Ultrafast harmonic rf kicker design and beam dynamics analysis for an energy recovery linac based electron circulator cooler ring

An ultrafast kicker system is being developed for the energy recovery linac (ERL) based electron circulator cooler ring (CCR) in the proposed Jefferson Lab Electron Ion Collider (JLEIC, previously named MEIC). In the CCR, the injected electron bunches can be recirculated while performing ion cooling for 10–30 turns before the extraction, thus reducing the recirculation beam current in the ERL to $1/10-1/30$ ($150\mathrm{mA}-50\mathrm{mA}$) of the cooling beam current (up to 1.5 A). Assuming a bunch repetition rate of 476.3 MHz and a recirculating factor of 10 in the CCR, the kicker is required to operate at a pulse repetition rate of 47.63 MHz with pulse width of around 2 ns, so that only every 10th bunch in the CCR will experience a transverse kick while the rest of the bunches will not be disturbed. Such a kicker pulse can be synthesized by ten harmonic modes of the 47.63 MHz kicker pulse repetition frequency, using up to four quarter wavelength resonator (QWR) based deflecting cavities. In this paper, several methods to synthesize such a kicker waveform will be discussed and a comparison of their beam dynamics performance is made using ELEGANT. Four QWR cavities are envisaged with high transverse shunt impedance requiring less than 100 W of total rf power for a Flat-Top kick pulse. Multipole fields due to the asymmetry of this type of cavity are analyzed. The transverse emittance growth due to the sextupole component is simulated in ELEGANT. Off-axis injection and extraction issues and beam optics using a multicavity kick-drift scheme will also be discussed.

Figure 1

Schematic of ERL-based bunched beam cooler (black solid) with option for future recirculation path (green dash).

Figure 2

Normalized continuous kicker waveform of 47.63 MHz repetition rate (bottom) resulting from the summation of a DC offset and 10 harmonics of the base frequency of 47.63 MHz (top).

Figure 3

Four different synthesized kick waveform schemes for a recirculating factor of 10 and bunch repetition frequency of 476.3 MHz.

Figure 4

Comparison of the kicking pulse curvature over $+/-3\sigma$ of 3 cm kicked electron bunch for the four waveform schemes with a recirculating factor of 10.

Figure 5

Two kickers tracking scheme in ELEGANT. Kicker 1 is for injection and Kicker 2 is for extraction.

Figure 6

Kicker scheme for 10-turn recirculation with two identical kickers, the alternating bunch kick is in one bunch spacing (left) and three bunch spacing (right).

Figure 7

The cancellation scheme of the residual kick with two identical kickers. A single particle in the phase space, receives two identical residual kicks with 180 degrees phase advance in x-x’ plane, and returns to its original phase space orbit.

Figure 8

Turn-by-turn normalized horizontal emittance for a single electron bunch in the two kicker scheme with 180 degrees betatron phase advance in between. The monitor is placed immediately after the injection kicker for the first 10 turns and after the extraction kicker for the last turn.

Figure 9

Compensation scheme for the nonuniform kicking pulse by using identical predistortion kicker (Kicker 3) and postdistortion kicker (Kicker 4).

Figure 10

Turn-by-turn normalized horizontal emittance growth for a single electron bunch with a predistortion kicker to compensate the nonuniform kicking pulse but no postdistortion kicker.

Figure 11

The distribution of electric field (left) and magnetic field (right) in the deflecting QWR.

Figure 12

10 harmonic modes in a four-cavity system with the highest harmonic electric field (left) and magnetic field (right) distribution shown in each cavity.

Figure 13

Normalized transverse voltage variations in x any y direction, $\mathrm{offset}=0$ is the beam line center.

Figure 14

Multicavity kick-drift tracking scheme with off-axis injection and extraction for flat-top waveform (the kick voltage is reversed at the last cavity as it is mainly used for curvature correction).