Electron Cloud Studies

Transcription

1 Electron Cloud Studies Tom Kroyer, Edgar Mahner,, Fritz Caspers, CERN LHC MAC, 7. December 2007

2 Agenda Introduction to electron cloud effects Overview over possible remedies surface coatings rough surfaces clearing electrodes Tests planned in the SPS Clearing electrodes impedance experiments in the PS Conclusion 2

3 Introduction Since the first observation of the electron cloud (EC) in the sixties and seventies this effect has been found in many high intensity machines. In the past years a large effect has been put into understanding the EC by numerical simulations, machine studies and dedicated EC diagnostics. The goal is to mitigate or better completely suppress the effect. By tuning the beam and machine parameters the EC build-up can be reduced, but this is usually undesired, as it makes operation more difficult and affects the machine performance. 3

4 Remedies 1 There are two main approaches for suppressing the EC effects: Modify the chamber surface such that the secondary emission yield (SEY) is minimized. This can be done by conditioning the chambers by beam scrubbing, which costs machine time surface coatings with low SEY with or without baking, such as TiN, nonevaporable getters (NEG), carbon,... artificial surface roughness to increase the probability of a secondary electron to be absorbed by the surface. This roughness can be macroscopically (mm range) or microscopically (micron range). 4

5 Remedies 2 The second approach is to change the dynamics of the EC by externally applied electric, magnetic or EM fields Solenoids have been shown to efficiently suppress the EC by confining the electrons close the chamber walls. However, solenoids can only be employed in straight sections Clearing electrodes allow to apply an electric field that changes the trajectories of the electrons and may disturb the EC build-up Electromagnetic fields were also proposed for electron cloud clearing Ideally we would prefer a passive means to suppress the EC that works even when the vacuum pipe has been vented and does not need baking 5

6 Studies for the SPS In the framework of the SPS Upgrade Study Team convened by E. Shaposhnikova various approaches to suppress the electron cloud effect in the SPS are being evaluated. Several test chambers are going to be installed in the machine before the 2008 run, including surface coatings, rough surfaces and clearing electrodes. Dedicated EC diagnostics, including strip detectors, shielded pick-ups and the microwave transmission method are going to be used to evaluate the achievable EC suppression with magnetic field Strip detector B field MBA chamber Beam screen with electron collecting holes Beam Holes (transparency 7%) Copper strips courtesy: J.M. Jimenez 6

7 Surface Coatings The SEY of a given surface is affected by the properties of the clean surface an oxide layer and adsorbed molecules In general adsorbed molecules increase the SEY, while some oxides may have a lower SEY than the base metal (e.g. Cu 2 O) Coatings for SEY reduction are being actively researched at CERN, in particular sputter-deposited NEG, TiN and carbon The LHC straight sections have been NEG coated, so there is a considerable body of experience with this technique Very low SEY have also been obtained with TiN, however there is a large scatter in the data reported in the literature. The challenge about TiN is to get a layer with low SEY with good reproducibility for large scale production. In addition to that is has to be examined if the obtained SEY degrades when the layer is exposed to air Other surfaces, such as carbon-like surfaces are also being considered courtesy: S. Calatroni, M. Taborelli, P. Chiggiato, presented at Beam 07: 7

8 Rough Surfaces In order to limit the increase in beam coupling impedance, macroscopic surface roughness should be in the shape of longitudinal grooves. Microscopic structures can have any shape as long as they are not much larger than the skin depth in the relevant frequency range. They should be mechanically stable and comply with vacuum requirements The challenge for rough surfaces is to produce such structures with a sufficient aspect ratio economically on an industrial scale Example below: rough Ti surface produced by chemical etching courtesy: S. Calatroni, M. Taborelli, P. Chiggiato 8

9 Clearing electrodes In several simulations and experiments is has been shown that localized clearing electrodes can effectively suppress electron multipacting close to the electrode Distributed clearing electrodes could be used to fight the electron cloud effect over longer regions of an accelerator. However, issues such as impedance, aperture and manufacturing get more important for larger lengths In the past, electrodes for electron or ion clearing have been employed in several machines: Ion clearing in the CERN AA (antiproton accumulator) machine where large stacks of p-bars had to be kept. The AA clearing system started off with 20 metallic electrodes and towards the end we had 50 ceramic ones with high resistive coating for beam coupling impedance issues Ceramic electrodes with highly resistive coating were applied in the CERN Electron Positron Accumulator (EPA) For the SNS floating-ground BPMs have been designed where a DC voltage can be applied for electron cloud clearing EPA clearing electrodes 9

10 Low-impedance electrodes Electrodes implemented as a resistive coating can be used to minimize the impedance.. They are basically invisible beam resistive layer electrodes in the sense that they are much thinner than the penetration depth and thus do not interact strongly with the beam fields. Another condition for a low coupling to the beam field is that the coating s s surface resistance is much dielectric larger than the free space impedance of 377 Ω. Such an electrode behaves almost like a dielectric layer. In practice, a solution could consist of a dielectric layer made of enamel or alumina for the dielectric isolator. A resistive coating as the actual electrode is deposited on top of the dielectric. Such a structure in particular has good mechanic stability and good g thermal contact to the beam pipe; for a thin dielectric the aperture reduction is small. 10

11 Resistive layers There are two conflicting requirements for the resistive layer: In order to minimize the beam coupling impedance the resistivity should be high However, to minimize the voltage drop on the electrode for a given clearing current the resistivity should be low Considering the clearing currents measured in the PS and the impedance calculations, the surface resistance should be in the range of R = 1 to 100 kωk The standard coating technology uses thick film paste (mainly Ruthenium oxides) which is fired at 800 deg C on alumina. Application of the thick-film paste onto enamel is being researched. 11

12 Longitudinal impedance The longitudinal impedance of resistive layer clearing electrodes was estimated analytically and using numercial simulations (CST Microwave Studio and HFSS) For thin dielectric layers Z/n is essentially imaginary and Im(Z/n) ) is proportional to the dielectric cross-section section Im(Z/n) ) increases slowly with ε Im(Z/n) ) is rather flat up to very high frequencies Estimation for one 0.1 mm thick electrode with ε r = 5 all around the SPS (pipe radius 25 mm, 20 mm dielectric width): Im(Z/n)= 0.3 Ω (entire machine today: Z/n~10 Ω) Wire simulation of two thin clearing electrodes Real part of long. impedance Imag part of long. impedance plots for rotationally symmetric structure, courtesy: B. Salvant 12

13 Transverse impedance The transverse impedance was estimated analytically for structures with rotational symmetry using the Burov-Lebedev formula and simulated using CST Microwaves Studio and HFSS [1] To first order the increase in Z TR is purely imaginary and frequency- independent; Preliminary results scaled to one 0.1 mm thick centered electrode with ε = 5 all along the SPS (pipe radius 25 mm, electrode width 15 mm): Im(Z TR,y ) = 4 MΩ/m M (entire machine today: Z TR ~ 20 MΩ/m) M [1] T. Kroyer, F. Caspers, E. Metral, F. Zimmermann, Distributed electron cloud clearing electrodes, Proceedings of the ECL2 Workshop, CERN, Geneva, 2007 plot courtesy: E. Metral 13

14 Recent experiments in the PS During the shutdown 2006/2007 a simple experiment was installed in the CERN PS [1,2] The goal was to research whether there is an electron cloud build-up up during the last few ms to μs, when the bunches get shortened before transfer to the SPS The experiment comprised a shielded button pick-up, vacuum diagnostics and a small dipole magnet. A stripline electrode was added to examine the properties of clearing electrodes. [1] [2] 14

15 Components of the PS electron cloud setup Penning gauge Shielded button pickups High-voltage feedthrough Clearing electrode 15

16 The PS electron cloud experiment in SS98 PS elliptical vacuum chamber with dimensions 1050 x 146 x 70 mm. Special antechamber for clearing electrode without aperture reduction. Material: stainless steel 316 LN BPU Stripline Vacuum gauge Beam Stripline upstream BPU 1 BPU 2 B Stripline downstream 16

17 Results A clear electron cloud effect was found during the last ~50 ms Indications: Vacuum pressure rise (see below) Current on the shielded pick-ups Clearing current on the stripline electrode The effect can be switched off with an appropriate clearing voltage Very fast pressure peaks with ~30 ms rise time, ~20 ms after onset of electron signal on the shielded buttons P [mbar] xLHC25 (72 bunches) ~ 3.6 s SS 98 ejection ~ 0.8 s transition t [s]

18 Shielded PU signals Bias voltage on pickup: +60 V No magnetic field Last three turns before ejection plotted With a -300 V on the stripline no more electron cloud build-up up is visible above the noise level The EC can be suppressed with positive clearing voltages, as well The clearing current on the stripline is large for positive clearing voltages (~500 μa/m) ) and small for negative clearing voltages (~2 μa/m) Stripline signal [100 V] Shielded PU2 [V], no clearing voltage Shielded PU2 [V], 300 V clearing voltage Time [μs] 18

19 Effect of the magnetic field The magnetic field B and the clearing voltage U SL were varied; the points where measurements were taken are marked on the axis of the plot The maximum EC signal on a shielded pick-up over one turn is plotted as a function of B and U SL Maximum EC signal on shielded PU1 [V], 20 ms before ejection, as a function of the magnetic field B and the clearing voltage U SL [V] The RF gymnastics in the PS decreases the bunch length,, which allows to characterize the EC build-up up as a function of bunch length (and long. profile) in one single shot The third bunch splitting takes place 27 to ~5 ms before ejection, giving 72 bunches with a 4σ4 bunch length of 14 ns. 5 ms to ~300 μs s before ejection: Adiabatic bunch compression, bunch length decreased to 11 ns Last ~300 μs: bunch rotation, bunch length reduced to 4 ns. 19

20 Islands with surviving EC EC signal from shielded PU1 plotted at different times before ejection ection Build-up up starts earlier with magnetic field; Islands with large EC appear in the parameter space. For large enough clearing voltages ( U SL > 1 kv) EC suppression was found in all cases t=-45 ms t=-20 ms t=-10 ms [1e-3] t=-1 ms t=-100 μs t=-2 μs 20

21 Conclusion The electron cloud effect is a serious issue for the CERN SPS and LHC and potentially for the PS with intensities beyond nominal LHC beam There are various means for combating the EC; at the end of the day we would like an economic and reliable solution that can be installed over large sections of the machine. Ideally, it should work even after venting the vacuum chamber and without baking Development work is ongoing on surface coatings, rough surfaces and clearing electrodes. These concepts will be tested in the SPS in In parallel, EC studies in the PS are going to be continued The concept of low-impedance enamel clearing electrodes was outlined and the efficiency of clearing electrodes demonstrated in the PS 21