I will present an update on the ECLOUD calculations at tomorrow's RFA meeting. A lot has been done lately, so in the interest of time, I will limit the presentation to three subtopics: 1) azimuthal distribution of p.e. hitting the beampipe wall and the REFL parameter 2) systematics of electric field gradient calculations 3) accelerations of the cloud macroparticles by the beam bunch passage and by the cloud space charge, and the effects on the distribution of kinetic energies in the cloud In the context of these results, I will seek your input on prioritizing the outstanding issues. 1) I have added some plots to the standard set of the cloud-characterizing analysis by extracting the positions and velocities of all particles hitting the wall, which at present is modelled as an ellipse with half-axes of of 4.5 cm and 2.5 cm. For example, here is a 2D plot of the positions for a baseline set of parameters which has the fraction of primary p.e. produced by far-upstream-multiply-reflected sr photons (uniformly distributed in azimuth) REFL=20%: http://www.lepp.cornell.edu/~critten/cesrta/ecloud/6may08/wall_219_3.jpg The horizontal, vertical and longitudinal position distributions are here: http://www.lepp.cornell.edu/~critten/cesrta/ecloud/6may08/wall_219_3.jpg Here is how it changes if REFL is set to 100%: http://www.lepp.cornell.edu/~critten/cesrta/ecloud/6may08/wall_224_3.jpg Now an equal number of particles hit the left and right sides of the wall. Offhand I do not know why so fewer macroparticles hit the wall with this value of REFL. As an aside, I mention that there is a substantial effect on the field gradients: dEy/y is similar, but dEx/x is much larger for the 100% REFL. This means that our tune measurements http://www.lepp.cornell.edu/~critten/cesrta/ecloud/6may08/e+qxqy.jpg favor the lower value for REFL. 2) Systematics of the electric field calculation: the preceding remark concerning the field gradients is supported by these plots: REFL=20% http://www.lepp.cornell.edu/~critten/cesrta/ecloud/6may08/ecloud_219_13.jpg REFL=100% http://www.lepp.cornell.edu/~critten/cesrta/ecloud/6may08/ecloud_224_13.jpg While it is encouraging that we obtain the expected vertical field gradient of 3000 V/m^2 corresponding to a tune shift of about 850 khz, there is clearly some source of instability in the calculation. The field calculation is made faster by summing over charges on a 21x21 grid rather than summing over the actual macroparticle positions. This is causing instability at the level we see, since going to a 51x51 grid changes the gradients by similar amounts. This makes the ecloud calculation much too slow however. 21x21 http://www.lepp.cornell.edu/~critten/cesrta/ecloud/6may08/qgrid_219_3.jpg 51x51 http://www.lepp.cornell.edu/~critten/cesrta/ecloud/6may08/qgrid_225_3.jpg Also, the ECLOUD field calculation requires an elliptical beampipe, and we know the calculated gradients depend strongly on the beampipe shape. I have some suggestions for improving the calculation which we can discuss. 3) Magnitudes of cloud space charge and beam kicks: effects on the energy spectrum hitting the wall. Some examples of trajectories and kicks can be instructive. Trajectories: http://www.lepp.cornell.edu/~critten/cesrta/ecloud/6may08/traj_219.jpg Momenta and energy: http://www.lepp.cornell.edu/~critten/cesrta/ecloud/6may08/mom_219.jpg Kicks: http://www.lepp.cornell.edu/~critten/cesrta/ecloud/6may08/ecloud_219_9.jpg Energy Distribution: http://www.lepp.cornell.edu/~critten/cesrta/ecloud/6may08/ecloud_219_9.jpg If we increase the current in the 10 bunches from 0.75 mA to 7.5 mA, the beam kicks get very large and the energy distribution above 100 eV becomes much more populated. Trajectories: http://www.lepp.cornell.edu/~critten/cesrta/ecloud/6may08/traj_221.jpg Momenta and energy: http://www.lepp.cornell.edu/~critten/cesrta/ecloud/6may08/mom_221.jpg Kicks: http://www.lepp.cornell.edu/~critten/cesrta/ecloud/6may08/ecloud_221_9.jpg Energy Distribution: http://www.lepp.cornell.edu/~critten/cesrta/ecloud/6may08/ecloud_221_9.jpg -- ======================================================== James Crittenden Tel. (607) 255-9424 Wilson Synchrotron Laboratory Fax (607) 255-8062 Cornell University Ithaca, New York 14853-8001 ========================================================