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ALPHA Storage Ring Indiana University Xiaoying Pang

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Summary Purpose Wiggler design Injection and Extraction RF cavity Touschek lifetime Vacuum Photon source

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Our Purpose Provide radiation effect experiments for NASA Debunch the rf linac beam bunches Compact X-ray photon source based on Inverse Compton scattering (ICSX) Advantage: low cost, easy operation. Difficulties: long damping time, negative horizontal damping partition, space charge effect, beam lifetime issue.

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Design Typical operational energy : 20MeV ~100MeV, maximum : 600MeV Dipole (existing) 1) effective length 2m 2) bending radius = 1.273m 3) edge angle 12⁰ vertical focusing Wiggler (three dipoles) 1) modify damping partition number horizontal betatron motion stable (without wigglers J x = -0.3 ) 2) tune momentum compaction factor C = 20 m T = 66.6 ns 3m

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Wiggler Design Three gradient rectangular dipoles with B 1 /B 0 =1.9 m −1, where B 1 =(dB z /dx) x=0 The minimum vertical gap is 40mm. Maximum field strength is 1.67 kG at 25MeV, 6.7kG at 100MeV. 0.1m 0.2m

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zero momentum compaction factor positive horizontal damping partition number Wiggler Design A linac beam can be debunched of its rf structure in one turn if | c |≥0.5 When the wiggler is turned off, w =∞ Qausi-isochronous condition

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Wiggler Design 25MeV At 50 MeV, the horizontal damping time can be 10s.

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Injection Two bumpers and a Lamberston septum are used Use electrostatic kickers with kicker rise time 10ns Beams are injected from a linac with 25MeV (up to 60 MeV) Phase space painting

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kicker 1kicker2 dipole septum

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At the kickers’ location: At the septum’s location: s = 0.818, mrad X co (septum) 32 mm beam pipe 25mm2-5mm

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1mm5mm or 2mm 2mm X=0 Septum A max = 25mm x Injection Scheme (Accumulation)

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Injection Efficiency vs Fractional Tune = 0.75 = 0.73 = 0.77 When tune is off 0.75, the ideal 4-injection-turn per closed orbit location is not guaranteed

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= 0.667 = 0.6 or 0.8 Number of Injection Turns per Closed Orbit Location

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x x ’ Septum 5 injections per ellipse x x ’ Septum 4 injections per ellipse More Ellipses! Number of Injection Turns per Closed Orbit Location

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Total number of Injection Turns

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Electrostatic kicker will be used: Kicker Strength For one turn injection and extraction, the integrated field strength is 0.60 MV at 25 MeV electron beam energy. Choosing a length of L=0.5 m, the applied voltage on two plate is 60 kV.

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Kicker Strength

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Sample Injection Watch the beam at septum

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Beam profile evolution around the ring

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Considering the aperture: Let’s take into the consideration of the apertures at the bending dipoles and electrostatic kickers. Set the aperture radius at dipoles to be 100mm = 0.1m, at kickers to be 25mm. The total injection numbers will decrease. With about 10 turns of injection, 50ns bunch length and 0.5A linac current, we can achieve:

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Extraction by Lambertson extraction magnet The Lambertson septum is used to extract beam. Injection Septum Extraction 1.4m 0.2m

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RF cavity Revolution frequency 15MHz In the operational mode of debunching no RF cavity is needed. For beam physics study with quasi- isochronous condition, we can modify the existing MPI cavity to make it operate at h=1, f=15MHz

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MPI Cavity Was built for proton acceleration with frequency from 2 to 10MHz A quarter-wave –like cavity, is loaded with 10 ferrite rings with quadrupole field bias. Major RF tuning is achieved by parallel external capacitors. With an external capacitance C ext =290 pF, the cavity was tested up to 11.4MHz, the resulting shunt impedance was about 1k . Diameter of the cavity ~0.55m; Length ~0.6m

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In the future For 15MHz operation, we need to reduce the external capacitance to about 120pF or the number of ferrite rings in the cavity. Reconfigure the ferrite rings to maximize the shunt impedance for a possible 3kV voltage. We will built a 90 MHz rf cavity for harmonic h=6 (or 494 MHz, h=33)in order to achieve a bunch length of the order of 10ps for short- pulse X-rays

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Touschek lifetime Toucheck lifetime is sensitive to the parameter: is the rf bucket height, is the horizontal momen- tum spread can range from 0.001 to 1. we will need a lifetime of 1h or more. It can also be varied by changing the momentum compaction factor where,

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Vacuum Emittances are dominated by pressure in low energy, become natural emittances at high energy.

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Compact Photon Source The X-ray is generated by laser- electron scattering at the chicane magnet.

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X-ray The energy of the scattering photon is: where, E L is the laser energy, c is the electron speed, is angle of the scattered X-ray photon, * is the crossing angle of the laser and the electron beam, for head on collision, *= . is a small correction term. The scattered X-ray photons are confined to a cone of 1/ with respect to the electron beam direction The bending angle of the chicane magnet can vary from zero to 110mrad. The scattered X-ray can easily be separated from the circulating electron beam at a distance 25cm from the collision point.

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Photon Brilliance The brilliance of the back scattering X-ray photon is: The X-ray flux is given by: where L is the luminosity. For head-on collision, the luminosity is: Brilliance 1/ x 2 z 2

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Quadrupole Triplet

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Fitting results Best resultsParameters

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Currently the ALPHA ring is under construction. We will start the experiments on RF cavity soon.

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