\subsection{Overview\label{ssec:ec_growth.ec_buildup.overview}} The buildup of high densities of low-energy electrons produced by the intense synchrotron radiation in electron and positron storage rings has been under active study since it was identified in the mid-90's in the KEK Photon Factory (PF) when operated with a positron beam \citep{PRL74:5044}. While this phenomenon did not present an operational limitation at the PF under nominal conditions, the observation raised immediate concerns for both B Factories, then under design, and triggered significant simulation efforts \citep{PRL75:1526} aimed at quantifying the phenomenon and designing mitigation techniques. Several years later, as the luminosity performance in the B Factories was pushed towards its specified goal, the electron cloud became at some point the most significant limitation. Mitigating this effect at both B Factories then became essential to reach, and then exceed, the design performance \citep{fukuma-BDNL48}. Almost concurrently with the above-mentioned developments at the PF and the B Factories, concerns arose in 1995-96, based on prior experience at the ISR, that electrons might spoil the LHC vacuum \citep{grobner-PAC97}. Early 1997 calculations showed that the LHC would be subject to an ECE \citep{SLAC:PUB7425}, chiefly because the beam emits copious synchrotron radiation upon traversing the dipole bending magnets, in analogy with \epem\ storage rings. Indeed, the LHC is the first proton storage ring ever built in which synchrotron radiation is significant: although the critical photon energy is only $ \sim44$ eV at 7 TeV beam energy (as opposed to several keV's in typical \epem\ storage rings), this value is well above the work function of the chamber metal, hence photoemission inside the chamber is unavoidable. A straightforward calculation yields an estimate of 0.4 photons emitted per proton per dipole magnet traversal, also at 7 TeV beam energy. Although $\sim50\%$ of these photons have energies below the work function of the metal, the remaining 50\% lead to a substantial number of photoelectrons. Further calculations \citep{SLAC:PUB7425, LHC:180}, including the effects of secondary electron emission, quickly revealed the possibility of a substantial ECE. In the case of the LHC, the primary concern from the ECE is the power deposited by the electrons on the beam screen as they rattle around the chamber under the action of the beam; this power must be dissipated by the cryogenic system if the LHC superconducting magnets are to work according to specifications. Since the cryogenic system was designed before the discovery of the ECE's in the LHC, a significant ``crash program'' was launched in 1997 to better estimate the power deposition, to identify the conditions under which the cooling capacity might be exceeded, and to devise mitigation mechanisms if necessary \cite{ruggiero-frascati97}. As part of this effort, the ECE has been experimentally studied at the SPS and the PS at the high beam intensities required for nominal LHC operation \cite{EC-LHC-website} (recent experience at the LHC has confirmed the expectation of a significant ECE, even though the beam energy is presently only 3.5 TeV \cite{jimenez-chamonix2011}). This crash program at the LHC was almost certainly the single most comprehensive effort to understand the electron cloud in a hadron machine, and was comparable in scope to the present program at CESRTA for an \epem\ machine. The above-mentioned ECE's are related to previously observed electron- proton dynamical effects such as beam-induced multipacting (BIM), first observed at the CERN proton storage ring ISR \cite{grobner- HEACC77} when operated with bunched beams. Closely related to BIM is trailing-edge multipacting observed at the LANL spallation neutron source PSR \cite{macek-PSR-status-ECLOUD04}, where electron detectors register a large signal during the passage of the tail of the bunch even for stable beams. All ECEs in \epem\ as well as in hadron storage rings have precursors in the e-p instabilities for bunched and unbunched beams first seen at BINP in the mid-60”Ēs \cite{dudnikov- PAC01-TPPH094}. The interest generated by the observations at the PF published in 1995 triggered a series of dedicated workshops the most recent of which was held at Cornell in 2010~\citep{ECLOUD10PROC}. The similarities of the ECE's in \epem\ and hadron storage rings is evidenced by the simultaneous and comparable participation, since 1997, of both communities in these workshops. Phenomena related to electron cloud buildup have been reported (and in some cases been a performance- limiting factor) at the Advanced Photon Source at ANL~ \citep{PRSTAB6:034402}, BEPC \cite{guo-PAC97}, the Spallation Neutron Source at ORNL~\citep{EPAC08:TUPP043}, the Relativistic Hadron Collider at BNL~\citep{PRSTAB11:041002}, the Proton Storage Ring at LANL~\citep{PAC03:ROAB003}, the \mbox{DA$\Phi$NE} $\Phi$-factory at the INFN-LNF in Frascati~\citep{PAC05:FPAP001}, PEP-II at SLAC~ \citep{MBI97:170,PAC01:TPPH100}, the Tevatron Main Injector at FNAL~ \citep{PAC07:THPAN117}, the CERN Proton Synchrotron~ \citep{PRSTAB5:094401} and Super Proton Synchrotron~ \citep{BEAM07:202to208} and the LHC \citep{jimenez-chamonix2011}. The enhanced understanding of electron cloud physics has identified the phenomenon as a primary potential limiting factor in the operation of damping rings for future \epem\ linear colliders~ \citep{PAC03:WOAA006,EPAC08:MOPP050,PAC05:ROPB001}. The determining physical phenomena governing the characteristics of the clouds are the generation of photoelectrons, their trajectories in the transient and ambient electric and magnetic fields, and the secondary electron yield properties of the vacuum chamber. Thus the RF structure of the beams, their intensities, the shape and dimensions of the vacuum chambers, and especially the surface physics properties of those chambers affect the production of electrons by incident electrons, and are therefore important factors in the rates of buildup and decay. A variety of mitigation techniques have been experimentally studied and implemented in operating beamlines, such as grooves on the chamber surface of the LHC arcs, the TiN coating in the PEP-II ring~ \citep{NIMA551:187to199} and at the Spallation Neutron Source, and weak solenoidal magnetic fields at both B Factories \citep{PAC01:TPPH100, PAC01:RPPH131}. The \cesrta\ program is almost certainly the single most extensive project for the study of mitigation techniques in \epem\ storage rings to date, including mitigation techniques based on low-emission coatings such as TiN, amorphous carbon and diamond-like carbon on aluminum chambers; grooves etched in copper chambers; clearing electrodes; and more. Combined with an extensive array of instrumentation and diagnostic tools such as retarding-field analyzers and shielded-pickup detectors, much has been learned to date about the physics governing the local buildup of electron clouds. In essentially all cases of practical interest, it is the secondary electron emission process that dominates the build-up of the electron cloud because this process leads to a compounding effect of the electron density under the action of successive bunches traversing the chamber: the more electrons are present in the chamber, the more electrons are generated upon striking the chamber walls. The CESRTA facility affords the unique and valuable possibility of studying the electron cloud formation and dissipation with a beam consisting of an almost arbitrary fill pattern and bunch intensity. This flexibility allows to tease out the contributions to the electron cloud due to photoemission from secondary electron emission. Short bunch trains (say, fewer than 10 bunches) lead to an electron cloud dominated by photoemission, while long bunch trains (say, more than 40) lead to an electron cloud dominated by secondary electron emission. As if this flexibility were not enough, CESRTA also allows the measurement of the electron cloud bunch by bunch, which provides another way to disentangle the two above-mentioned effects: the formation of the electron cloud at the beginning of the train is dominated by photoemission, while secondary emission dominates the formation towards the tail. In addition, an isolated ``witness bunch'' can be placed at varying distances from the end of the train, thus affording the possibility of studying the dissipation of the electron cloud as a function of time.