1. Introduction
An accelerator-based THz source prototype for pump-probe experiments at the European XFEL was recently constructed at the Photo Injector Test Facility at DESY in Zeuthen (PITZ), which was established for the commissioning and testing of electron sources with various diagnostic systems from measurement of beam charge, beam momentum to transverse and longitudinal phase space characterization [
1]. At PITZ, a space charge-dominated electron beam from the radio-frequency (RF) gun is accelerated to a beam momentum of 15–20 MeV/c by the Cut Disk Structure (CDS) booster cavity. In 2022, new elements such as an undulator, THz test stations, and a bunch compressor were installed at the Photo Injector Test Facility at DESY in Zeuthen (PITZ) beamline as parts of the THz source prototype.
To achieve a THz Self-Amplified Spontaneous Emission (SASE) Free Electron Laser (FEL) with pulse energy in the mJ-range, electron beams of 1.5–2.5 nC compressed by the bunch compressor to a peak bunch current near 200 A are considered.
Figure 1 shows a brief PITZ layout for the THz SASE FEL experiments, including the RF gun, the booster, the bunch compressor, the coherent transition radiation (CTR) measurement station, and the undulator. Furthermore, the bunch compressor has been foreseen and assigned as a part of the prototype for various applications such as the SASE FEL, seeded FEL, superradiant radiation, etc. [
2].
The bunch compressor using a magnetic chicane consists of four rectangular dipole magnets (with the pole gap of 8 cm and the pole size of 15.5 × 30 cm) obsoleted from the HERA beamline [
3]. The magnets are reused, as the pole shoes are added with four shims optimized by the program CST Studio Suite 2018 [
4] to flatten the magnetic field profile of the magnet. Thus, the electron beam experiences a magnetic field that is <0.1% different from the maximum field throughout the chicane dipole magnet within the constant field region (5 × 7 × 20 cm in the horizontal, vertical, and longitudinal axis, respectively). Moreover, these chicane dipole magnets have identical strength and length and a vertical bending plane with a deflection angle of 19 degrees. Given the angle, the beam remains within this “good field” region during transport through the magnet. In addition, the angle is designed due to the limited installation space along the original beamline. As the effective length of each chicane dipole magnet of 0.327 m is estimated from the magnetic field simulation via the program CST,
of this chicane is 0.215 m. However, there is no tuning knob for varying the chicane height (or
), as
is fixed to one value. Note that in general, the vertical chicane structure has few benefits. First, the emittance growth from collective effects in the vertical axis could be beneficial to an undulator with a large vertical pipe size (not in our case). Secondly, there is no interference from the earth magnetic field in bending (vertical).
At PITZ, the space charge-dominated electron beam is transported throughout the beamline via the use of periodic and dense focusing elements. However, in addition to the space charge-dominated beam dynamics, this type of bunch compressor design becomes a challenge for beam dynamics and beam transport due to the impact of high bunch charge, high bending angle, and low beam energy on coherent synchrotron radiation (CSR) effects, resulting in energy spread and projected emittance growth [
5,
6]. Furthermore, such collective effects—both space charge and CSR effects—introduce dispersion leakage due to the imbalance in the longitudinal dynamics of particles [
7,
8]. According to our previous study via the particle tracking simulation [
2], the compression performance (compression ratio of bunch length) is dropped, and projected vertical emittance is increased drastically when increasing bunch charge. While the beam is fully compressed and the beam current is increased, space charge and CSR effects limit the bunch compressor performance and distort the current profile. To satisfy our goal for THz SASE FELs in the simulation, for instance, a 2-nC beam current profile with booster phase of −39 degrees can be compressed (in under-compression mode) into an average bunch current of ∼200 A with normalized vertical emittance under 20 μm.
A commissioning method was developed to achieve the chicane dipole magnet currents to guide the electron beam throughout the reference path. Moreover, the two commissioning objectives are to ensure proper electron beam transport through the vacuum chamber of the chicane and to acquire the vertical bending angle closest to 19 degrees by each chicane dipole magnet, thereby achieving minimum energy (or momentum) dispersion.
Figure 2 shows a concept of the first commissioning method based on the dispersion measurements at the screen downstream of the chicane. The method uses two cerium-doped ytterbium aluminium garnet (Ce:YAG) screens called HIGH2.SCR2 and HIGH2.SCR3, located upstream and downstream of the chicane, respectively. The chicane dipole magnets are named CHICANE.D1, CHICANE.D2, CHICANE.D3, and CHICANE.D4, respectively.
2. Commissioning Goal
The bunch compressor will be used to maximize the FEL pulse energy with the booster phase as a beam-energy-chirp tuning knob. However, the bunch compressor was installed downstream of a longitudinal bunch profile measurement station, and the bunch profile after compression cannot be directly measured. In order to predict the bunch compression outcome via the start-to-end simulation results, both beam momentum chirps d
p/d
t upstream and coherent transition radiation (CTR) downstream of the chicane are measured to benchmark the simulation results in [
2].
Firstly, the generation of the beam momentum chirps upstream to the chicane via tuning booster phases was verified to benchmark ASTRA [
9] tracking simulations with space charge effects for bunch charges of 10 pC, 30 pC, and 2 nC with the booster phase between 0 and −25 degrees [
2,
10]. The momentum chirps are extracted from the longitudinal phase space (LPS) measurement at PITZ with the setup consisting of a transverse deflecting system (TDS) and a dipole spectrometer (HEDA2), respectively [
11]. Note that in the case of 2 nC, the 50-μm-slit at a distance of 5.3 m upstream of the TDS is used to reduce horizontal emittance influence in order to enhance the LPS resolution. Thus, the beamlet of 2 nC beam momentum chirps is benchmarked ASTRA simulations.
Secondly, the CTR is measured at the measurement station between the chicane and the LCLS-I undulator. The CTR measurement is expected to indicate the fully compressed-bunch booster phases for the beam with the bunch charge up to a few hundred pC, which are simulated [
2] by the combination of the simulation programs ASTRA, IMAPCT-t [
12], and OCELOT [
13,
14]. In these simulations, program ASTRA with space charge effects tracks the beam from the RF gun to the chicane entrance, and then program IMPACT-T [
15,
16] with both space charge and CSR effects tracks the beam throughout the chicane. Ultimately, program OCELOT with only CSR effects also tracks the beam throughout the chicane to benchmark with program IMPACT-T.
The CTR signals from compressed bunches can be measured after the chicane dipole magnet currents are obtained by the commissioning method. This benchmarking also represents that the commissioning method results in bunch compressor properties such as , as expected from the simulations.
3. Background
In theory, an equal beam bending angle of all dipole magnets throughout the chicane results in zero beam momentum dispersion
. However, the challenge arises regarding how to achieve zero dispersion in practice, due to imperfection in the chicane installation, the inexactly identical magnetic field of each chicane dipole magnet, magnetic fields from other beamline components, etc. Additionally, at PITZ, the chicane was installed with a mechanical constraint for positioning the four dipole magnets, resulting in offsets in the magnet positions. In terms of the mechanical constraint, the second and the third chicane dipole magnet are installed above the original beamline components—an energy measurement station including a horizontal dipole magnet, a vertical beam dump, etc.—with clearance of 3–4 mm. Such clearance is very small due to the attempt to minimize the bending angle for the CSR effects. A vertical offset of any chicane dipole magnets results in the fringe field difference depending on the vertical position at the dipole magnet entrance due to the finite width of the magnets. As a result, the offset could introduce energy dispersion. For example, according to ASTRA tracking simulation results, the dispersion is increased by 0.07 mm/MeV/c, once both the first and fourth chicane dipole magnet vertically are moved by −1 mm from the designed positions.
Table 1 represents the energy dispersion for different bending angles offset to 19 deg, where
and
denote bending angle offsets (errors) of the chicane dipole magnets CHICANE.D1, CHICANE.D2, CHICANE.D3, and CHICANE.D4, respectively. Therefore, in this study, the magnitude of the energy dispersion is ultimately minimized via direct measurements.
According to
Table 1, the magnitude of the energy dispersion in the cases of
and
is smaller than the case of the single bending angle offset. This suggests the possibility of performing the commissioning by tuning the chicane dipole magnet currents with some constraints.
4. Dipole Magnet Current Constraints
To simplify our first commissioning with an assumption, the currents (
) of the chicane dipole magnets CHICANE.D1 and CHICANE.D3 are set to equal magnitude, and the same goes to the currents (
) of the chicane dipole magnets CHICANE.D2 and CHICANE.D4, which are written as
The assumption is that the electron beam experiences a fringe field of CHICANE.D1 and CHICANE.D3 in the similar beam path as the beam straight enters them, and the same goes for CHICANE.D2 and CHICANE.D4 as the beam enters them with the same opening angle of 19 degrees.
Figure 3 shows the evolution of absolute value of offset momentum in the
y-axis of a reference particle throughout each chicane dipole magnet, which is simulated by program IMPACT-T. The simulation is implemented by the 1D magnetic field distribution (
Figure 4) along the beamline longitudinal axis [
2], simulated by CST Studio Suite [
4], as the particle enters and leaves at a beamline longitudinal distance of
m offset from the center of each chicane dipole magnet, respectively. Note that the constant field region (longitudinal length) of each chicane dipole magnet of approximately
m is implemented in the program IMPACT-T. In order to achieve the bending angle of 19 deg at all chicane dipole magnets, current
is set to 1.155 A, while
is set to 1.220 A for the beam momentum of 17 MeV/c.
Furthermore, beam transport throughout the chicane can be manipulated such that the beam follows the beam reference path by a simulation of beam tracking in program ASTRA, which is implemented by the 2D magnetic field distribution. Note that the simulation is performed without space charge effects. For instance, the chicane dipole magnet currents with the constraints in Equation (
1) can be found to achieve both minimum momentum dispersion and
y-position
of 17-MeV/c beam at the screen HIGH2.SCR3 (
Figure 2) located downstream of the chicane in the simulation results shown in
Figure 5.
Figure 5 displays both the contour line of the
y-position (or offset position
) and density plot of momentum dispersion as a function of its chicane dipole magnet currents with the constraints in Equation (
1), which determine at least one set of the chicane dipole magnet currents to achieve both the zero offset position and zero dispersion. In other words, the constraints in Equation (
1) can be applied to the commissioning.
6. Summary
A commissioning procedure based on the dispersion measurements at the screen downstream from the chicane has been developed. In order to achieve optimum bunch compressor operation, the first PITZ bunch compressor commissioning was performed to obtain the set of dipole magnet currents that provides zero momentum dispersion for a bunch charge of 50 pC. The procedure also yielded correlations between dipole magnet currents. By applying these chicane dipole current settings, the first bunch compression experiments were performed, where pyroelectric detector signals from the CTR station downstream from the bunch compressor were used to find a maximum compression phase of the booster cavity.
In other words, we demonstrated zero-dispersion transport of the low-energy electron beam throughout the chicane with the high bending angle, where space charge and CSR effects drastically affect bunch compressor performance. With limited beam diagnostic stations downstream from the bunch compressor, we finally obtained the experimental results to verify the simulation study.
7. Future Plans
Bunch compressor commissioning will be performed to achieve the set of dipole magnet currents that provides zero momentum dispersion for a bunch charge higher than 50 pC to reach THz application requirements. For example, the superradiant radiation experiment at PITZ requires a bunch charge around 250 pC.
The momentum dispersion derivative () component is planned to be measured once a new beam position monitor near HIGH2.SCR3 is installed and ready to use. However, during this stage of this commissioning, there was no other position monitor nearby HIGH2.SCR3 to conveniently use for the dispersion derivative measurement. Moreover, the CTR station screen approximately one meter far from HIGH2.SCR3 is used to observe the position and beam focusing prior to the CTR measurement. When introducing the momentum chirp by changing the booster phase (under compression mode), the beam remains focused at the same position. This could indicate a negligible introduction of the momentum dispersion and the dispersion derivative.