Beam-based vacuum observations and their Beam-based vacuum observations and their consequencesconsequences
Sergei NagaitsevAugust 18, 2003
Recycler vacuum shutdown work review - Nagaitsev 2
Design goalsDesign goals
Stochastic cooling only Emittance growth rate (n, 95%): 2 μm/hr Lifetime of 10-μm emittance pbar beam with
cooling: ≥ 200 hrsStochastic+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:
%95,%95,
30)(
6nn
A
AA
AAΛ
yx
yx
Recycler vacuum shutdown work review - Nagaitsev 3
Design goalsDesign goals
Stochastic cooling only Emittance growth rate (n, 95%): 2 μm/hr Lifetime of 10-μm emittance pbar beam with cooling: ≥ 200 hrsStochastic+electron cooling Emittance growth rate (n, 95%): 4 μm/hr Lifetime of 10-μm emittance pbar beam with cooling: ≥ 200 hrs
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%)
0 10 20 30 400.01
0.1
1
Normilized 95% emittance [mm mrad]
Rel
ativ
e be
am li
fetim
e
Recycler vacuum shutdown work review - Nagaitsev 4
Today’s menuToday’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 5
Today’s menuToday’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 6
Today’s menuToday’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 7
Today’s menuToday’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 8
Observations (coasting beam) Aug 2, 2003Observations (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 9
Emittance growth rateEmittance 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 10
Emittance growth rateEmittance 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:
i i
ieiniiave
p
Z
ZZZn
crn
)()(12 2
2
%95,
Recycler vacuum shutdown work review - Nagaitsev 11
Observations (coasting beam) Aug 2, 2003Observations (coasting beam) Aug 2, 2003
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
-12 MeV/c 12 MeV/c -12 MeV/c 12 MeV/c
Recycler vacuum shutdown work review - Nagaitsev 12
Observations (coasting beam) Aug 13, 2003Observations (coasting beam) Aug 13, 2003
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.
dB
Frequency
Recycler vacuum shutdown work review - Nagaitsev 13
The low-energy tailThe low-energy tail
We’ve proved that the low-energy tail comes from ionization losses.
1.9E11 protons before scrape 0.9E11 protons after scrape
-12 MeV/c 12 MeV/c
Recycler vacuum shutdown work review - Nagaitsev 14
The low-energy tailThe 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 15
The low-energy tailThe 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 16
Proton collision with a stationary electronProton collision with a stationary electron
Max. energy transfer Tmax= 91 MeV
0 20 40 60 80 1000
1
2
3
4
5
Tra
nsve
rse
mom
entu
m, M
eV/c
pperp ( )
t ( )
Energy transfer per collision, MeV
Outside of ±0.3% momentum acceptance
Recycler vacuum shutdown work review - Nagaitsev 17
Proton collision with a stationary electronProton collision with a stationary electron
avep
n cM
pJ
22
2
2
1
0 5 10 15 200
10
20
30
40
Bet
atro
n ac
tion,
mm
-mra
d
J ( )
t ( )
Energy transfer per collision, MeV
Scraped to 7 mm-mrad
Recycler vacuum shutdown work review - Nagaitsev 18
Observations (coasting beam) Aug 2, 2003Observations (coasting beam) Aug 2, 2003
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
-12 MeV/c 12 MeV/c -12 MeV/c 12 MeV/c
Recycler vacuum shutdown work review - Nagaitsev 19
The rms momentum spread increaseThe rms momentum spread increase
IBS
Lc - is the Coulomb logarithm, C – is the ring circumference, σ – is the rms beam size and θ – is the rms angular spread.
21
2||
233
2
2
2||2
||
C
cNLr
p
p
dt
d
dt
d Cp
Recycler vacuum shutdown work review - Nagaitsev 20
IBS (coasting beam)IBS (coasting beam)
The rms momentum growth rate (MeV/c per hour) as a function of the rms momentum spread (MeV/c) for various transverse (n,95%) emittances.
N = 9x1010 protons Measured:
Initial 1.0 MeV/c Final 1.3 MeV/c
Need to add rms spreads in quadrature!
The ibs can explain a portion (0.1-0.2 MeV/c) of the measured momentum spread.
1 105
1 106
1 107
1 105
1 106
1 p( )
2 p( )
3 p( )
4 p( )
p Mp
dWithout any cooling the momentum spread diffusion rate is (for a 10 pi emittance):
εn =5 μm
10 μm
15 μm
20 μm
Recycler vacuum shutdown work review - Nagaitsev 21
0 2 105
4 105
6 105
8 105
1 106
0
2 106
4 106
6 106
8 106
1 105
1.2 105
g p( )
p
Energy loss, MeV
The rms momentum spread increaseThe rms momentum spread increase
IBS
Energy-loss straggling
Landau probabilitydensity
Mean energy loss: 0.42 MeVMost probable loss: 0.3 MeV
Recycler vacuum shutdown work review - Nagaitsev 22
Momentum distributionMomentum distribution
A Gaussian(σ=1 MeV/c) distr. is folded with the Landau probability density.
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
04.8 106
9.6 106
1.44 107
1.92 107
2.4 107
60
49.6
39.2
28.8
18.4
8
FD jj
GD jj
jj
Momentum spread: +/- 12 MeV/c
Recycler vacuum shutdown work review - Nagaitsev 23
Summary of beam-based measurementsSummary of beam-based measurements
Emittance growth rate:
Beam average energy loss: 0.40±0.04 MeV/hrThis corresponds to a mean energy loss of 0.42±0.04
MeV/hr
m/hr 1 10)(
)(12 22
%95,
i i
ieiniiave
p
Z
ZZZn
crn
i ii
i
i
I
Tx
A
ZMeV 0.040.42ln][g/cm
307.0MeV 2max2
2
Recycler vacuum shutdown work review - Nagaitsev 24
Partial pressures modelPartial 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 25
Partial pressures modelPartial 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 26
Partial pressures modelPartial 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 27
Partial pressures modelPartial 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 28
What could have increased the water content?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 29
So, what’s the problem with this model??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
1.0E-13
1.0E-12
1.0E-11
1.0E-10
1.0E-09
0 500 1000 1500 2000 2500 3000 3500 4000 4500 5000
Distance (cm)
Pres
sure
(To
rr)
Total
IG Readings
Hydrogen
Water
Carbon Dioxide
Carbon Monoxide
Methane/Ethane
Argon
Unknown
Recycler vacuum shutdown work review - Nagaitsev 30
So, what’s the problem with this model??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
TorrRecycler IG Readings
1.E-11
1.E-10
1.E-09
1.E-08
Location
Pres
sure
(T
orr)
1/10/2003 7/11/2003
Recycler vacuum shutdown work review - Nagaitsev 31
So, what’s the problem with this model??So, what’s the problem with this model??
1. Terry’s vacuum model predicts an order of magnitude low pressures.
There is only one ion pump that show pressures above 1x10-
9 Torr.Recycler 30 - 40 House Ion Pump Pressure Profile
1.E-10
1.E-09
1.E-08
1.E-07
R:I
P220
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C
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A
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B
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C
R:I
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1
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2
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R:I
P425
Location
Pre
ssur
e (T
orr)
1/10/2003 7/11/2003
Recycler vacuum shutdown work review - Nagaitsev 32
So, what’s the problem with this model??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 33
ConclusionsConclusions
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.