Electron Beam Systems
The ATF offers users access to two 75 MeV, high-brightness electron beam lines. These beam lines can be used in conjunction with the range of laser capabilities at the facility.
Electron Beam Systems Capabilities
Photocathode Electron Gun
Electron beams at ATF are produced from a water-cooled 1.6 cell S-Band (2856 MHz) photocathode radio frequency (RF) electron gun, equipped with a water-cooled emittance compensation solenoid. The typical gun operating pressure is ~1e-10 Torr.
Two ports symmetrically placed in the first 0.6 cell are used for laser irradiation and imaging of the cathode. One port in the full cell is used for power coupling, and a second port is placed to preserve the symmetry. RF coupling into the half-cell is through the iris.
The gun is powered by a SLAC XK-5 klystron. The maximum RF input power at the gun power coupler is 10 MW in pulses up to 2.5 microseconds long. The corresponding peak accelerating field is 130 MV/m. The center of the removable back flange serves as the cathode. The quantum efficiency of the laser cleaned copper cathode is typically in the 1e-4 range.
One can find more information in “Initial Commissioning Results of the Next Generation Photoinjector”, D.T. Palmer, X.J. Wang, R.H. Miller, I. Ben-Zvi, C. Pellegrini, J. Sheehan, J. Skaritka, H. Winick, M. Woodle and V. Yakimenko, in AIP Conf. Proc. 398, 695–704 (1997), https://doi.org/10.1063/1.53071
S-band Linac
The majority of experiments at the ATF require much higher energies than those which are available from the » 6 MeV photocathode electron gun. Two 3 m long traveling wave LINAC structures, known as “SLAC sections”[1] are used to accelerate the electron bunches to kinetic energies typically in the range of 50-75 MeV.
The microwave drive power, at a frequency of 2856 MHz, is provided by a single TH2128A Thales klystron. RF output pulses up to 2 microseconds long are split evenly between the two accelerating sections.
The electron beam can operate at repetition rates of up to 6 Hz, and the bunch train structure can also be varied. The standard configuration used by most user experiments consists of the electron beam containing all the electrons produced during a single RF pulse in a single bunch, with a repetition rate equal to the RF pulse repetition rate. The electron beam can also be separated into a bunch train, with each bunch containing a portion of the total charge and with a spacing between bunches of 24 or 48 ns. Each RF pulse then supports this entire train of bunches. Trains of up to 20 bunches have been demonstrated although longer trains with up to 100 bunches are possible.
RF System
[1] From the famous 2-mile SLAC LINAC; see SLAC Blue Book.
A simplified block diagram of the RF system is shown above. In order to synchronize the electron gun, LINAC sections, and lasers, all RF signals at the ATF are derived from one Master Oscillator (MO). Then, for instance, some low-level RF is sent to the YAG laser systems to phase-lock the laser oscillator, while some is sent through I/Q modulators and amplified to drive the klystrons which provide high power RF for the electron beam acceleration and manipulation. For diagnostics, RF signals are picked up from directional couplers installed on RF cables, RF waveguides, and cavities and can be monitored at different locations.
Experimental Hall (EH)
A high-brightness- up to 75 MeV electron beam can be delivered to the experimental hall where user experiments can be executed on two beam lines. Typical operating parameters for the electron beam (and related information) are shown in the tables in the next sections. The experimental beam lines are equipped with diagnostics and experimental chambers that provide space for experimenters to install their own equipment, e.g., for testing advanced acceleration or beam manipulation. The unique capabilities available at the ATF include the possibility to combine the electron beam with synchronized multi-TW long-wave infrared (LWIR) and near infrared (NIR) lasers. Longitudinal phase space manipulation is available for beams delivered to Beamline #2 [2].
[2] See, for instance, “Generation of trains of electron microbunches with adjustable subpicosecond spacing”, P. Muggli, V. Yakimenko, M. Babzien, E. Kallos and K. P. Kusche, Phys. Rev. Lett. 101, 054801, https://doi.org/10.1103/PhysRevLett.101.054801
Electron Beam Parameters
Table: Electron beam parameters
| Parameter | Units | Typical Values |
|---|---|---|
| Beam Energy a | MeV | 50-75 |
| Bunch Charge b | nC | 0.1-2 |
| Bunch Length (FWHM) c | fs | Down to 100 fs |
| Transverse Size at IP (RMS) d | µm | 30-100 |
| Normalized Emittance e | mm-mrad | 1 |
| Repetition Rate f | Hz | 1.5 |
a contact us if values outside this range are desired b bunch length and emittance vary with the charge; c depends on the peak current; d with permanent magnets optics 6 microns were demonstrated; e at 0.3 nC (see note b); f 3 and 6 Hz are available
Beam Line Parameters
Table: Magnetic Element Parameters
| Name | Aperture, mm | Maximum Current, A | TYP. Voltage Drop, V/A | Field strength, or Gradient | Effective Length, cm |
|---|---|---|---|---|---|
| BEPC quad 1X | 40 | 25 | 0.06 | 22.1 G/cm/A | 10.4 |
| BEPC quad 2X | 40 | 25 | 0.1 | 22.1 G/cm/A | 20.4 |
| 6Q40 “Small” | 40 | 10 | 0.63 | 50 G/cm/A | 7.6 |
| Dipole, 20 degree | 15 | 60 | 0.19 | 50 G/A | 40.0 |
| Dipole, 90 degree spectrometer | 62.8 | ||||
| Solenoid | 80 | 200 | 0.1 | 14.0 G/A | 19.5 |
| Steering Coils | |||||
| Inside solenoid, IN | 40 | 1 | 49.97 G/A | 10.2 | |
| Inside solenoid, OUT | 40 | 1 | 40.83 G/A | 10.5 | |
| “Slim” Trim | 70 | 4 | 8.8 G/A | 13.4 | |
| “Old” Trim | 5 | 7 G/A | 17 |
Beam Diagnostics
Energy Spectometers
Both beamlines have a spectrometer section near the end where the beam can be redirected with a dipole, enabling measurements of the beam’s energy spectrum.
On beamline 1, the spectrometer consists of a 90-degree, downward-facing dipole magnet and a diagnostic chamber with a viewscreen for measuring the energy spectrum and a Faraday cup to measure charge. The dipole vacuum chamber has a “zero-degree” port which allows installation of X-ray windows for X-ray radiation measurements.
On beamline 2, the spectrometer consists of a 20-degree dipole magnet, four quadrupole magnets, and a diagnostic chamber with a viewscreen and a Faraday cup. The quadrupoles are used to achieve a small horizontal beam size and high dispersion at the location of the viewscreen to achieve high energy resolution (< 30 keV).
Viewscreens
The ATF has numerous pop-in viewscreens for monitoring the electron beam which are combined with CCD/CMOS cameras. The viewscreens are typically composed of Ce:YAG (Cerium-doped Yttrium Aluminum Garnet) or a phosphor. The resolution of ATF’s standard viewscreen systems is approximately » 10 micrometers with a dynamic range of 12 bits. Depending on the type of experiment, special screens or cameras for higher sensitivity, resolution or dynamic range can be selected.
Longitudinal Beam Profile Measurements
At the ATF, to measure longitudinal beam profiles, coherent transition radiation (CTR) from the electron beam is used. The CTR signal is monitored in a setup that is comprised of an interferometer, bolometer or pyrometric detector. An example of an interferometer signal obtained in PWFA experiments is shown below.
