Instrumentation for conducting experiments with laser produced plasmas
- Micro controller based S-20 optical Streak camera:
Streak camera is an important instrument for studying very fast (sub-nanosecond) events in a single-shot mode. The important features of a streak camera are: 1) high time resolution (of the order of few pico-seconds), 2) high sensitivity (near single photon) and 3) good dynamic range (typically 10-100). A streak camera is specially required in studies of laser plasma interaction due to very short time scales of the processes involved.
Laser Electronics Support Division has recently developed a micro-controller based S-20 optical streak cameras using indigenous components. The optical components like streak tube, de-magnifier unit and image intensifier unit were procured from General Physics Institute. The optical imaging system, mechanical housings and mountings, as well as various electronic circuits were fabricated at RRCAT. An 8 bit MCS family microcontroller is employed to generate all control signals for the streak camera. All biasing voltages required for various electrodes of the tubes are generated using DC-to-DC converters. A high voltage ramp signal is generated through a step generator unit followed by an integrator circuit and is applied to the camera’s deflecting plates. The slope of the ramp can be changed by varying values of the capacitor and inductor through stepper motor controller. A programmable digital delay generator has been developed for synchronization of ramp signal with the optical signal. A microcontroller independent hardwired interlock circuit has been developed for machine safety. A LABVIEW based GUI has been developed which enables the user to program the settings of the camera and capture the image. The image is displayed with intensity profiles along horizontal and vertical axis.
The control electronics of streak camera involves high voltage bias to streak tube, de-magnifier tube and image intensifier tube, High voltage variable speed ramp generation for deflection plates, generation of synchronized trigger signals for the ramp and gating of micro-channel plate (MCP) with respect to optical input and a hardwired safety circuit for monitoring various operations.
Biasing of the streak tube assembly is implemented using a programmable DC-DC converter (EMCO make 4150N) with a -15 kV output. The microcontroller provides a TTL signal to activate the -15 kV power supply. A high voltage resistor divider network is then used to bias various electrodes of the streak tube, de-magnifier tube and image intensifier tube. The streak tube assembly is shown in Figure.1
Figure 1: The streak tube assembly
Optical output of the image intensifier was coupled to a CCD camera followed by a PC based frame grabber for capturing the images. A LABVIEW based graphical user interface (GUI) has been developed for the streak camera. The GUI has three panels; setting/status, execute and image. A typical GUI is shown in Figure 2.
The dynamic testing of this unit and their streak speed calibration was done using nano-second laser and femto-second laser pulses from a Ti:Sapphire laser system. The laser system provides 50 fs duration laser pulses at 790 nm at 10 Hz repetition rate. For this purpose, the laser beam was split into two beams using different lengths of two fiber cable introduced just before the entrance to the input slit of the camera. The typical speeds are 700 ps, 6 ns, 12 ns, 20 ns, 24 ns, 28 ns for 25 mm of phosphor screen.
For each speed, several shots were recorded for checking the linearity as well as repeatability of the unit. The shots were captured such that the entire phosphor screen was covered by changing the delay. It was observed that the linearity was changing from 2% to 5% from one end to other. A piece wise linearization technique is employed to remove this nonlinearity.
A typical streak image of a 50 fs laser pulse and the corresponding intensity profile is shown in Figure 3.
Figure 3: A typical streak image of 50 fs laser pulse and the corresponding intensity profile
The specifications of these cameras are as follows:
- Photo-cathode Type: S-20 (Na-O-Sb-Cs)
- Spectral Sensitivity: 350-800 nm with peak at 500 nm
- Streak sweep time for 25 mm screen : 700 ps, 6 ns, 12ns, 18 ns, 24 ns, 28 ns
- Temporal Resolution: ~4.5 ps
- Spatial Resolution: ~175 μm
- Magnification: 1.375 X
It was found that the fastest streak speed of this streak camera is 3.57 cm/ns. The temporal and spatial resolution for the camera was measured as 4.5 ps and 175 μm respectively. A picture of complete assembled streak camera is shown in Figure 4.
Figure 4: A picture of complete assembled streak camera