Building block. sources: paracle species + intensity (charge) acceleraaon process: energy & intensity

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1 Building block sources: paracle species + intensity (charge) acceleraaon process: energy & intensity pre- accelerator (someame): boost paracle energy for injecaon in accelerator front- end use: target, collisions, radiaaon producaon 1

2 Radiofrequency (RF) linacs most commonly encountered Ame dependent field sustained in a resonant cavity (see L3) a linac consists of many resonant caviaes the frequency choice is a compromise between: maximum field and size, achievable bunch length at injecaon linac composed of secaon operaang at different (harmonic) frequency is someame used 2

3 (see also HW3) Induc9on linacs typically used to accelerate very long (up to µs) high current pulses (typically can accelerate ~mc bunches) very few inducaon linacs based on Faraday s law I E = e E.dl pulse current provided by a HV pulse forming network can be viewed as a transformer Z S B.dS pulsed current to produce t- dependent magne1c field 3 H. Davis, LINAC 06, 208 (2006)

4 Induc9on linacs (cnt d) state of the art is DARHT at Los Alamos H. Davis, LINAC 06, 208 (2006) 4

5 Linac configura9on focusing laace interspaces with caviaes, not subject to radiaaon loss (synchrotron radiaaon). A limit can be formulated to be limited by radiaaon we would need an acceleraang gradient de dz 1014 MeV/m 5

6 Linac configura9on no need for laace periodicity (the beam sees the laace once), choice of cavity frequency depends on bunch length to be accelerated, one typically wants to insure: bunch length z 2 RF wavelength before linac z acer linac z z z z /(2 ) /(2 ) 6

7 longitudinal mo9on in a linac final energy for a paracle downstream of an acceleraang secaon: acceleraang iniaal energy final posiaon (w.r.t. the reference paracle) assuming relaavisac beam z = z 0 iniaal posiaon within bunch voltage E = E 0 + V cos(kz 0 + ) V = qt E 0 L off- crest phase (some ballisac moaon can occur and is ignored here; see L09 and PHYS 790- D HW2) Special topics in Beam Physics, 7

8 rewri9ng in (z, ) remember that take the reference paracle to coincide with bunch barycenter p ref = hpi then final fracaonal momentum spread he 0 i ' 0 hei + V where = p 1 p ref ' E hei hei [cos(kz 0 + ) cos ] hei he 0 i + V cos 1 8

9 Some applica9ons of linacs elementary- paracle physics: linear collider (e+/e- ) (decommissioned SLC, ILC?) Nuclear Physics: electron- on- target experiment (CEBAF, JLab) high- intensity beam for neutrino factories, etc accelerator- based light source: short wavelength free- electron laser, high- power infrared radiaaon for defense applicaaon sub- mm wave producaon (THz) 9

10 linac can produce short bunches bunch length in storage ring is limited by staasacal effects to an equilibrium bunch length; APS typically produce 10 ps bunches linacs are not subject to such a limitaaon bunching occurs by introducing an inseraon beamline that provides an energy dependent path length variaaon 10

11 bunch compression incoming is bunch is chirped (like for ballisac bunching; see L09) a paracle with iniaal coordinate (z 0,δ 0 ) is delayed by z = z 0 + R 56 0 high energy low energy now R 56 depends on the geometry of the bunch compressor. 11

12 how short? one can follow the exact same treatment as for ballisac bunching and show that the final (downstream of the bunch compressor beamline) longitudinal coordinate is given by z f = z 0 (1 + CR 56 ) if there is an uncorrelated (stochasac) energy spread the minimum final achievable bunch length is: z,f = R 56,0 12

13 choice of linac frequency beyond beam dynamics consideraaon, the linac frequency also has implicaaon on the maximum peak field achievable in the resonant cavity This is the Kilpatrick criterion f [MHz] 1.64(E max [MV/m]) 2 exp 8.5 E max [MV/m] the criterion is not strictly valid but provides guidance. 13

14 choice of linac frequency (CNT D) Kilpatrick s criterion provides some incenave to go to higher frequency Take E max f 1/2 1/2 Then the net acceleraaon from a pillbox cavity with length /2 E/ 1/2 for a total linac length L E total / 1/2 L / 1/2 operaang at higher frequency can lead to higher energy gain for a given linac length 14

15 frequency bands RF linac W THz 15

16 SLAC linear collider (SLC)

17 high- frequency linacs 94- GHz linac (W band) /2 17

18 high- frequency (X- band) linacs W. S. Graves et al. arxiv: [physics.acc- ph] (2014) electrons 18

19 limita9ons of normal conduc9ng RF (NCRF) linacs finite resistance of the conductor used to make the resonant cavity leads to losses (Joule): P = 1 2 R s H 2 ds shunt impedance R s = 1 (!) surface impedance depends on skin- depth (field penetraaon) 19

20 superconduc9ng RF (SCRF) linacs skin depth is replaced by London penetraaon depth typically L 50 nm ideally conducavity is!1 pracacal limitaaon (some e- in NC state) leads to residual surface impedance data measured for niobium (T c =9 K) [TESLA coll. DESY, Hamburg] 20

21 superconduc9ng RF (SCRF) linacs quality factor of superconducang resonator is very high typically energy can remained stored for a very long Ame (ms) and enable the acceleraaon of a conanuous (or pulsed) train of bunches This feature is put at use in: high- intensity proton/ion source (e.g. spallaaon neutron source), high average brightness accelerator based light source (MW free- electron laser, next- generaaon X- ray light source) nuclear and elementary paracle physics increase collision rates and staasacs. 21

22 next linear colliders internaaonal linear e+/e- collider 22

23 CEBAF: conanuous electron beam accelerator facility beam is recirculated in a pair of linacs for many passes recircula9ng linac this behaves like a long linac (though recirculaaon arcs set a limit on achievable bunch length, ) 23