Damping-ring-free electron injector proposal for future linear colliders

The current designs of future electron-positron linear colliders incorporate large and complex damping rings to produce asymmetric beams for beamstrahlung suppression. Here, we present the design of an electron injector capable of delivering flat electron beams with phase-space partition comparable to the electron-beam parameters produced downstream of the damping ring in the proposed International Linear Collider (ILC) design. Our design does not employ a damping ring but is instead based on cross-plane phase-space manipulation techniques. The performance of the proposed configuration, its sensitivity to jitter along with its impact on spin-polarization are investigated. The proposed paradigm could be adapted to other linear collider concepts under consideration and offers a path toward significant cost and complexity reduction.

Figure 1

Block diagram of the proposed damping-ring-free injector concept. The emittance partitions at the various stages along the injector are also listed. We defined ${\widetilde{ϵ}}_m\equiv [{\widetilde{ϵ}}_u^2+{\tilde{\mathcal{L}}}^2{]}^{1/2}$. See text for details.

Figure 2

Overview of the emittance manipulation beamline combining the RFBT (skew-quadrupole magnets SQ1, SQ2, and SQ3) and EEX (from dipole magnet B1 to B4) insertions. The label “$\mathrm{SQ}i$” and “$\mathrm{Q}i$” refer to skew- and normal-quadrupole magnets, “$\mathrm{B}i$” and “$\mathrm{S}i$” are dipole nd sextupole magnets. The elements ”$\mathrm{TDC}i$” and “$\mathrm{HCAV}i$” refer to transverse-deflecting and 3.9-GHz SRF cavities; “SOL3” is a solenoidal magnetic lens.

Figure 3

Photoinjector diagram (upper schematics) and snapshots of the LPS distribution at $z=1.88$ (a), 7.48 (b), and 9.3 m (c) from the photocathode. Evolution of the beam energy and rms bunch length (d) and corresponding 4D transverse and longitudinal emittances (e). In the upper block diagram, SOL1 and SOL2 refer to the solenoidal magnetic lenses, ACC1-5 is the 1.3-GHz SRF cavities, and HCAV1-3 represent the 3.9-GHz SRF cavities. In plots (a–c) and throughout this paper, $\zeta >0$ corresponds to the head of the bunch.

Figure 4

Evolution of the horizontal (a), vertical (b), and longitudinal (c) emittance (blue traces) and bunch size (green dashed traces) along the emittance manipulation beamline (combining the RFBT and EEX transformations). The vertical shaded bands indicate the locations for the RFBT’s skew-quadrupole magnets (gray band at distances $<10\mathrm{m}$ are for SQ1-3) and dipole magnets (red bands from $\sim 14\mathrm{m}$ to the end of the beamline are for B1-4) associated with the EEX beamline; see Fig. 2.

Figure 5

Horizontal (a,d,g), vertical (b,e,h), and longitudinal (c,f,i) phase-space upstream of the RFBT (a,b,c), upstream of the EEX (d,e,f), and at the exit of the EEX (g,h,i).

Figure 6

Histogram of final horizontal (a) and vertical (b) emittances simulated downstream of the EEX beamline for 1000 realizations of SRF-cavity random phase and amplitude jitters.

Figure 7

Relative emittance dilution $\frac{\delta ϵ}{{ϵ}_0}$ (here ${ϵ}_0$ represents the nominal emittances reported in Table 2 obtained with the ellipsoidal distribution ${\nu }_{\perp }=2$) as a function of the exponent ${\nu }_{\perp }$ value in Eq. (23) (a). Spatiotemporal distribution $(\beta ct,r)$ at the cathode surface (b–d) and longitudinal phase spaces (e–g) for ${\nu }_{\perp }=0.5$ (b,e), 2 (c,f), and 8 (d,g). In plots (b,d), $\beta$ represents the reduced velocity at emission (i.e., associated with the excess kinetic energy) and $t$ is the emission time.

Figure 8

Evolution of the spin components along the emittance-manipulation beamline. Spin components associated with the reference particle ${\mathbf{S}}^T=(S_x,S_y,S_z)$ (a), statistical average $⟨\mathbf{S}⟩$ (b) and rms value $⟨{\mathbf{S}}^2{⟩}^{1/2}$ (c) computed over the macroparticle distribution. The vertical shaded bands indicate the locations for the RFBT’s skew-quadrupole magnets (gray band at distances $<10\mathrm{m}$ are for SQ1-3) and dipole magnets (red bands from $\sim 14\mathrm{m}$ to the end of the beamline are for B1-4) are associated with the EEX beamline; see Fig. 2.

Figure 9

Snapshots of the LPS distributions at the exit of the photoinjector (a), after acceleration to 5 GeV (b) and downstream of a single-stage bunch compressor (c) and current distribution (d) at the injector exit (“injector”) and downstream of the bunch compressor (“after BC”).