations Meeting Wilson Lab 16 July 2008 This version includes some corrections and annotations resulting from discussion during the meeting.

2 16 July 2008 Electron Cloud Simulations/ J.A.Crittenden Comparison shown at ILCDR08 Input Parameter Set Ten 0.75 mA bunches 1.9 GeV 30 empty witness bunches No magnetic field Elliptical chamber 4.5x2.5 cm 0.1 s.r. photon per beam particle per m 20% reflected s.r. photons 10% photoelectron per s.r. photon Peak secondary yield of 2.0 Peak SEY energy of 310 eV ➢ Predicted vertical tune shift 50% lower than measured even if the entire ring were B-field-free ➢ Impressively similar time structure ➢ Vertical gradient factor 5 higher ➢ Horizontal gradient bipolar Positron Beam Field gradients averaged over a 9 mm x 5 mm region at the center of the beam pipe dE Y /dY (V/m 2 ) dE X /dX (V/m 2 )

3 16 July 2008 Electron Cloud Simulations/ J.A.Crittenden Gradients along bunch length Input Parameter Set Same as previous slide ➢ Appreciable gradient change during bunch passage Positron Beam dE Y /dY (V/m 2 ) dE X /dX (V/m 2 )

4 16 July 2008 Electron Cloud Simulations/ J.A.Crittenden Gradients along bunch length for electron beam Electron Beam ➢ Large relative variation for smaller gradients dE X /dX (V/m 2 )dE Y /dY (V/m 2 ) These points precede the bunch passage T

5 16 July 2008 Electron Cloud Simulations/ J.A.Crittenden Gradients along bunch length for positron beam ➢ The integrated-Gaussian shape in bunch 1 shows the time development expected from a longitudinal Gaussian bunch shape. Positron Beam These points immediately preceding the bunch passage are the gradient values shown at ILCDR08. Small differences are due to the fact that 15 recalculations per bunch passage are made here, while only 6 were made in the ILCDR08 calculation. No point was shown for bunch 1 at ILCDR08. ➢ Bunch 1 shows the gradient dominated by the charge at the source point with very little cloud migration. Note the equal and opposite vertical and horizontal tune shifts (c.f. Ohmi). Deviations from this sum rule are due to nonzero cloud density in the beam. ➢ The gradient calculations exhibit a jump between the pre-passage time slice and the first recalculation during the passage. The gradient is recalculated 10 time slices into the passage, and a total of 15 times during the 150 time slices during the passage. This jump is not understood at present.

6 16 July 2008 Electron Cloud Simulations/ J.A.Crittenden Gradients along bunch length for electron beam Electron Beam ➢ For electrons, bunch 1 shows the same source-dominated gradients as calculated for positrons, while the later cloud development is very different, owing to opposite-sign cloud kicks from the beam. If the source point dominated throughout, one would expect horizontal (vertical) gradient after ten bunches of about -(+)500 V/m 2. All contributions to the gradient which do not produce equal and opposite horizontal and vertical tune shifts must be caused by charge density in the beam region. The mysterious jumps at the beginning of the bunch are even more pronounced for the electron beam.