New halo formation mechanism at the KEK compact energy recovery linac

The beam halo mitigation is a very important challenge for reliable and safe operation of a high-energy machine. A systematic beam halo study was conducted at the KEK compact energy recovery linac (cERL) since non-negligible beam loss was observed in the recirculation loop during a common operation. We found that the beam loss can be avoided by making use of the collimation system. Beam halo measurements have demonstrated the presence of vertical beam halos at multiple locations in the beam line (except the region near the electron gun). Based on these observations, we made a conjecture that the transverse beam halo is attributed to the longitudinal bunch tail arising at the photocathode. The transfer of particles from the longitudinal space to a transverse halo may have been observed and studied in other machines, considering nonlinear effects as their causes. However, our study demonstrates a new unique halo formation mechanism, in which a transverse beam halo can be generated by a longitudinal bunch tail due to transverse rf kicks from the accelerating (monopole) fields of the radio-frequency cavities. This halo formation occurs when nonrelativistic particles enter the cavities with a transverse offset, even if neither nonlinear optics nor nonlinear beam effects are present. A careful realignment of the injector system will mitigate the present halo. Another possible cure is to reduce the bunch tails by changing the photocathode material from the present GaAs to a multi-alkali that is known to have a shorter longitudinal tail.

Figure 1

Layout of cERL and locations of measurement equipment. The collimators COL1 and COL2 are located at the end of the injector before the first bending magnet, and between the second and the third bending magnets of the merger section, respectively. COL4 is located in the first arc section. The positions of screen monitors SCM8, 16, 17, and 21A are chosen to observe beam halo profiles in the merger section, at the first arc, at the exit of the first arc, and at the end of the south straight section, respectively.

Figure 2

Betatron and dispersion functions of the recirculation loop from the exit of the main linac cryomodule to the beam dump.

Figure 3

Schematic of the screen monitor system with a 12-bit CCD camera and a YAG:Ce screen (top). Comparison of sizes of the YAG:Ce screens and the CCD camera screen images (bottom).

Figure 4

Schematic of the collimators COL1 and COL2.

Figure 5

Measured beam halo profiles at different beam line locations without (left) and with (right) collimation, respectively. (a),(b): Screen monitor SCM8 at the burst mode with gain 22 dB and integration time of $10\mu \mathrm{s}$. (c)–(h): screen monitors SCM16, 17, and 21A at the long pulse mode with gain 22 dB and integration time of 2 ms, respectively.

Figure 6

Time response measurement of the bulk GaAs cathode at laser wave length of 520 nm (dots) by the projection of the bunch traces taken by the beam profile monitors. The solid line shows the fitting curve obtained by the least squares method using the model cathode response function.

Figure 7

Probability density function of the longitudinal bunch size obtained via convolution of the $\sigma =3.3\mathrm{ps}$ Gaussian signal with the cathode emission current function with $\tau =0.757\mathrm{ps}$.

Figure 8

The horizontal (solid line) and the vertical (dotted-dashed line) emittances from the electron gun up to the exit of the main linac cavities for the Gaussian with $\sigma =3.3\mathrm{ps}$ core with longitudinal tail $t_{\text{cutoff}}=100\mathrm{ps}$ longitudinal distribution.

Figure 9

The horizontal (solid line) and the vertical (dotted-dashed line) dispersion functions from the electron gun up to the exit of the main linac cavities for the Gaussian with $\sigma =3.3\mathrm{ps}$ core with longitudinal tail $t_{\text{cutoff}}=100\mathrm{ps}$ longitudinal distribution.

Figure 10

The horizontal (solid line) and the vertical (dotted-dashed line) betatron functions of the recirculation loop from the electron gun up to the exit of the main linac cavities.

Figure 11

Schematic of cERL injector cryomodule. The cryomodule includes three 1.3 GHz 2-cell superconducting cavities. Each cavity has two input couplers and five loop-type HOM couplers.

Figure 12

Radial kick values for the horizontal trajectory offsets of 1 mm (solid line), 3 mm (dashed line), and 5 mm (dotted-dashed), respectively, as a function of the cavity phase (the vertical offset is fixed to 2 mm).

Figure 13

Radial components of the electric fields in the cavities for different longitudinal offsets of 1 mm (solid line), 0.5 mm (dashed line), and 0 mm (on-crest acceleration) (dotted-dashed line), respectively.

Figure 14

Longitudinal component of the electric fields in the cavities.

Figure 15

Transverse component of the magnetic fields in the cavities.

Figure 16

Photograph of cERL injector steering coils. The electron beam travels from the left to the right. Two pairs of rectangular-shaped coils are placed in parallel to the beam line. A pair of rectangular-shaped coils on the left and on the right is used for vertical steering. Those at the top and at the bottom are for horizontal steering.

Figure 17

Schematic of cERL injector steering coils wound on the poles of quadrupole magnets with the bore radius of 3 cm and core length of 10 cm. Simulated magnetic field lines are also shown.

Figure 18

Layout of steering coils ZHV1–ZHV8 in cERL injector line (before the main linac starts).

Figure 19

Influence of the steering coils on the electron beam trajectory. The trajectory is shown on the $z\text{−}y$ plane. It is simulated for two cases: when all coils are turned on (dotted-dashed line) and off (solid line), respectively. The total effect of ZHV1–ZHV4 on the beam trajectory is a vertical offset of $\mathrm{\Delta }y=1.69\mathrm{mm}$ at the first injector cavity location. The corresponding entry angle is $\alpha =0.138°$.

Figure 20

Time response measurement results of the bulk GaAs cathode at laser wavelength of 520 nm in the interval from $-100\mathrm{ps}$ to $-10\mathrm{ps}$ (dots). The solid line shows the fitting curve obtained by the least squares method using the model function. The area under the fitting curve is about 1.5% of the total area of the longitudinal beam distribution.

Figure 21

Simulated beam halo profiles at screen monitor SCM8 location. The steering coils are on. The horizontal offset of the cavity No. 2 is 2.6 mm, and the common vertical offset of 2 mm is assumed for the cavities Nos. 1–3. The collimators COL1 and COL2 are taken out. (a) Demonstration of those for the 3.3 ps Gaussian core. (b) For the 3.3 ps Gaussian core with 100 ps bunch back tail. (c) For the 3.3 ps Gaussian core with back and forward tails.

Figure 22

Simulated beam halo profiles at screen monitor SCM8 location. Horizontal offset of the cavity No. 2 is 2.6 mm. The longitudinal bunch distribution includes 3.3 ps Gaussian core, and the back and forward tails. The steering coils are turned on. The collimators COL1 and COL2 are taken out. (a) Demonstration of those for the collective vertical offset of cavities Nos. 1–3 of $-2\mathrm{mm}$. (b) For those with the collective vertical offset of 0 mm. (c) For the offset 2 mm.

Figure 23

Simulated beam halo profiles at different screen monitor locations without (left) and with (right) collimation COL1 and COL2, respectively. Tracking was performed for the Gaussian beam with $\sigma =3.3\mathrm{ps}$ core in the presence of the longitudinal bunch back and forward tails. All the steering coils are turned on, with the horizontal offset of 2.6 mm at the second cavity and the common vertical offset of 2 mm at all three cavities.

Figure 24

Comparison of beam loss rates along the beam line for the high-current operation ($J=0.95\text{mA}$) where the collimators COL1, 2 and 4 are inserted in. Loss rates are given in percentage of the bunch population. Loss rates estimated from the measured secondary photons are denoted by blue numbers. Simulation results (italic red numbers) are for particles tracking with the total number of particles $N={10}^6$, with the Gaussian distribution of the $\sigma =3.3\mathrm{ps}$ core in the presence of longitudinal bunch back and forward tails. In the simulations, all the steering coils are turned on. The measured horizontal offset of the second cavity (2.6 mm), and the common vertical offsets of the cavities nos. 1–3 (2 mm) are included.

Figure 25

Geometry of a single rectangular loop.

Figure 26

Geometry of a pair of rectangular loops and the $z\text{−}y$ plane.

Figure 27

$B_y$ component of the magnetic flux density on the $z\text{−}y$ plane.

Figure 28

$B_y$ component along the longitudinal coordinate $z$.

Figure 29

$B_y$ component along the transverse coordinate $y$.