Thermal effects in PEP-IIConsequences for BABAR
• The thermal effects in PEP-II– Origin and characteristics
• Before and after the ROD• The drivers of BABAR background:
VGCC3027, VP3044, VP3075 and VP3147
• Detailed time history of the sensivity coefficients
• A possible model
Overview of Thermal effects in PEP2
• The sustained running at high currents in the LER causes heating in some PUMPS which then OUTGAS (mainly Ion pumps but 150°F has been also observed on NEG pumps)
• The source of the heating is High Order Modes getting into the core of the pump
• The associated time constants are order of HOURS• The pressure rise causes higher beam-gas background• The single beam LER background is now a function of :
f(I_Ler, Time_since_refill)
Beam current
VGCC3027
From Artem Koulikov
0
5
10
15
20
1.7 1.8 1.9 2 2.1 2.2 2.3
VGCC3027Pr
essu
re,
nTor
r
LER current, APreROD data, from Artem Koulikov
0
5
10
15
20
25
30
35
vgcc
4033
vp31
81vp
3164
vp31
47vp
3131
vp31
15vp
3075
vgcc
3075
vp30
54vg
cc30
54vp
3044
vgcc
3027
vgcc
2187
vp21
76vp
2167
vgcc
2099
vgcc
1143
Pres
sure
, nT
orr
A
IP
Beam
8.212.9
15.415.9
2459.68.2
12.9
PreROD, Artem Koulikov
Before and after the ROD• This effect has always been present. IT rises critically with current • BUT before the ROD, even at 2.2 A, BABAR was not very sensitive
to it• During the ROD on March 24, the NEG were activated (ie heated
around 500°C: this throws gas way which is then reabsorbed with higher efficiency)
• Just after the ROD. It was noticed that the pressure on the pumps has increased (ex VP3147) while most of them decreased
• BUT the main effect was that BABAR sensitivity to the thermal HEATING was INCREASED by a huge factor
• The main sensor is BAckward East Diode/East Diamond• Since then, the operators try to process the extra gas by constantly
running at the maximal allowed value of SIG9• The current was then raised from 1.5A on March 25 to 2.23 today
Pump HistoryThe pressures before and after the ROD
show in general no dramatic changes
VP3181
VP3164
VP3147
VP3131
VP3054
VP3044
VGCC3027
VP3075
VP3115
Rod
Diamond History
Diamond East History
ROD
Time history From March 17 to March 31
LER current
2.2 A
Study of LER vacuum problems using detailed time evolutions
• Most ideas discussed here stem from discussions/presentations by Witold and Artem
• Identification of the sources using very detailed time histories thru BABAR ambient database, fill by fill.
• Characterization of Babar background as ftcn of LER pressures in pre-ROD data using the 4 pumps sensitive to HOM and showing heat-related effects (plus a constant term*LER)
• VP3044, VGCC3027, VP3075 , VP314• Post ROD data
-Identical sources BUT Large changes in sensivity (Underlying mechanisms: Thermal outgassing , large current
sensivity when equilibrium reached
Electron/gas separationUse of the end of the fill
VP3181
VP3147VGCC3027
VP3044
Fit of Electron component2nd order polynomial
(but cubic term present)
LER current (A)
VP3181 VP3164
VP3115VP3147
A typical preROD fill VP3044March 17, between 3pm and midnight
LER VP3044
DIAM-E DIAM-E vs VP3044
Time in hours
Thermal outgassing!
New method (least square fit)
Diam_E as function of time
VP3044
VGCC3027
VP3075
VP3147
COnstant
Just after the ROD
VP3044VGCC3027VP3075VP3147COnstant
Main Discrete events
• March 29 12 pm Orbit change• March 30 12.45 pm Gap Voltage• March 31 23 pm Collimator change• April 3 9 am Collimator change• April 3 17 pm Bunch pattern change
Orbit changePressure history for March 29
Time in hours
VP3044
VP3075
50 nT
VGCC3027
VP3147
30 nT
70 nT
2 nT
Gap Voltage change, March 30 at 12.45 pm
VP3044
VP3075VP3147
VGCC3027
Time, in hours
Collimator March 31 at 11 pm
VP3044VGCC3027
VP3147VP3075
Time in hours
Diamond evoution
Days in March (32= April 1st)
1 point per fill
PreROD Level
Success on April 3
ROD
Sensitivity Coefficients history
VP3044
VP3075
VGCC3027
VP3147
Days in March
1 point per fill
RF voltage gap change
RODROD
X200 sensitivity change just after the ROD!!!
Coll open.
Fractions of the Diamond signal for each pump , fill by fill
Days in March
VP3044
VP3075
VGCC3027
VP3147
Zooming in on April 3
Days in March (34= April 3)
Asymptotic Diamond level
Pre ROD level
After collimator opening
After bunch pattern change
Fits April 3Before :
Large VP3044 and VP3147 components
Time , 10 sec units
DiamEast
VP3044VGCC3027VP3075VP3147COnstant
After coll opening, no more VP3044 component but higher VGCC3027
APril 3, after collimator AND bunch pattern changes
Negligible VP3044 AND VGCC3027!
Pressures History in April 3Once again, the story is not in the pressure
but in the sensitivity coefficients
VP3044
VP3075
VGCC3027
VP3147
Coefficients on April 3Coefficients
VP3044
Coll opening reduced VP3044 dramatically
Bunch pattern change reduced VP3147 by a factor 2
VGCC3023 coeff. Increased when coll was open but it was processed away
VP3075
Dead time issue on April 3 VGCC2187 drives BABAR dead time. It was reduced a) on
coll opening b) on bunch pattern change
VGCC2187 pressure
BABAR Dead time (in %)
Time in hours Time in hours
Babar Dead time
VGCC2187
Conclusions• PEP exhibits thermal outgassing in (at least) 4 locations related to
HOM losses which then induce BABAR beam gas background• BABAR sensitivity was significantly increased after the ROD: Best
hypothesis: outgassing of heavy Gas, although the pressures themselves did not change much
• A detailed fit to the time evolution of the background signal as a linear combiantion of each pump alows to extract the sensitivity coefficients fill by fill.
• The main contributors were:• VP3044 and VGCC3027 in th week following the ROD• VP3147 came into play after 1 week , triggered by the RF voltage
change• The fix to the background problem was
-processing-reduction of HOM heating by opening collimators and change in bunch
pattern