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					Electron clouds and vacuum pressure rise
                in RHIC
                  Wolfram Fischer
                            Thanks to
     M. Blaskiewicz, H. Huang, H.C. Hseuh, U. Iriso, S. Peggs,
           G. Rumolo, D. Trbojevic, J. Wei, S.Y. Zhang




     33rd ICFA Advanced Beam Dynamics Workshop on
      High Intensity & High Brightness Hadron Beams
                Bensheim, 19 October 2004

                                                                 1
                                 Contents


  • Pressure rise observations
        – Injection
        – Transition
        – Store
  • Pressure instabilities
  • Counter measures




Wolfram Fischer                         2
                                                        Pressure rise observations


                  1st fill with 110 Au79+ bunches N=0.50·109 Oct. 2001

                                                            Beam losses
                                                            during acceleration



                                               next fill N=0.44·109




                              10-7 Torr abort limit




Wolfram Fischer                                                                   3
                                            Location of pressure rise problems in 2003/04
                                   Blue sector 10/11:
                                    Increased gas density
IP10: PHOBOS                        in cold arcs (with p)
 Be beam pipe
 (high background)




                                Run-4 Au-Au
                              Nov. 2003 to Apr. 2004


                          No of bunches: 61, 56, 45
                         Ions per bunch: 0.5-1.1109

                                                                   Yellow sector 4:
                                                                    Unbaked stochastic
 Blue sector 8:                                                     cooling kicker
  Unbaked collimator                                                (high pressure)
  (vacuum instability)
Wolfram Fischer                                                                         4
                                                                   RHIC pressure rise observation to date


                                                 Au79+                     d+                 p+
 Pressure rise locations                                     only warm                   warm / cold

                                                                       Injection
 Pressure rise observed                             Yes                    Yes                Yes
 E-clouds observed directly                         Yes                    Yes                Yes

                                                                      Transition
 Pressure rise observed                             Yes                    Yes               N/A
 E-clouds observed directly                         Yes                    No                N/A
                                               with large losses

                                                                          Store
 Pressure rise observed                             Yes                    No                 No
                                                                                          no rebucketing

 E-clouds observed directly                          No                    No                 No
 Pressure rise observed Yes = pressure rise  1 decade
 E-clouds observed directly = observed with electron detector
Wolfram Fischer                                                                                            5
                                                           Pressure rise mechanisms

  Pressure rise mechanisms considered so far
  • Electron cloud  probably dominating for operational problems
        – Coherent tune shift in bunch train
        – Electron detectors
        – Comparison with simulations
  • Ion desorption  tolerable for operation
        – Rest gas ionization, acceleration through beam
        – Ion energies ~15eV for Au, ~60 eV for p
        – Visible pressure rise, may lead to instability
          in conjunction with electron clouds (Au only)
  • Beam loss induced desorption  tolerable for operation
        – Need large beam loss for significant pressure rise
        – New desorption measurements in 2004
            (H. Huang, S.Y. Zhang, U. Iriso, and others)
Wolfram Fischer                                                                   6
                                                              Electron cloud observation at injection

Indirect observation – coherent tune shift along bunch train

 33·1011 p+ total, 0.3·1011 p+/bunch, 110 bunches, 108 ns spacing (2002)


                                                                (1) From measured tune
                                                                shift, the e-cloud density
                                                                is estimated to be
                                                                0.2 – 2.0 nC·m-1
                     DQ2.5·10-3
                                                                (2) E-cloud density can be
                                                                reproduced in simulation
                                                                with slightly higher charge
                                                                and 110 bunches
                                                                (CSEC by M. Blaskiewicz)




     [W. Fischer, J.M. Brennan, M. Blaskiewicz, and T. Satogata, “Electron cloud measurements and
     observations for the Brookhaven Relativistic Heavy Ion Collider”, PRSTAB 124401 (2002).]
Wolfram Fischer                                                                                     7
                                                              Electron cloud observation at injection

            Direct observation – electron detectors                                 U. Iriso-Ariz
                                                               Observation:
                                                                88·1011 p+ total
                                                                0.8·1011 p+/bunch
                                                                110 bunches
        bunches with                                            108 ns spacing
        lower intensity



                                                               Simulation:
                                                               Variation of SEYmax: 1.7 to 2.1
                                                               Keep R=0.6
                                                                (reflectivity for zero energy)

                                                               Good fit for
                                                                SEYmax = 1.8 and R=0.6

                                                               Code: CSEC by M. Blaskiewicz


       [U. Iriso-Ariz et al. “Electron cloud and pressure rise simulations for RHIC”, PAC’03.]
Wolfram Fischer                                                                                     8
                                                               Electron cloud observation at injection

Electron cloud and pressure rise                                                    U. Iriso-Ariz
86·1011 p+ total, 0.78·1011 p+/bunch, 110 bunches, 108 ns spacing

          e-cloud and pressure

                                                                             Clear connection
                                                                             between e-cloud
                                                                             and pressure at
                                                                             injection

                                                                          Estimate for he
                                12 min
                                                                          assuming pressure
          total beam intensity                                            caused by e-cloud:
                                                                            0.001-0.02
                                                                          (large error from
                                                                          multiple sources)


     [U. Iriso-Ariz et al. “Electron cloud observations at RHIC during FY2003”, in preparation.]
Wolfram Fischer                                                                                      9
                                         Pressure rise at injection – cold regions

Gas density increase observed in cold regions with intense proton beams
                                                         1.5e13 protons
        intensity




        pressure in gauge                                            1e-7 Torr




Wolfram Fischer                                                                10
                                                            Cold bore pumps and gauges

 H.C. Hseuh

 CCG and Sorption pump every other 15m
    17 CCGs per arc (~ one per 30m)
 Most CCGs read mid 10-10 Torr                             Gauge conduit




Sorption Pumps with 300g charcoal        CCGs with a 1.5mx1” Φ conduit to cold bore
@ ~ 10K, S(He) ~ 2 l/s                   Q ~ 10-9 Torr.l/s, C < 1 l/s
P (He) 10-10 Torr @ Q(He) > 30 Torr.l     P ~ 10-9 Torr
 Wolfram Fischer                                                                      11
                                              Pressure rise at injection – cold regions


  Cold surface not very clean:

  • Before cool down without pumping: 1e-1 Torr
       (in almost all cold regions in the past)
        About 10-100 monolayers

  • Pumping 500m arcs with turbo for 2 weeks: 1e-3 Torr
       (10 days after stop pumping: 1e-2 Torr)

  • Need to install more turbo pumps
     1e-3 Torr initial pressure results in 1 monolayer


Wolfram Fischer                                                                     12
                                                          Pressure rise at transition

    Typical transition pressure rise in Au-Au operation

                                        transition
                                        bunch length 4ns (18ns at injection)
                  beam intensities




                                                                     1e-7 Torr
                                                 IR12
                  IR4

                                 IR10

Wolfram Fischer                                                                   13
                  Presssure rise at transition




                        Strongly
                        intensity
                        dependent



                        No correlation
                        with beam loss

                         suggests
                        electron clouds


Wolfram Fischer                           14
                                                                Pressure rise at transition


            Stronger pressure rise for fewer bunches in IR12
      (independent of pattern in IR10 – same as Run-3, S.Y. Zhang)




    Behavior can be quantitatively reproduced in e-cloud simulation (U. Iriso)

Wolfram Fischer                                                                         15
                                                    Pressure rise at transition

  With fewer bunches, cloud density and current into wall is reduced,
  but electron impact energy is increased, and can lead to larger
  desorption. (simulation by U. Iriso)
                 45 bunches/ring 56 bunches/ring 61 bunches/ring
   e-cloud
   density



   current
  into wall


  electron                                                     400 eV
                         500 eV
impact energy
Wolfram Fischer                                                             16
                                                              Pressure rise at store – IR10 PHOBOS

PHOBOS background increase after rebucketing, drops after minutes to 2 hours
(most severe luminosity limit in Run-4)
                          Rebucketing, bunch length reduced to 50%



                                                                  intensity


                                                                   vacuum




                             background

     [Some thoughts on switch-off: U. Iriso and S. Peggs, “Electron cloud phase transitions”,
     BNL C-A/AP/147 (2004). Can e-cloud codes create 1st order phase transitions?]
Wolfram Fischer                                                                                 17
                                                     Pressure rise at store – IR10 PHOBOS

            Pressure before and after rebucketing (50% bunch length reduction)




                                  Run-4 physics stores


        Did not find narrow range that triggers problem for
        • average bunch intensity
        • peak bunch intensity
        • pressure before rebucketing
        No good correlation with any parameter and duration either
Wolfram Fischer                                                                        18
                                                     Pressure rise at store – IR10 PHOBOS

              Average bunch intensity at rebucketing/pressure drop,
            and duration of increased pressure sorted by bunch patterns




Wolfram Fischer                                                                        19
                    Pressure rise at store – IR10 PHOBOS, simulations G. Rumolo, GSI



                                         12m ~ 40ns

                                Be pipe




         Considered 2 cases:
         At IP:
         nominal bunch spacing (~216ns) and double intensity
         At end of the beryllium pipe:
         normal intensity, spacing of 40ns then 176ns
     [G. Rumolo and W. Fischer, “Observation on background in PHOBOS and related electron
     cloud simulations”, BNL C-A/AP/146 (2004).]
Wolfram Fischer                                                                             20
                  Pressure rise at store – IR10 PHOBOS, simulations G. Rumolo, GSI

        Can calibrate Be surface parameters:
        • No e-cloud before rebucketing (10ns bunch
        length)
        • E-cloud after rebucketing (5ns bunch length)N. Hilleret, LHC-VAC
                                                          Technical Note 00-10




                                                            Modified to match
                                                            observation




Wolfram Fischer                                                                 21
                  Pressure rise at store – IR10 PHOBOS, simulations G. Rumolo, GSI

              Important result:
               After surface parameter calibration find e-clouds
               at end of 12m Be pipe, but not in center
               May be sufficient to suppress e-cloud at ends

                         Emax=400 eV and dmax=2.5




            Center of Be pipe                           End of Be pipe
Wolfram Fischer                                                                 22
                                                      RHIC vacuum instabilities


  • In a number of cases vacuum instabilities were
    observed (pressure grows exponentially without bound)
  • Vacuum instabilities seen
        –   Only with Au79+ beam
        –   Only at locations with unbaked surfaces
        –   At injection (previous runs)
        –   At store, after rebucketing (Run-4)
        –   May be at transition
            (growth time large compared to transition crossing time)
  • Growth times range from 2 to 12 sec

Wolfram Fischer                                                              23
                                                              RHIC vacuum instability




                                                               11.7 sec growth time




                  Location of unbaked collimator (unbaked due to scheduling conflict)
Wolfram Fischer                                                                    24
                                                      RHIC vacuum instabilities


  • Need feedback mechanisms for instability
       (gas load Q proportional to rest gas pressure P):
        – Rest gas ionization by cloud electrons


        – Rest gas ionization by beam



  • Define parameter (analog ISR instability)


                            e-cloud        beam
                                 ionization                                  25
Wolfram Fischer
                                       RHIC vacuum instabilities


  • Critical desorption coefficient for ionized
    and accelerated rest gas molecules
                                        c – conducdance
                                        L – half distance
                                         between pumps


  • Growth time (very approximate)

                                         r – pipe radius




Wolfram Fischer                                               26
                                                 RHIC vacuum instabilities

  Parameter                       Unit   Au79+            p+
  Reported hH2     [CERN 99-05]   …       0.4            1.5
  Reported hCO [CERN 99-05]       …       0.3            1.2
  Calc. hH2 critical              …       42             131
  Calc. hCO critical              …       2.8             14
  Meas. growth time t             sec    6-12
  Calc. growth time tH2           sec     23
  Calc. growth time tCO           sec     5.4

  • e-clouds contribute about 20% of effect
  • Instability may be possible for Au79+ and CO like molecules.
  • Still signifcant descrepancy between reported h and calc. hcrit
    (higher charge state rest gas ions important?)
  • Note: No h measurements available for ion energies below 100 eV.
Wolfram Fischer                                                         27
                                                 Electron counter measures

  • In-situ baking of warm elements
       (>95% of 700m/ring warm pipes baked)
     Occasionally installation schedules were too tight
  • NEG coated pipes
     Installed 60m last shut-down for test,
            about 250m of NEG coated pipes now
  • Optimized bunch patterns
     Most uniform along circumverence, used this year
  • More pumping of before cool-down
     Need more turbo pumps for full coverage
  • Solenoids
     Tested, no large scale implementation planned near term
  • Scrubbing
     Tested, no large scale implementation planned near term
Wolfram Fischer                                                         28
                                                                               Summary

  • All operational relevant dynamic pressure rises in
    RHIC can be explained with electron clouds
       (an abnormal large beam losses can still lead to inacceptable vaccum)
  • Electron cloud driven pressure rises observed
        – With all species (Au79+, d+, p+),
        – In warm and cold regions
        – At injection, transition and store
  • Pressure instabilities only for Au79+ and unbaked wall
       (likely caused by rest gas ionization by beam and electron cloud)
  • Current counter measures:
        – Complete baking of all elements
        – NEG coated warm beam pipes
        – Optimized bunch patterns


Wolfram Fischer                                                                     29

				
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