# The DREAM Endstation at the Linac Coherent Light Source
## 2 Hardware
### The Optical Laser Setup  
At TMO, the optical excitation of gas-phase samples in the target chamber is established by a synchronized, femtosecond pulsed laser source situated in the NEH laser hall. These pulses are generated from an Optical Parametric Chirped Pulse Amplification (OPCPA) system, which is developed in-house and is capable of providing 800 nm, 100 kHz, 35 W, \(<\)20 fs pulses [25]. Ultimately, the goal is to reach a laser pulse repetition rate of 1 MHz for complete synchronization with the 1 MHz repetition rate of LCLS II. The optical laser hall is upstairs of TMO and the 800 nm fundamental is transported via vacuum tubes towards the hutch. To achieve a sub-10 \(\upmu\)m optical laser spot size at 800 nm, the in-coupling of the laser is located as to the interaction point as possible and finally focused with an in-vacuum off-axis parabolic (OAP) mirror mounted on a piezo motor driven by a hexapod--a motorized stage with 6 degrees of freedom. The designed focal length of the OAP at 800 nm is 110 mm. For beam diagnostic reasons, the optical laser beam is collimated directly behind the interaction point and out-coupled directly behind the spectrometer, as shown in Figure 5, and sent to the laser diagnostic mounted on the granite stand. Figure 4: The DREAM gas jet and skimmer assembly; indicated on the left-hand side are the targeted pressure ranges and on the right-hand side the gas jet (1), the gas jet manipulator (2), the first skimmer chamber (3), the first skimmer pumping (4), the position of the first skimmer (5), the gate valve (6), the second skimmer pumping (7), the position of the second skimmer (8), the third skimmer chamber pumping (9), the third skimmer position (10), the fourth skimmer pumping (11), the skimmer assembly motion flexure (12), the fourth skimmer position (13), the main chamber (14), the position of the first catcher aperture (15), and the position of the second catcher aperture (16). The TIXEL Detector  
To meet the detection challenge posed by high-power, high-repetition-rate FEL, SLAC is actively developing a novel front-end ASIC detector [26] based on silicon technology, known as TIXEL. Unlike conventional TOF detectors, this detector is designed to have high spatial and time resolution with a low damage threshold, without needing a charge amplifier MCP. The ability to detect the Time over Threshold (TOT) and Time of Arrival (TOA) allows this detector to detect charged particles with spatial, energy, and arrival time information. When fully commissioned, this pixelated charged particle detector can accommodate \(>\)300 hits at a repetition rate of up to 1 MHz. Thus, it is possible to perform, for the first time, massive multi-particle coincidence experiments. Since the number of combinations grows exponentially with the number of hits, detecting and analyzing large densities of multi-particle hits at a high repetition rate will soon be possible. The readout rate of this detector is variable and scalable since only an active or triggered pixel is read out with the maximum readout rate of 5 kHz for the full array. This capability is ideal for an equally spaced bunch pattern at a high repetition rate. All the described parameters make the TIXEL detector superior to state-of-the-art detection schemes and the ideal detector for COLTRIMS types of experiments. Table 2 shows the main parameters for the TIXEL detector, and Table 3 shows the performance parameters of the new DREAM endstation.