RF background simulations

1. RF background simulations MICE collaboration meeting Fermilab 2006-06-09 Rikard Sandström

2. Outline • Reminder • How it was done • The results • Future plans

3. RF background • At high fields, electrons are emitted from irregularities in the RF cavities. • Electrons are accelerated along beam line according to RF phase settings. • Energy loss in beryllium windows, vacuum and absorber windows. • Electrons lose energy in form of ionization and bremsstrahlung in absorbers. They all die there. • Bremsstrahlung photons leave absorbers and creates hits in trackers, or low energy electrons in windows etc (typically high Z material). • Very rare, but RF electron emission frequent. • Dark currents from RF cavities can have a negative impact on the tracker performance. • To a lesser extent also a problem for TOF2. • TOF1 is shielded by the diffuser.

4. RF electron acceleration • Time and energy of RF electron were calculated numerically in Matlab. • The RF phases were optimized for a mu+ at 200 MeV/c on axis. • Assumed phase difference between neighboring cavities was constant. • Phase diff = 2.0498 rad = 1.621 ns. • This gave the muon an energy gain of 10.8 MeV per set of four RF-cavities, including energy loss in Be windows. • Electrons were assumed to be emitted when the E-field is at the extremes. • Gave starting time and phase for each cavity.

5. Accelerating e- in the RF, 2 cavs Downstream direction Upstream direction

6. Accelerating e- in the RF, 4 cavs Downstream direction Upstream direction

7. Please note: • The RF phases are set such that • electrons have higher energy in upstream direction. • some electrons turn around if starting with downstream direction. • hence, both rates and energies higher upstream. • If MICE is optimized for mu-, most of the background will be in downstream direction. • Malcolm showed trackers are more sensitive to background downstream, since the electrons travel with the expected direction.

8. From Matlab to G4MICE • Generating the background as calculated in Matlab at red locations. • Extracting data at green locations.

9. Initial state of e- in G4MICE • The electrons were distributed evenly over 21 cm in radius just outside the outer beryllium windows. • Particles were assumed to be parallel to beam line initially. • B-field curves trajectories. • Matlab gave time of arrival at absorbers, background from different RF periods was achieved by repeating with an integer RF period offset in time. • Off crest emission • If the particles were allowed to be emitted somewhat off crest E peak, energies might change.

10. Implementation of absorbers • Absorbers and vacuum windows supports different geometrical shape. • Flat, spherical, torispherical. • All optional absorber shapes had the central window thickness and central liquid hydrogen thickness set to most up to date design (fall 2004).

11. Result • Only flip-mode studied. • Non flip could defocus electrons less, and thus more photons in trackers. • Particles at upstream tracker (worst direction): • 5.8x10-6 e- per RF electron. • 8.3x10-4 gamma per RF electron. • With existing emission data from MTA, this means 0.15 MHz of electrons and 21.2 MHz of photons. • Energies up to 7.0 MeV achieved, typically much lower. • Only ~20% of photons leaving cooling channel enter trackers.

12. Future plans • New 201 MHz measurements in MTA will give more accurate information on situation for MICE. • Total rates. • Angular & spatial distributions. • With improved knowledge on RF emitting phenomena, the simple assumptions previously used could be more elaborate and accurate. • For example emission angle, and off crest emission. • Other possible improvements: • Can we implement a more accurate absorber shape in reasonable time? • Should also non-flip field mode be simulated? • Or even only non-flip mode?

13. Geant 4 versions • Geant4.5 (basis of old study) • Simple models for multiple scattering, ionization and related processes. • The stuff we care about! • For example, G4.6 showed 22% more photons in trackers. (small statistics) • Could not simulate an electron decelerating to zero energy, and then reaccelerate. • It was assumed dead at the zero crossing. • Geant4.7/8 • Implementations of recent papers on multiple scattering models, and other improvements in the tracking of particles. • Not sure if reaccelerating zero energy particles is yet supported. • I spoke with them fall 2004 about this. • Hence, differences in underlying model suggests the RF background should be resimulated if we take the problem seriously.

14. A future simulation • Simulating the problem in 2004 was very demanding on processing power. • Due to rare nature of RF induced bremsstrahlung events. • Now, I have access to a GRID cluster in Geneva. • Would be helpful to get the MICE VO started / fully functional. • Still, timescale is weeks, not days or hours of processing time. • Also disk space could prove a problem. • Terry Hart and Malcolm Ellis are checking if the code I and Yagmur Torun wrote in 2004 is still doing what it is supposed to. • Nobody has touched it ever since, so nothing should be broken that we know of.

15. Sub jobs of the simulation • Implement updated model, with fresh data from MTA. • If possible, cross check that electrons arrive at absorbers at the same energy and time in • G4MICE simulation (depends on whether G4 can do it). • Matlab calculation • Use spectrum at absorber entries to shoot through absorbers, collect particles on the other side. • The most time consuming part. • Use spectrum of particles which made hits on other side of absorbers to generate background for detectors.

16. Summary • The RF background problem has not been touched for some time. • New data from MTA will give useful information. • Geant4 model changes suggests a new simulation should be run.