Free Electron Laser

1 FEL beam line

The first phase of the DLS facility at IUAC aims to produce both ultra-short electron beam bunches of energy 2 to 8 MeV and intense THz radiation in the frequency range of 0.18 to 3.0 THz. For this a compact beam line has been designed and installed inside a class-10000 clean room. Conceptual design of the beam line and detail about its components will be described in the following sections.

The Phase-I consists of two beam lines i.e. radiation beam line and electron beam line. Both the beam lines will have their own experimental stations inside the clean room (as shown in figure-1).

In the conceptual design, a detailed beam line components has been planned in a schematic diagram with their actual dimensions. for phase-I of the DLS project has been done. The detail schematic of the above said beam line is shown in figure-2.

During the last academic year, the Undulator Magnet along with its narrow vacuum chamber had been precisely aligned and the beam energy was measured by using the dipole magnet installed at the achromatic bend. In this academic year another dipole magnet along with magnetic quadrupole triplet and steering magnets were installed to complete the achromatic bending section of the electron beam line. The sixty degree (60) bending section of the second dipole magnet was aligned with the experimental chamber along with the quadrupole doublets and steering magnet for the delivery of electron beam to the experimental chambers to be dedicated for electron beam related experiments. The zero-degree line of the first dipole magnet was also aligned with an additional experimental chamber for atomic physics experiments. The BPM along with ICT are being installed in this beam line for tuning of the beam to the experimental chamber. The view of the installed beam line post undulator section along with design drawings of the entire beam line is shown in Figure 3.

Beam Optics Simulation

The beam optics simulation of DLS is done in two stages, firstly from PC gun to undulator exit to optimize for radiation output of the THz frequencies and secondly from undulator exit through an achromatic bend to guide the electron beam to electron experimental stations. For the first section ASTRA (A space charge tracking algorithm) and GPT (General particle tracer) softwares has been used for the simulations, whereas for the second stage GICOSY has been used in addition to GPT. After first order optimization from GICOSY, the GICOSY parameters was used for simulation in GPT. The GPT results for the complete beamline is shown for 4MeV beam in figure 1.

Figure 1: The results of GPT simulation for 4MeV beam showing the variation of beam transverse rms size with Z, from PC gun to electron experimental station. The transverse beam profile at Undulator exit and electron experimental station is also shown.

The simulation for DLS has been done by trying to optimize the Bunching factor(BF) of the bunch through the Undulator for a given frequency. This is achieved by maximizing sum of BF square inside the Undulator. The simulation results for 3 THz frequencies using 16micro-bunches is shown in figure2.

High Power RF system

The specification high power RF system for the Delhi Light Source project of IUAC is derived from the beam requirement of the system. The photoinjector cavity resonating at 2860 MHz (i.e. S-band) with a maximum gain of 8 MeV is required to accelerate the laser induced photo-electron beam to generate THz radiation in the frequency range of 3 THz. To produce such a gain, accelerating fields of the order of ~115 MV/m are required which translates to ~19 MW of RF power. The RF power source for the photoinjector cavity, with the above specification is based upon a 25 MW peak power klystron operating in pulsed mode for maximum 4 µs duration. The main components of High Power RF system are High Voltage Pulse Modulator, Solid State Driver Amplifier, S-band Klystron, RF Waveguide System and Ion Pumps. The system has been commissioned at IUAC and the solidworks drawing of whole assembly and actual site picture is shown in figure 1.

A 2.6 cell S-band copper cavity is being used as the RF gun to work as photoinjector cavity. It has been fabricated in collaboration with KEK, Japan, tested with low power RF. The cavity is under evacuation in the DLS beam line at present and has been aligned at its correct position. The high power RF system has been installed and commissioned based on final location of this cavity. The resonance frequency of the cavity is 2860 MHz and the measured Q-value is ~ 15200 with low power RF. The cavity and its electric field profile from Superfish and from the bead pull measurement are shown in figure 2.

Photocathode Deposition System

In the DLS facility which is under development at Inter University Accelerator Centre, New Delhi, initially, the electron beam will be generated from copper photocathode and subsequently from semiconductor photocathode like Cs₂Te and other advanced photocathode materials. To deposit the thin film of photocathode material on metal substrate, a deposition system has been designed at IUAC and fabricated at Brookhaven National Laboratory (BNL),USA under joint collaboration. The system consists of four vacuum chambers which will be interconnected with vacuum manipulator with the option of isolating by gate valves (shown in figure 1). The system will have provision for cleaning of photocathode metal substrate (to be called subsequently as PC plug), deposition of photocathode film on the PC plug, storage of the PC plug without residual gas poisoning and insertion of the PC plug into the RF electron gun. The operating vacuum in all the chambers in the facility will be in the order of 10^-11 mbar for ensuring minimum residual poisoning of photocathode thin film.

At present, the production of electron beam is being continued by striking the laser beam on the Cu photocathode. In order to improve the quality of the electron beam in terms of bunch width, emittance, energy spread, etc., semiconductor photocathode (Cs₂Te) deposition system had been installed in last academic year. After installation of photocathode deposition system along with other accessories like vacuum elements, magnetic manipulators, source chambers etc., the evacuation was done and a leak rate of 5x10^-12 mbar mbar had been achieved. Installation of source assembly has been completed and temperature calibration of cathode substrate is completed. The positioning of the Quartz Crystal Monitor (QCM) with respect to the substrate was optimized by measuring ~1/5^th of film thickness measured by QCM compared to the deposition thickness on the substrate. A UV LED system was installed just outside the deposition chamber and the laser beam was aligned so that it would hit the substrate kept inside the deposition chamber to measure the quantum efficiency of the photo cathode during deposition.

Fiber Laser system

The laser system is one of the most crucial subsystem of the Delhi Light Source (DLS) facility for the production of THz. The concept of DLS stems from the fact that the coherent emission from the pre-bunched electron beams can be achieved immediately from the injection point of the compact undulator magnet. The pre-bunched electron beam will be produced by the laser micro-pulses striking the photocathode. A state-of-the-art Fiber Laser system to produce ultra-short laser pulses with energy ~1.5 Joule at UV (255 nm) with a pulse width of  500 femto-seconds was developed as a collaborative project between IUAC and High Energy Accelerator Research Organization (KEK), Japan. The development of the laser system is successfully completed and the production of electron beam is demonstrated by using the Fiber Laser system of IUAC.

Undulator

Undulator is a periodic arrangement of magnets in such a way that the particle passing along its axis experience a sinusoidally varying magnetic field (Fig. 1).

The relativistic electron beam will be injected into the undulator magnet and produce electromagnetic radiation. The wavelength λ of nth harmonic of radiation is given by the famous undulator equation,

where λ_u is the undulator period, γ is the relativistic factor for electron, K= 0.0934*B[T]*λ_u [mm] is the undulator parameter, B is the on- axis peak magnetic field of undulator and θ is the angle at which the radiation is emitted (or the observation angle) with respect to z-axis.

The DLS is designed to produce the radiation wavelength in the range of 0.18 THz to 3 THzfrom electron beam of 4 MeV to 8 MeV. The undulator for this light source was received from HZB, Germany to use their spare undulator whose specifications closely match with the designed one. Few refurbishments were done at DESY,Germany to make it suitable for the installation at IUAC. Table 1 shows the specificationsof the undulator now being used.

The undulator has been shifted from DESY and has been installed in the beam line. The alignment was done using the Taylor – Hobson Fudicial. Vaccum chamber inside the undulator has been aligned and connected to the beam line (Figure 2). Electron beam has been successfully transmitted through the undulator chamber with 80% of transmission efficiency. The short and long coil are also has been powered. The gap will be varied to get the different radiation wavelength of the generated THz. Field Integral and correction coil currentmeasurement data has been shown in figure 3.

THz Transport and Detection:

The terahertz radiation has to be captured just at the exit of the undulator to reduce its further loss and attenuation while propagating through the vacuum chamber. At the end of the undulator, an extraction chamber equipped with a Titanium foil is installed. The foil will reflect the THz radiation in perpendicular direction and allow the electron beam to pass through. The reflected THz radiation will be collected using a Teflon lens and parabolic mirrors, and then transported to the experimental stations with the help of mirrors and lenses. The schematic is shown in figure 1 to detect the THz using Michelson interferometer set up.

Table 1 shows the calculated THz power generated using the Lienard Wiechert formulation when the electron microbunches passes through the undulator.

Figure 2 shows the installed extraction chamber in the DLS beam line along with parabolic mirrosrs and pyroelectric detector set up to detect the generated THz.