(4) vacuum pressure & gas desorption in the IRs ( A.

Transcription

1 Electron Cloud Effects in the LHC Frank Zimmermann,, SL/AP (1) heat load on the beam screen inside the s.c. magnets (4 20 K) (2) heat load on the cold bore (1.9 K) (3) beam instability at injection (4) vacuum pressure & gas desorption in the IRs ( A. Rossi s presentation) Thanks to: G. Arduini, V. Baglin, O. Bruning, F. Caspers, A. Chao, I. Collins, K. Cornelis, H. Fukuma, M. Furman, O. Grobner, S. Heifets, N. Hilleret, M. Jimenez, K. Ohmi, E. Perevedentsev, A. Rossi, G. Rumolo, F. Ruggiero, L. Wang, and many others

2 Electron Build Up e production mechanisms: residual gas ionization; typical rate e m s in the LHC at 7 TeV synchrotron radiation and photo-emission; typical rate e m s 7 orders of magnitude more than from ionization! secondary emission: (1) true secondaries & (2) elastically reflected or rediffused; exponential growth; amplification of primary electrons

3 5 ns γ γ γ 20 ns 5 ns 20 ns photoelectron secondary electron 10 ev 10 ev 10 ev 2 kev 200 ev 200 ev 200 ev 2 kev 5 ev secondary electron 5 ev 5 ev LOST or REFLECTED time Schematic of electron-cloud build up in the LHC beam pipe. [Courtesy Francesco Ruggiero] Proper multipacting: (O. Gröbner, 1977)

4 Simulation parameters for LHC, SPS, and PS. symbol LHC (init.) LHC (fin.) SPS PS [GeV] [mm] , , 1.3 [cm] [m] [mm] 22, 18 22, 18 70, , 35 [ev] [%] ] m [

5 Evolution of electron line density in units of m vs. time during the passage of a 72-bunch LHC batch through an LHC dipole chamber for.

6 /N dn/de E (ev) Energy distribution of electrons incident on LHC chamber wall for a chamber radius mm (left) and for mm (right).

7 ./ 54 : CP M - *,+ % %& ) "& (' &%$ $!#" 3! =?>9 < ; 7: L K GM I,J H R P EM C QEK D N#O Transverse aperture in the LHC arcs compared with SPS vacuum chambers. Vertical dimension of SPS dipole is similar to LHC arcs.

8 Evolution of electron line density in units of m vs. time during the passage of three 72-bunch LHC batches through an LHC dipole chamber, separated by gaps of 8, 24, 48, and 68 missing bunches, for. Gap larger than 2 s needed for reset.

9 S UT Heat load per unit length in the LHC as a function of bunch population, for various magnetic fields. Other parameters:, electron reflection is included. Dipole field is best. V ev,,, and elastic V UT

10 S UT Simulated average LHC arc heat load and cooling capacity as a function of bunch population, for various. Other parameters are electron reflection included. Average:,,. V UT V ev,,, and elastic WXZY W U _^ WZ[ X\]

11 Schematic of LHC beam screen operating at 5 20 K. (Ian Collins, 2001).

12 delta_max=1.3, emax=450 ev, Y=0.025, R=0.1 TU TU 0.03 ev, ECLOUD 02 Workshop: Electron Cloud in the LHC V F. Zimmermann Snapshot of transverse e distribution in an LHC dipole chamber (F.Z., 1997). Parameters:,, and. Two vertical stripes emerge! V

13 Effect of detector or pumping slot transparency on electron flow through the slots (solid) compared with the flow in the absence of the slots (dashed).

14 ] estimated TMCI thresholds accelerator PS SPS LHC (450 GeV) LHC (7 TeV) e osc./bunch density enhancement TMCI threshold m ] [ density ratio For SPS and LHC, saturation e densities exceed TMCI threshold [ for 2-part. model [K. Ohmi & F.Z., PRL 85, 3821]. Mode coupling calculation [K. Ohmi et al., PRE 65, ]. Heat load may set tighter tolerance.

15 LHC Recipe in arc dipoles: use sawtooth chamber to reduce photon reflections coat all warm sections with getter material TiZr (low secondary emission yield) rely on surface scrubbing during the commissioning to reduce the maximum secondary emission yield to a value of 1.1 back up solutions: larger bunch spacing or satellite bunches

16 Sawtooth chamber protoype; the sawtooth reduces the photon reflectivity to 1.3% [co-laminated Cu: ]. (Ian Collins).

17 Comparison of dose dependence of the Secondary Emission Yield as measured at and SLAC (N. Hilleret et al., 2001).

18 Average LHC arc heat load & cooling capacity as a function of bunch population, for 25 and 50 ns bunch spacing, and. Other parameters are reflection is included. ev,, ; elastic electron

19 Electron cloud build up in the LHC with (left) and without (right) two satellite bunches of various intensity placed one bucket (5 ns) before and after the main bunches. m. Elastic e reflection included.

20 Electron Cloud Heat Load for Shorter Bunch Spacing (LHC Luminosity Upgrade) Average arc heat load as a function of bunch population for bunch spacings of 12.5 ns, 15 ns, and 25 ns, and a maximum secondary emission yield. Elastically reflected electrons are included.

21 V and various bunch populations. UT Average arc heat load as a function of bunch spacing, for

22 Electron Cloud Heat Load for Superbunches (LHC Luminosity Upgrade) Average energy deposition per proton vs. full bunch length for LHC dipole; line density m with 10% rising and falling edge.

23 Conclusions for LHC the most worriesome effects: heat load on the beam screen and through the pumping slots at injection, single-bunch instability could also become a problem simulation results are sensitive to model parameters (, refl. e,...) cloud prevents LHC bunch spacings shorter than 25 ns; superbunches promising alternative