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multipactoring and the ece have been detrimental to the functioning of onboard space devices as well as klystrons and accelerators . in the latter ,
the ece was characterized and dealt with at the cern isr ( intersecting storage rings ) @xcite . with the construction of high current colliders ,
e.g. , the b factories , or accelerators producing a high amount of photons per ( e@xmath0 , h@xmath0 ) bunch , multipacting and the ece are again being studied extensively @xcite .
methods to suppress multipacting or the formation of electron cloud ( ec ) are still similar to the ones used for radio frequency ( rf ) components .
one can allow the secondary electron to be produced and then get rid of them afterwards .
for example by the use of electron clearing electrodes or , by use of low magnetic field solenoids that return the secondary electrons to the surfaces from which they have been produced .
when photons are responsible for the creation of the ec , it is also possible to confine the photoelectrons to a place which is non - detrimental to the accelerator , for example in an ante - chamber .
finally , one may modify the surface to produce less than one secondary electron per incident electron .
this can be done , for example , by using a rough surface , by using an emission suppressing coating , or by cleaning the surface in - situ ( thermal treatment , plasma glow discharge etc ... ) to remove high sey adsorbed gas and oxide layers .
these remedies can also be mixed together to give the best results for solving the problem in an existing machine or to be applied in a forthcoming accelerator where the ece problem is expected to occur .
of course , lowering the circulating beam intensity can retard cloud formation but that also affects luminosity . in this summary ,
we report sey measurements obtained at slac on flat surfaces , aluminium , tin , tizrv non evaporable getter ( neg ) and ticn .
the system used to measure the sey is shown in fig.[figsketchsetup ] . the experimental methodology used to measure
the secondary electron yield has been described in @xcite    1 analysis chamber - 2 loadlock chamber    3 sample plate entry - 4 sample transfer plate    5 rack and pinion travel - 6 sample plate stage    7 xyz@xmath1 omniax@xmath2 manipulator - 8 sample on xyz@xmath1    9 electrostatic energy analyzer - 10 x - ray source    11 sey / sem electron gun - 12 microfocus ion gun    13 sputter ion gun - 14 to pressure gauges and rga    15 to vacuum pumps - 16 gate valve    the sey ( @xmath3 ) definition is the number of electrons leaving the surface over the number of incident electrons ( primary electrons ) , which becomes @xmath4 . with @xmath5 the primary electrons and @xmath6 the total sample current being the difference between the primary and secondary electron current .
a natural method for reducing the sey of a material is thermal heating .
usually this is achieved during an in - situ bakeout . however , to be efficient the bake should be above 150@xmath7c .
nevertheless , any increase of surface temperature has an effect on the sey @xcite . on some materials like cu or ag , certain oxides have an sey below that of the atomically clean metal .
hence , growing an in - situ oxide by heating the surface , in presence of oxygen , is also a possibility @xcite .
many examples can be found in the literature , and an excellent summary is available @xcite .
another way of processing a surface to lower its sey is to expose it to energetic particles : photons , electrons or ions .
usually the sey of metals obtained after exposure to energetic particles is close to that of an atomically clean surface .
this trend seems also not to be observed in the case of exposure to very energetic ions , mev range per nucleons @xcite .    in the laboratory
we have quantified the reduction of the sey as a function of electron or ion bombardment . by ion bombardment we mean an ion beam , not a plasma glow discharge .
a plasma glow discharge is very effective in cleaning the surface in a few minutes , but plasma gas pressure required for a stable discharge is above a mtorr @xcite .
moreover , performing an in - situ glow discharge of in an accelerator vacuum beam pipe is far from trivial .
tin and activated tizrv getter coatings are good candidates for suppressing the ece .
tin coating is known to have a sey max ( @xmath8 ) below or close to 1 when freshly deposited @xcite . however
, when the `` as - deposited '' film is exposed to air , its sey maximum varies between 1.5 to 2.7 @xcite .
sputtering of air exposed tin by ar@xmath0 ions or exposing it to a high dose of electrons will return @xmath8 to around 1 @xcite .    in the ece
the energy gain of the secondary electrons is typically lower than 300  ev . in the case of the international linear collider ( ilc )
positron damping ring , the average energy was computed to be 130 ev @xcite .
the effect of 130  ev electron conditioning on the sey and @xmath8 are shown in fig.[figseylblall ] and [ figseymaxelectron ] , respectively .
it can be seen that after a surface has been cleaned by a bake ( fig.[figseylblall ] ) or by electron conditioning the sey will increase even when the surface is left under a good vacuum ( 5.10@xmath9 torr ) , fig.[figseymaxvacuum ] .
any scrubbed or `` clean '' surface will adsorb molecules from the residual gas .
the increase of the sey is linked to the oxidation of the surface by the presence of oxygen atoms in these molecules .
this was directly verified by observing the evolution of the xps ( x - ray photoelectron spectroscopy ) carbon spectrum of tizrv during exposure to residual gas @xcite .    during the passage of the circulating positron beam
, ions will be created from the residual gas .
their energy , for the ilc damping rings , is less than 200  ev .
bombardment from these ions can also contribute to sey reduction . to probe this effect ,
we have submitted three surfaces to ion bombardment .
the results are summarized in table.[tabseyiondose ] .
the experimental parameters are described in @xcite .
energy@xmath10 & dose @xmath11 + ticn & h@xmath12 & 1.29 & 280 & 1.11 + tin & n@xmath12 & 1.09 & 260 & 3.39 + tizrv & n@xmath12 & 1.15 & 300 & 2.29 +    [ tabseyiondose ]    comparing the effect of conditioning to tin , a gas - saturated tizrv neg was conditioned with a 130  ev electron beam , fig.[figseymaxelectron ] , and to a 250  ev ion beam , table.[tabseyiondose ] .
an n@xmath13 ion dose of 0.96  @xmath14c / mm@xmath15 reduces @xmath8 from 1.45 to 1.18 , further exposure up to 2.29  @xmath14c / mm@xmath15 , causes only a @xmath8 decrease from 1.18 to 1.15 @xcite .
aluminium is one of the common metal used in fabricating the accelerator vacuum chambers .
the sey of atomically clean al ranks among the best material with a @xmath8 around 1 .
clean al is extremely reactive to oxygen , however , upon air exposure , it will form a thick oxide with a @xmath8 well above 2 @xcite . as discussed earlier
, electron conditioning will bring the @xmath8 of the metal to its atomically clean value .
we repeated that measurement with oxidized aluminium . in the laboratory , we conditioned three different air - exposed technical aluminium surfaces and observed that the sey decreases at first and then re - increases .
results shown in fig.[figseyalu ] are similar for the two other samples @xcite .    during conditioning the xps spectra
show the c1s peak shifting toward lower binding energy ( be ) ) , signaling reduction of the oxide surface . with further conditioning ,
the trend stops and the peak broadens .
atomically clean al shows one peak at 73  ev ( metallic ) and another , al@xmath12o@xmath16 , at 76  ev .
two of our samples show this double peak structure .
thus , during conditioning , the peaks evolve from oxide to clean al and then reverse again .
the third sample was extremely oxidized but , again , the broad peak shifted to lower be and then broadened further , consistent with the other samples @xcite .
activated getter surfaces @xcite , like st707@xmath17 , tizr and tizrv have a @xmath8 below 1.3 @xcite .
they also provide linearly - distributed pumping capacity .
the main interest in tizrv coating over the other neg alloys is that its activation temperature is lowest , 180@xmath7c @xcite . during residual gas pumping the sey of neg increases ( fig.[figseymaxvacuum ] ) , just as it does for initially - clean tin .
the sey increase of neg was followed with xps measurement by monitoring the evolution of the carbon peak @xcite .
significantly , during residual gas atmosphere saturation of neg , the @xmath8 exceeds 1.3 , the maximum value obtained when saturating neg with individual common residual gases present in an accelerator environment @xcite .
this behaviour is seen for a freshly activated neg as well as from a 11.2  mc / mm@xmath15 electron conditioned surface , fig.[figseymaxvacuum ] .
this suggests that fast saturation by a single species is different from slow saturation by multiple species , i.e. , that time may play a role or co - adsorption of multiple species , may enhance the surface oxidation mechanism , as it can be seen in some surfaces @xcite .
the residual gas composition of a baked uhv is mainly composed of h@xmath12 which readily diffuses into the neg , co , co@xmath12 , h@xmath12o , and ch@xmath18 which is negligibly - pumped by tizrv @xcite .
the co - adsorption of the three oxidizing species enhances oxidation , similar to an air - oxidation , compared to oxides formed by dosing with a single specie .
thicker surface oxide has generally higher sey .
the process involved in building this thick oxide might be somewhat equivalent to cryogenic co - adsorption @xcite .
it is planned to test this hypothesis in our setup .
we have investigated a series of flat surface materials for suppressing the ece .
the most promising remedies are ti - based coatings .
tin has historically been the choice for successfully reducing multipacting . upon conditioning exposure to low energy ions or electrons ,
its atmosphere - oxidized surface @xmath8 returns to 1 .
ticn was developed as an oxidation - resistant replacement for tin ; however , it behaves similarly to tin .
another coating option is low temperature activated neg , tizrv . following an activating - bake
, its @xmath8 is 1.2 . as a bonus ,
neg coating provides distributed beam chamber wall pumping . while pumping toward film gas saturation , the @xmath8 increases . to return the sey to low value and restore the pumping capacity ,
the film can be thermally re - activated multiple times .
the sey of the surface itself may also be restored by electron or ion bombardment , which will also recreate some surface pumping capacity .
technical al surfaces were investigated under electron exposure .
we found that @xmath8 will not go consistently below 1.8 .
however , the behaviour at very large doses , above 3.10@xmath19  @xmath20 , was not measured .
the sey may increase further , stabilize or oscillate .    in an accelerator environment ,
synchrotron radiation , ions and electrons not only desorb molecules but also produce electrons which can lead to the formation of the ec .
as we have seen , electron exposure is very efficient in reducing the sey .
however , as the ec conditions the surface , the number of secondary electrons diminishes , hence the ec can oscillate between being on or off .
nevertheless , photons and ions directly created by the beam may ensure that an ec does not develop , but only direct measurements in beam chambers will confirm this @xcite .
most of the thin film sample were graciously produced by d. lee and a. wolski at lbnl , p. he and h.c hseuh at bnl .
our first tizrv getter was produced at cern , thanks to v. ruzinov of the est group .
the electron cloud effect ( ece ) causes beam instabilities in accelerator structures with intense positively charged bunched beams . 
 reduction of the secondary electron yield ( sey ) of the beam pipe inner wall is effective in controlling cloud formation . 
 we summarize sey results obtained from flat tin , tizrv and al surfaces carried out in a laboratory environment . 
 sey was measured after thermal conditioning , as well as after low energy , less than 300  ev , particle exposure .