Accelerator, Laser, and Coherent Optoelectronics R&D Lab (ALCOL)
Beam-Optics
Beam Profile Measurement of 3.9 GHz 5 Cell Deflecting Mode Cavity (A0-HBESL)
As a part of the plan to re-commission A0-Photoinjector Lab (A0-PI) to High Bright Electron Source Laboratory (HBESL), a 3.9 GHz 5 cell deflecting mode cavity and its operating system were upgraded and re-installed in the HBESL beamline. The re-commissioning project includes upgrade of cryo-vessel and LN2-supply system, high power RF system, and beamline control system. For the RF system upgrade, the cavity was tested with a vector network analyzer to measure room temperature resonant frequencies and it was characterized by tuning parameters, which was compared with simulation data. The cavity was installed with the cryo-vessel and LN2 hookups at the new position closer to the photo-injector and reviewed by the Fermilab cryo-pannel to authorize its LN2-operation. The review panel recommended installing a burst disk to prevent a catastrophic vaporized nitrogen explosion with thermal insulation to give full authorization. The shielding foams were thus wrapped around the cavity and a burst disk was installed in the beam line. The cryo-operation was fully authorized by second panel review, cavity RF performance was re-examined in LN2-operation mode. Some sub-components in the klystron modulator, including a RF driver (TWT amplifier), were replaced with new ones as well.
The 5 cell cavity in the cryo-vessel, after completely shielded by thermal insulation foams, was tested at room (297 K) and LN2-temperature (80 K). The cold cavity parameters were measured twice after 0 day and 20 days long cooling operations to observe temporal variation of thermal stability. We measured RF-characteristics (amplitude and phase) of the warm and cold cavities and also thermal variations of accelerating mode (TM110). The resonant frequency is up-shifted with ~ 12.1 MHz by the thermal transition from T = 297.2 K to T = 80 K, corresponding to 55.74 kHz/K, under the condition that cavity frequency varies linearly with temperature. The resonant frequency further rose to 3.899922 GHz after 20 days cooling, corresponding to converged thermal frequency variation of ~ 57 kHz/K, which is close to the simulation result, 64 kHz/K. The return loss remains steady at the initial cooling, while it is gradually increased to ~ - 10 dB, corresponding to 0.0137 dB/K. Even the reduction of coupling level leaves the system fairly operational as 90 % of driving power is still coupled in the system. The instantaneous phase deviation is ~ 0.18 degree/K, but it is also gradually increased up to 0.447 degree/K after 20 days cryo-operation. Therefore, the measurement implies that in the early stage of LN2-operating mode the system can operate with temporal change of < 5 K in terms of amplitude/phase deviation. However, as the vessel loses cooling efficiency with increasing time, acceptable temperature range for the system drops down to < 2 K, which might exceed ambient thermal fluctuation beyond controllable range of the foam-insulated system. For a better thermal management, a cryo-vessel should thus be designed with a vacuum-insulation, which is currently planned with the ASTA 5-cell deflecting mode cavities.
The beam test was completed with the LN2-cooled deflecting cavity, which ended up with p ~ 3.5 MeV/c and bunch length (sz) ~ 5 ps. We scanned RF launching phases to extrapolate transverse kick strength, k = eV^/Er. The drift distance of the beam monitor measuring a peak-to-peak deflection ~ 17 mm is ~ 1.22 m from the TDC, which corresponds to the deflection voltage of ~ 24.4 kV. The kick strength, k, from the data is calculated to be ~ 0.41 m-1. Some of the beam test result was presented at international conferences (IPAC, NA-PAC, FEL, and AACW), and published to the Physics of Plasmas.
Development of Bunch Modulation Technique with a Slit-Mask (ASTA)
A wide range of electron beam application such as free-electron lasers (FELs) and particle accelerators employs a bunched beam for improvement of machine performance in the relativistic or quasi-relativistic regimes. It is well known that the short electron pulse can lead to an appreciable improvement of energy conversion efficiencies or power growths of coherent light sources and high gradient accelerators. Generally, electron bunches can be shortened by magnetic bunch compressor (e.g. chicane, S-chicane, dogleg, a-magnet) or continuous velocity bunching process. The bunching techniques are commonly used in electron accelerators. However, it is not easy to reduce the bunch length to sub-ps range. Normally, when a beam is modulated, it is more strongly coupled with an undulating or accelerating structure at the resonance condition with a fundamental or higher order mode. It would enable more sophisticated beam control in energy-phase space. The modulation (or microbunching) thus enables photo-electron interaction or electro-optical transition in a smaller time scale, preferably in femto-second range. One of the easiest ways for the beam-modulation is to mask the beam in a chicane with a slit-mask or a wire-grid. The basic concept was first suggested by D. C. Nguyen and B. Carlsten in 1996 in the effort to reduce the length of FEL undulators. Also, the Brookhaven National Laboratory (BNL) demonstrated the generation of a stable train of micro-bunches with a controllable sub-ps delay with the mask technique using a wire-grid. The main advantage of the masking technique is to readily control micro-structured density profiles, including the energies and phases.
We have been investigating the masked chicane technique with the available beam parameters such as the 50 MeV photoinjector of the Advanced Superconducting Test Accelerator (ASTA). Downstream of the ASTA 50 MeV photoinjector beamline, a magnetic bunch compressor, consisting of four rectangular dipoles, is adopted and a slit-mask is designed and inserted in the middle. Based on this slit-masked chicane, the bunching performance and the ability of sub-ps microbunch generation are explored. In order to evaluate bunching performance with nominal beam parameters, the masked chicane has been analyzed by the linear bunching theory in terms of bunch-to-bunch distance and microbunch length. Two simulation codes, CST-PS and Elegant, are employed to examine the theoretical model with the ASTA nominal beam parameters (RMS bunch length sz,i is 3 ps and energy ratio τ is around 0.1). The Particle-In-Cell (PIC) simulation (CST-PS) includes space charge and CSR effect and nonlinear energy distribution over macro-particle data. For Elegant simulations, bunch charge distribution and the beam spectra are mainly investigated with three different bunch charges, 0.25 nC, 1 nC, and 3.2 nC, under two RF-chirp conditions of minimum and maximum energy spreads. The corresponding bunch length for the maximally chirped beam is 2.25, 3.25, and 4.75 ps and the correlated energy spread is 3.1, 4.5, and 6.2 % respectively for bunch charge of 0.25 nC, 1 nC, and 3.2 nC. The simulation analysis results were presented at the international conferences (IPAC, NA-PAC, FEL) in 2013 – 2014 and a paper is submitted to PRST-AB for peer-review. We already installed a slit-mask to X115 in the low energy bunch compressor (BC1), which is waiting for beam test. Once the first beam test is complete in the ASTA injector beamline, the slit-mask for the bunch modulation will be tested subsequently. Also, we also have a new slit-mask already machined and it will replace the one in the BC1 for further parameter study. Currently we are planning to test masked bunch modulation at the Fermilab-ASTA injector beamline to be commissioned within a month.
Development of Quadrupole Scan Emittance Measurement and Beam Control Technique (ASTA)
Transverse emittance measurements based on the quadrupole scan technique have been widely used to characterize the beam phase space parameters in linear accelerators. We have been investigating the quad-scan technique at the Advanced Superconducting Test Accelerator (ASTA) at Fermilab. The flexible implementation permits an operator to select the quadrupole associated analyzing screen to measure the beam emittance. A GRA (Andrew Green, Z1623397), who has been supported by the DoD (Air Force) project (Contract #: FA8650-13-C-1604) since 2014 summer (May, 2014), has been working on the research as his master degree thesis topic and I have been advising him on the research in the collaboration with Prof. Piot. We have been mainly developing implementation of the quad-scan technique by coding Python scripts combined with Fermilab’s control system ACNet and running beam tracking simulations (Elegant). The applicability of the quadrupole scan method at 50 MeV injector beamline has been investigated with the range of operating charge (20 pC to 3.2 nC) available at ASTA and eventually, the beam emittances from the quad-scan process will be verified by measuring beam parameters in the ASTA beamline. Progress on this work will be reported to IPAC2015.