Editing Compact Linear Collider (section)

== Beams and accelerators ==
To reach the desired 3&nbsp;TeV beam energy, while keeping the length of the accelerator compact, CLIC targets an accelerating gradient up to 100&nbsp;MV/m. CLIC is based on normal-[[Electrical conductivity|conducting]] acceleration cavities operated at room [[temperature]], as they allow for higher acceleration gradients than [[superconductivity|superconducting]] cavities. With this technology, the main limitation is the [[High voltage|high-voltage]] breakdown rate (BDR), which follows the [[empiricism|empirical]] law <math>BDR \propto E^{30}\tau^5</math>, where <math>E</math> is the accelerating gradient and <math>\tau</math> is the RF pulse length.<ref name=Grudiev_Calatroni_Wuensch_LocalFieldQuantity_CERN-2009>
{{cite journal
 |last1=Grudiev|first1=A.
 |last2=Calatroni|first2=S.
 |last3=Wuensch|first3=W.
 |title=New local field quantity describing the high gradient limit of accelerating structures
 |journal=Physical Review Special Topics: Accelerators and Beams
 |volume=12
 |issue=10
 |pages=102001
 |year=2009
 |doi=10.1103/PhysRevSTAB.12.102001 |doi-access=free
 |bibcode=2009PhRvS..12j2001G
}}</ref> The high accelerating gradient and the target BDR value (3&nbsp;×&nbsp;10<sup>−7</sup>&nbsp;pulse<sup>−1</sup>m<sup>−1</sup>) drive most of the beam [[parameter]]''s'' and [[machine]] design.

{| class="wikitable"
|+<small> Key parameters of the CLIC energy stages.<ref name="Burrows_CLIC_CERN-2018"/> </small>
|-
! Parameter !! Symbol !! Unit !! Stage 1 !! Stage 2 !! Stage 3
|-
| Centre-of-mass energy || <math>\sqrt{s}</math> || GeV || 380 || 1500 || 3000
|-
| Repetition frequency || ƒ<sub>rep</sub>|| Hz || 50 || 50 || 50
|-
| Number of bunches per train || ''n''<sub>b</sub> ||  || 352 || 312 || 312
|-
| Bunch separation || Δ''t'' || ns || 0.5 || 0.5 || 0.5
|-
| Pulse length || <math>\tau</math><sub>RF</sub> || ns || 244 || 244 || 244
|-
| Accelerating gradient || ''G'' || MV/m || 72 || 72/100 || 72/100
|-
| Total luminosity || ''L''|| 10<sup>34</sup> cm<sup>−2</sup>s<sup>−1</sup>|| 1.5 || 3.7 || 5.9
|-
| Luminosity above 99% of <math>\sqrt{s}</math> || ''L''<sub>0.01</sub> || 10<sup>34</sup> cm<sup>−2</sup>s<sup>−1</sup> || 0.9 || 1.4 || 2
|-
| Total integrated luminosity per year || ''L''<sub>int</sub> || fb<sup>−1</sup> || 180 || 444 || 708
|-
| Main linac tunnel length ||  || km || 11.4 || 29.0 || 50.1
|-
| Number of particles per bunch || ''N'' || 10<sup>9</sup> || 5.2 || 3.7 || 3.7
|-
| Bunch length || ''σ''<sub>''z''</sub> || μm || 70 || 44 || 44
|-
| IP beam size || ''σ''<sub>''x''</sub>/''σ''<sub>''y''</sub> || nm || 149/2.9 || ~60/1.5 || ~40/1
|-
| Normalised emittance (end of linac) || ''ε''<sub>''x''</sub>/''ε''<sub>''y''</sub> || nm || 900/20 || 660/20 || 660/20
|-
| Final RMS energy spread ||  || % || 0.35 || 0.35 || 0.35
|-
| Crossing angle (at IP) ||  || mrad || 16.5 || 20 || 20
|}

In order to reach these high accelerating gradients while keeping the power consumption affordable, CLIC makes use of a novel two-beam-acceleration scheme: a so-called Drive Beam runs parallel to the colliding Main Beam. The Drive Beam is decelerated in special devices called Power Extraction and Transfer Structures (PETS) that extract energy from the Drive Beam in the form of powerful [[radio frequency|Radio Frequency]] (RF) waves, which is then used to accelerate the Main Beam. Up to 90% of the energy of the Drive Beam is extracted and efficiently transferred to the Main Beam.<ref name=Adli_CLIC_CERN-2009>
{{cite thesis
 |last=Adli |first=E.
 |date=2009
 |title=A Study of the Beam Physics in the CLIC Drive Beam Decelerator
 |url=https://inspirehep.net/record/887068/files/CERN-THESIS-2010-024.pdf
 |publisher=University of Oslo
 |degree=PhD
}}</ref>
[[File:CLIC complex 3tev woarrows.jpg|alt=|thumb|400x400px|Overall layout of the CLIC accelerator complex for the 3&nbsp;TeV stage, in which one can identify the two Drive Beam and two Main Beam injector complexes<ref name=Burrows_CLIC_CERN-2018/>]]

=== Main beam ===
The electrons needed for the main beam are produced by illuminating a [[gallium arsenide|GaAs]]-type [[cathode]] with a Q-switched polarised [[laser]], and are longitudinally polarised at the level of 80%.<ref name="2018_proj_imp">
{{cite journal |last1=Aicheler |first1=M. |last2=Burrows |first2=P.N. |last3=Catalan |first3=N. |last4=Corsini |first4=R. |last5=Draper |first5=M. |last6=Osborne |first6=J. |last7=Schulte |first7=D. |last8=Stapnes |first8=S. |last9=Stuart |first9=M.J. |title=The Compact Linear Collider (CLIC) – Project Implementation Plan |date=20 December 2018 |journal=CERN Yellow Reports: Monographs |volume=4 |doi=10.23731/CYRM-2018-004|arxiv=1903.08655 |doi-access=free
}}</ref> The [[positron]]''s'' for the main beam are produced by sending a 5&nbsp;GeV electron beam on a [[tungsten]] target. After an initial acceleration up to 2.86&nbsp;GeV, both electrons and positrons enter damping rings for [[Beam emittance|emittance]] reduction by [[radiation damping]]. Both beams are then further accelerated to 9&nbsp;GeV in a common booster linac. Long transfer lines transport the two beams to the beginning of the main [[CERN Hadron LINACs|linacs]] where they are accelerated up to 1.5&nbsp;TeV before going into the Beam Delivery System (BDS), which squeezes and brings the beams into collision. The two beams collide at the IP with 20&nbsp;m[[radian|rad]] crossing [[angle]] in the horizontal plane.<ref name=2018_proj_imp/>

=== Drive beam ===
Each Drive Beam complex is composed of a 2.5&nbsp;km-long linac, followed by a Drive Beam Recombination Complex: a system of delay lines and combiner rings where the incoming beam pulses are interleaved to ultimately form a 12&nbsp;GHz sequence and a local beam [[electric current|current]] as high as 100[[ampere|A]].<ref name=2018_proj_imp/> Each 2.5&nbsp;km-long Drive Beam linac is powered by 1&nbsp;GHz [[klystron]]''s''. This produces a 148&nbsp;μs-long beam (for the 1.5&nbsp;TeV energy stage scenario) with a bunching [[frequency]] of 0.5&nbsp;GHz. Every 244&nbsp;ns the bunching phase is switched by 180 degrees, i.e. odd and even buckets at 1&nbsp;GHz are filled alternately. This phase-coding allows the first factor two recombination: the odd bunches are delayed in a Delay Loop (DL), while the even bunches bypass it. The [[time of flight]] of the DL is about 244&nbsp;ns and tuned at the picosecond level such that the two trains of bunches can merge, forming several 244&nbsp;ns-long trains with bunching frequency at 1&nbsp;GHz, separated by 244&nbsp;ns of empty space. This new time-structure allows for further factor 3 and factor 4 recombination in the following combiner rings with a similar mechanism as in the DL. The final [[time]] structure of the beam is made of several (up to 25) 244&nbsp;ns-long trains of bunches at 12&nbsp;GHz, spaced by gaps of about 5.5&nbsp;μs. The recombination is timed such that each combined train arrives in its own decelerator sector, synchronized with the arrival of the Main Beam. The use of low-frequency (1&nbsp;GHz), long-pulse-length (148&nbsp;μs) klystrons for accelerating the Drive Beam and the beam recombination makes it more convenient than using klystrons to directly accelerate the Main Beam.<ref name=2018_proj_imp />[[File:CLIC two beam module installed in the CLIC Test Facility at CERN (CTF3).jpg|thumb|Image of the CLIC Two Beam Module in the CLIC Test Facility, CERN ([[CTF3]]). The beam travels from left to right.|alt=|280x280px]]

=== Test facilities ===
The main [[technology]] challenges of the CLIC accelerator design have been successfully addressed in various test facilities. The Drive Beam production and recombination, and the two-beam acceleration concept were demonstrated at the [[CTF3|CLIC Test Facility 3 (CTF3)]]. [[X-band]] high-power [[klystron]]-based RF sources were built in stages at the high-gradient X-band test facility (XBOX), CERN.<ref name=Hamdi_XBandPowerSource_CERN-2012>
{{cite conference
 |editor1-last=Hamdi|editor1-first=A.
 |display-editors=et al.
 |id=C1205201
 |url=http://accelconf.web.cern.ch/AccelConf/IPAC2012/papers/THPPC060.pdf
 |title=Commissioning of the First Klystron-based X-band Power Source at CERN
 |book-title=Proceedings of IPAC2012, New Orleans, Louisiana, USA
 |number=3428
 |year=2012
 |isbn=978-3-95450-115-1
}}</ref><ref name=CatalanLasheras_ProceedingsLinac16_CERN-2012>
{{cite conference
 |editor1-last=Catalan Lasheras|editor1-first=N.
 |display-editors=et al.
 |id= Proceedings, 28th International Linear Accelerator Conference (LINAC16): East Lansing, Michigan
 |title=Commissioning of XBox-3: A Very High Capacity X-band Test Stand
 |book-title=Proceedings of LINAC2016, East Lansing, MI, USA
 |volume=LINAC2016
 |url=http://inspirehep.net/record/1633214/files/tuplr047.pdf
 |year=2016
 |last1=Catalán Lasheras
 |first1=Nuria
 |last2=Argyropoulos
 |first2=Theodoros
 |last3=Esperante Pereira
 |first3=Daniel
 |last4=Eymin
 |first4=Cedric
 |last5=Giner Navarro
 |first5=Jorge
 |last6=McMonagle
 |first6=Gerard
 |last7=Rey
 |first7=Stephane
 |last8=Solodko
 |first8=Anastasiya
 |last9=Syratchev
 |first9=Igor
 |last10=Volpi
 |first10=Matteo
 |last11=Woolley
 |first11=Benjamin
 |last12=Wuensch
 |first12=Walter
 |isbn=978-3-95450-169-4
}}</ref> These facilities provide the RF power and infrastructure required for the conditioning and verification of the performance of CLIC accelerating structures, and other X-band based projects. Additional X-band high-gradient tests are being carried out at the NEXTEF facility at [[KEK]] and at [[SLAC National Accelerator Laboratory|SLAC]], a new test stand is being commissioned at [[Tsinghua University]] and further test stands are being constructed at [[Laboratori Nazionali di Frascati|INFN Frascati]] and SINAP in Shanghai.<ref name=Burrows_Wuensch_Argyropoulos_XBandRF_CLICL_CERN-2017>
{{cite book
 |last1=Burrows|first1=Phil
 |last2=Wuensch|first2=Walter
 |last3=Argyropoulos|first3=Theodoros
 |title=Proceedings of 38th International Conference on High Energy Physics — PoS(ICHEP2016)- Chapter : High-gradient X-band RF technology for CLIC and beyond.
 |year=2017
 |pages=829
 |doi=10.22323/1.282.0829 |doi-access=free
 |chapter=High-gradient X-band RF technology for CLIC and beyond
}}</ref>
{{clear}}