VIEWS: 4 PAGES: 33 POSTED ON: 3/10/2012 Public Domain
Beam-based vacuum observations and their consequences Sergei Nagaitsev August 18, 2003 Design goals Stochastic cooling only Emittance growth rate (n, 95%): 2 μm/hr Lifetime of 10-μm emittance pbar beam with cooling: ≥ 200 hrs Stochastic+electron cooling Emittance growth rate (n, 95%): 4 μm/hr Lifetime of 10-μm emittance pbar beam with cooling: ≥ 200 hrs The lifetime cannot be greater than: Ax Ay A 6Λ 30 n , 95% ( Ax Ay ) n , 95% Recycler vacuum shutdown work review - Nagaitsev 2 Design goals Stochastic cooling only Emittance growth rate (n, 95%): 2 μm/hr 1 Lifetime of 10-μm emittance pbar beam with cooling: ≥ 200 hrs Stochastic+electron cooling Relative beam lifetime Emittance growth rate (n, 95%): 4 μm/hr of Lifetime0.1 10-μm emittance pbar beam with cooling: ≥ 200 hrs 0.01 0 10 20 30 40 Normilized 95% emittance [mm mrad] Relative Coulomb scattering loss rate for the Recycler beam, assuming a 40-μm acceptance in both planes. The design emittance is 10 μm (n, 95%) Recycler vacuum shutdown work review - Nagaitsev 3 Today’s menu I will present the beam-based measurements and demonstrate that the results of these measurements are consistent with Coulomb scattering off residual gas. Recycler vacuum shutdown work review - Nagaitsev 4 Today’s menu I will present the beam-based measurements and demonstrate that the results of these measurements are consistent with Coulomb scattering off residual gas. Assuming that all scattering is elastic, I will derive the partials pressures. These are an order of magnitude higher than what we obtain from our vacuum model. Recycler vacuum shutdown work review - Nagaitsev 5 Today’s menu I will present the beam-based measurements and demonstrate that the results of these measurements are consistent with Coulomb scattering off residual gas. Assuming that all scattering is elastic, I will derive the partials pressures. These are an order of magnitude higher than what we obtain from our vacuum model. I will critique my own model and consider uncertainties. Recycler vacuum shutdown work review - Nagaitsev 6 Today’s menu I will present the beam-based measurements and demonstrate that the results of these measurements are consistent with Coulomb scattering off residual gas. Assuming that all scattering is elastic, I will derive the partials pressures. These are an order of magnitude higher than what we obtain from our vacuum model. I will critique my own model and consider uncertainties. Finally, I will demonstrate that the goal of 4 μm/hr could be achieved with a successful bakeout. The goal of 2 μm/hr is likely to be unattainable with the present vacuum system. Recycler vacuum shutdown work review - Nagaitsev 7 Observations (coasting beam) Aug 2, 2003 One hour of observations with 9E10 protons (no MI ramps) Initial emittance: 10 μm (95%, Schottky) 7 μm (100%, scraper) Final emttance: 20 μm (95%, Schottky) 17 μm (95%, scraper) Growth rate: 10 ± 1 μm/hr Zero-current, pencil beam lifetime: 90 hours Acceptances: Ax = 60 μm, Ax = 40 μm The emittance growth rate and the lifetime are selfconsistent! Recycler vacuum shutdown work review - Nagaitsev 8 Emittance growth rate Our measurements demonstrate that: The transverse emittance growth is linear with time The rate is insensitive to beam intensity and bunching structure The rate does not depend on beam emittance The rate does not depend on tunes (within limits) Recycler vacuum shutdown work review - Nagaitsev 9 Emittance growth rate Our measurements demonstrate that: The transverse emittance growth is linear with time The rate is insensitive to beam intensity and bunching structure The rate does not depend on beam emittance The rate does not depend on tunes (within limits) This all points to Coulomb scattering I will be using the following growth rate expression for my analysis: crp2 e (Z i ) 12 ave ni Z n ( Z i ) 2 Zi n , 95% i i Recycler vacuum shutdown work review - Nagaitsev 10 Observations (coasting beam) Aug 2, 2003 -12 MeV/c 12 MeV/c -12 MeV/c 12 MeV/c 1 hour, 9E10 p’s, long. Schottky at 1.75 GHz (no MI ramps) The average revolution frequency has decreased by 32 mHz This corresponds to a 0.37-MeV energy shift The rms momentum spread has increased (1.0 to 1.3 MeV/c) The low energy tail has developed Recycler vacuum shutdown work review - Nagaitsev 11 Observations (coasting beam) Aug 13, 2003 dB Frequency 30 min, 1E10 p’s, long. Schottky at 79 GHz (no MI ramps) Energy loss: 0.42 MeV/hr The rms momentum spread has increased. The low energy tail has developed. Recycler vacuum shutdown work review - Nagaitsev 12 The low-energy tail We’ve proved that the low-energy tail comes from ionization losses. -12 MeV/c 12 MeV/c 1.9E11 protons before scrape 0.9E11 protons after scrape Recycler vacuum shutdown work review - Nagaitsev 13 The low-energy tail The scrape was done with a horizontal scraper at a location with a horizontal beta-function of 52 m, zero dispersion (~20 cm) and with equal tunes. The scraper was stopped 6.2 mm away from the beam center, which corresponds to a 7-μm acceptance, and then withdrawn. How can one scrape the low energy tail at zero- dispersion location? Recycler vacuum shutdown work review - Nagaitsev 14 The low-energy tail The scrape was done with a horizontal scraper at a location with a horizontal beta-function of 52 m, zero dispersion (~20 cm) and with equal tunes. The scraper was stopped 6.2 mm away from the beam center, which corresponds to a 7-μm acceptance, and then withdrawn. How can one scrape the low energy tail at zero- dispersion location? The answer is: proton-electron collisions. Recycler vacuum shutdown work review - Nagaitsev 15 Proton collision with a stationary electron Max. energy transfer Tmax= 91 MeV 5 4 Transverse momentum, MeV/c Outside of ±0.3% 3 p perp ( ) 2 momentum acceptance 1 0 0 20 40 60 80 100 t( ) Energy transfer per collision, MeV Recycler vacuum shutdown work review - Nagaitsev 16 Proton collision with a stationary electron 2 1 p Jn ave 2 M p c 2 2 40 30 Betatron action, mm-mrad Scraped to 7 mm-mrad J( ) 20 10 0 0 5 10 15 20 t( ) Energy transfer per collision, MeV Recycler vacuum shutdown work review - Nagaitsev 17 Observations (coasting beam) Aug 2, 2003 -12 MeV/c 12 MeV/c -12 MeV/c 12 MeV/c 1 hour, 9E10 p’s, long. Schottky at 1.75 GHz (no MI ramps) The average revolution frequency has decreased by 32 mHz This corresponds to a 0.37-MeV energy shift The rms momentum spread has increased (1.0 to 1.3 MeV/c) The low energy tail has developed Recycler vacuum shutdown work review - Nagaitsev 18 The rms momentum spread increase IBS p2 || d 2 dt || d || dt p 2 rp2 cNLC 2 C 3 3 2 1 2 Lc - is the Coulomb logarithm, C – is the ring circumference, σ – is the rms beam size and θ – is the rms angular spread. Recycler vacuum shutdown work review - Nagaitsev 19 IBS (coasting beam) The rms momentum growth 1 10 6 rate (MeV/c per hour) as a function of the rms εn =5 μm momentum spread (MeV/c) for various transverse 1( p ) 10 μm (n,95%) emittances. 2( p ) 15 μm N = 9x1010 protons 3( p ) 4( p ) 20 μm Measured: Initial 1.0 MeV/c Final 1.3 MeV/c Need to add rms spreads in 1 10 5 1 10 5 1 10 6 1 10 7 quadrature! p M p d The ibs can explain a Without any cooling the momentum spread diffusion rate is (for a 10 pi emittance): portion (0.1-0.2 MeV/c) of the measured momentum spread. Recycler vacuum shutdown work review - Nagaitsev 20 The rms momentum spread increase IBS Energy-loss straggling 1.2 10 5 1 10 5 Landau probability density 8 10 6 Mean energy loss: 0.42 MeV g( p ) 6 10 6 Most probable loss: 0.3 MeV 4 10 6 2 10 6 0 2 10 4 10 6 10 8 10 1 10 5 5 5 5 6 0 p Energy loss, MeV Recycler vacuum shutdown work review - Nagaitsev 21 Momentum distribution A Gaussian(σ=1 MeV/c) distr. is folded with the Landau probability density. 8 18.4 28.8 FD jj GD jj 39.2 49.6 60 2.4 10 1.92 10 1.44 10 9.6 10 4.8 10 7 7 7 6 6 0 jj Momentum spread: +/- 12 MeV/c The result is that by fitting the energy loss alone, I am able to reproduce both the low-energy tail and the rms spread increase Energy loss due to resistive impedance is negligible Recycler vacuum shutdown work review - Nagaitsev 22 Summary of beam-based measurements Emittance growth rate: crp2 e (Zi ) 12 ave ni Z n ( Z i ) 2 10 1 m/hr Zi n , 95% i i Beam average energy loss: 0.40±0.04 MeV/hr This corresponds to a mean energy loss of 0.42±0.04 MeV/hr Tmax MeV 0.307 Zi 2 A xi [g/cm ] I 2 ln 2 0.42 0.04 MeV i i i Recycler vacuum shutdown work review - Nagaitsev 23 Partial pressures model Assume that the residual gas consists only of Z=1 and Z=8 atoms (H2 and H2O) Solving equations for 10±1 μm/hr and 0.42±0.04 MeV results in: pH = 3.3±1.7x10-9 Torr pW = 1.0±0.2x10-9 Torr Recycler vacuum shutdown work review - Nagaitsev 24 Partial pressures model Assume that the residual gas consists only of Z=1 and Z=8 atoms (H2 and H2O) Solving equations for 10±1 μm/hr and 0.42±0.04 MeV results in: pH = 3.3±1.7x10-9 Torr pW = 1.0±0.2x10-9 Torr If I assume that one of the gases is hydrogen with a know concentration nH, and then try looking for another gas with a new Z (A = 2Z), nZ ≤ nH, I am unable to find any solution unless pH > 1.5x10-9 Torr and Z ~ 5. Recycler vacuum shutdown work review - Nagaitsev 25 Partial pressures model Assume that the residual gas consists only of Z=1 and Z=8 atoms (H2 and H2O) Solving equations for 10±1 μm/hr and 0.42±0.04 MeV results in: pH = 3.3±1.7x10-9 Torr pW = 1.0±0.2x10-9 Torr “Water” alone contributes 8.3 μm/hr to the emittance growth Recycler vacuum shutdown work review - Nagaitsev 26 Partial pressures model Assume that the residual gas consists only of Z=1 and Z=8 atoms (H2 and H2O) Solving equations for 10±1 μm/hr and 0.42±0.04 MeV results in: pH = 3.3±1.7x10-9 Torr pW = 1.0±0.2x10-9 Torr “Water” alone contributes 8.3 μm/hr to the emittance growth Before Jan 2003 shutdown the measured emittance growth rate was 5 μm/hr. There were no beam energy loss measurements. Present measurements are consistent with the water content doubled after the shutdown. Recycler vacuum shutdown work review - Nagaitsev 27 What could have increased the water content? Work done during Jan 2003 shutdown: Installed new ion pumps, capable of pumping Ar at a higher rate. Installed some diagnostics (RGAs, ion gauges). Out of 27 vacuum sectors, 20 were vented with “dry” nitrogen and then only 5 sectors baked to 120C. Of these five, three have been vented since then without a bake. Many new ion pumps were not baked in situ. Unrelated to the Jan 2003 shutdown: BPMs and bellows (total equiv. length of about 500m) were never baked at all. Recycler vacuum shutdown work review - Nagaitsev 28 So, what’s the problem with this model?? There are two problems with my pressure model. 1. Terry’s vacuum model predicts an order of magnitude low pressures. Recycler Pressure Profile Using Average RGA Data from Pump Locations Total 1.0E-09 IG Readings Hydrogen Water 1.0E-10 Carbon Dioxide Carbon Monoxide Pressure (Torr) Methane/Ethane 1.0E-11 Argon Unknown 1.0E-12 1.0E-13 0 500 1000 1500 2000 2500 3000 3500 4000 4500 5000 Distance (cm) Recycler vacuum shutdown work review - Nagaitsev 29 So, what’s the problem with this model?? 1. Terry’s vacuum model predicts an order of magnitude low pressures. None of the ion gauges show pressures above 5x10-10 Torr Recycler vacuum shutdown work review - Nagaitsev 30 Pressure (Torr) 1.E-10 1.E-09 1.E-08 1.E-07 R:IP220 R:IP221 R:IP222 R:IP224 R:IP225 R:IP226 R:IP227 R:IP228 R:IP229 R:IP230 R:IP231 Torr. R:IP232 R:IP301A R:IP301B R:IP302A R:IP302B R:IP303A R:IP303B R:IP304A R:IP304B R:IP305A R:IP305B R:IP306A R:IP306B R:IP306C R:IP307 R:IP308 R:IP309 R:IP310 R:IP311 R:IP312 R:IP313 R:IP314 R:IP315 R:IP316 R:IP317 R:IP318 R:IP320 R:IP321 R:IP323 R:IP324 R:IP325 magnitude low pressures. R:IP326 R:IP327A R:IP327B R:IP328 Location R:IP329 R:IP330 R:IP331 R:IP332 R:IP333 R:IP334 R:IP335 R:IP336 Recycler 30 - 40 House Ion Pump Pressure Profile R:IP337 R:IP338 R:IP339 R:IP340 R:IP341 R:IP400 R:IP401 R:IP402A Recycler vacuum shutdown work review - Nagaitsev R:IP402B R:IP402C R:IP403 R:IP404 R:IP4051 R:IP4052 1. Terry’s vacuum model predicts an order of R:IP406 R:IP407 So, what’s the problem with this model?? R:IP408 R:IP409 R:IP410 R:IP411 1/10/2003 R:IP412 There is only one ion pump that show pressures above 1x10-9 R:IP413 R:IP414 R:IP415 R:IP416 R:IP417 R:IP418 R:IP419 R:IP420 7/11/2003 R:IP421 R:IP422 R:IP423 R:IP424 31 R:IP425 So, what’s the problem with this model?? 2. We know that the total gas capacity of our TSPs is 0.2-0.4 Torr-L. Assuming we understand the TSP’s pumping speed at the beam pipe, pressures of 3x10-9 Torr would saturate the TSPs in 10-20 days, yet the TSPs last on the average 100 days. Recycler vacuum shutdown work review - Nagaitsev 32 Conclusions The evidence for the residual gas scattering from beam-based measurements is overwhelming. I can not reconcile the beam-based measurements with the instrument measurements. All beam-based measurements point to the fact that amount of heavy molecules (most likely water) in the system has doubled after the Jan 2003 shutdown. The model can not answer how much CH4, CO or CO2 is in the system. If I assume that all heavy molecules are water, eliminating them completely reduces the emittance growth rate to 2 μm/hr. It is likely that we will never reach this value with a present system. I estimate that reaching a 4-μm/hr rate is possible with a successful bakeout. Recycler vacuum shutdown work review - Nagaitsev 33