Low-noise Amplifier for Photomultiplier Tube Detectors for Plasma Diagnostics

P. Nowak vel Nowakowski, D. Makowski, A. Mielczarek (Lodz Univ. of Techn., Poland)

Photomultiplier Tube (PMT) detectors are commonly used for plasma diagnostics in fusion devices such as tokamaks or stellarators. These detectors measure spectrum of soft and hard X-ray radiation. Modern spectroscopy systems use Digital Pulse Processing (DPP) in contrary to analogue systems [1]. Scintillator used together with PMT converts X-ray radiation to electrical signal. PMT is superior in response speed and detection of low-light-level sources such as single photons [2]. This type of sensor is often used in application that rely on fast digital pulse processing. Low output voltage level is the main concern when dealing with PMT. Depending on supplied biasing voltage the output voltage of pulse coming out of PMT can be in range of mV or even lower [2]. Such low output voltage levels require low-noise amplification to keep good signal to noise ratio (SNR) before digitalisation. Fall-time of pulses coming from PMT can be as low as 1 ns, that require amplifier to have respectively bandwidth of at least 300 MHz. What is more large size fusion devices such as ITER tokamak require sensitive electronic device to be placed far away from reactor due to high radiation and magnetic field levels. PMT assembly has to be installed ca. 20 m away from reactor, then signal is sent for long distance reaching over 100 m using coaxial cables. Amplifier have to supply power to drive signal over such long distance. Environment near fusion reactor is heavily polluted by strong magnetic fields and electromagnetic noise. Electromagnetic interferences can couple into signal cables. Cable should be well shielded and amplifier ought provide high amplitude output, to assure high signal-to-noise ratio. Therefore, sending fast analogue signal from PMT via long coaxial cable is a challenging task. Due to ITER specification a dedicated Photomultiplier Tube amplifier is required. A suitable ultra low-noise amplifier with 300 MHz bandwidth, gain of at least 100x (with possible adjustment), high voltage output and which will be able to drive long coaxial cables is required. Market search was carried to find amplifier suitable for low-noise application and fulfilling requirements ITER specification. Various available solutions were considered, but none of them did meet all requirements at once. Finally, low-noise two stage amplifier was designed from scratch to meet all specification required by PMT-involved applications. Paper discusses how to design a low noise amplifier suitable for Photomultiplier Tube applications. Characteristics and measurements of the prototype are presented in this paper. After evaluation and simulation of different amplifier circuits, two-stage topology has been chosen, as it allow to achieve wide bandwidth, high output voltage and yet be low noise. Amplifier is based on ultra low-noise preamplifier (0.69 nV/√Hz) and high power second stage amplifier (output voltage swing 28 Vpp). Amplifier circuit board is enclosed in aluminium RF-sealed enclosure to protect circuitry from electromagnetic interferences. The first prototype of the PMT amplifier was developed and evaluated at Department of Microelectronics and Computer Science (DMCS). After series of tests and tuning of the prototype, device was finally connected to PMT. Developed amplifier will be used in ITER Hard X-Ray Monitor diagnostic system.