Finding a new methodology of PMT signal transmission might be required for plasma diagnostic systems of future tokamaks. PMT signal has to be transmitted to the digitizer via a relatively long coaxial cable (more than 100 m). The signal will travel through an electromagnetically (EM) polluted environment of the tokamak where radio-frequency (RF) noise can couple into the cable. To minimize the influence of noise on the signal special care in signal transmission chain design and component selection has to be taken.
The PMT signal has a relatively low amplitude (1–100 mV) in the order of hundreds of mV. Signal amplitude should be increased to maintain a satisfactory Signal-to-Noise Ratio (SNR) of the signal transmitted through the RF noisy environment of tokamak. The RF noise is present near the tokamak where the cable is routed. Amplification of the signal before feeding it into a coaxial cable (at PMT output) can improve the ratio between signal and noise amplitude, and effectively decrease the impact of the RF noise coupled to the cable on the signal .
To further improve noise performance, a noise coupling to the cable can be decreased. A coaxial cable consists of two inductors separated by dielectric, one for ground and the second for signal transmission. Introducing an additional metal shielding layer could decrease the amount of noise coupled to the inner layers of the cable. There is a special type of a coaxial cable with an additional shielding layer called triaxial cable. The outer conductor of the triaxial cable is connected to the earth and is not involved in PMT signal transmission. Application of triaxial cable can improve the interference rejection.
Still, both coaxial and triaxial cables are a type of lossy medium. For long distances, losses caused by transmission-line effects become significant. For instance, for a triaxial cable Belden 9222, which can be used for PMT signal transmission, the attenuation can reach up to 16 dB for a 100 MHz signal . For tokamak applications, special radiation-tolerant grade cables have to be used. Such cables could have even higher attenuation. Cable attenuation is an important factor and has to be compensated by the amplifier gain.
Due to stray capacitance, long triaxial cables require a dedicated amplifier, that is able to drive capacitive loads. The parasitic capacitance can reach up to 10 nF for a 100 m long cable . An amplifier with a high current output is required to drive a long coaxial cable without signal degradation .
On one hand, the amplitude of the signal should be as high as possible to achieve the best SNR. On the other hand, the signal levels should be adjusted to the digitizer input range. The high-speed digitizers (more than 1 GS/s) accept voltages in the range of ± 5 V at max (with respect to 50 Ω load) [15,16,17,18]. Pulses generated by a scintillator coupled with a PMT have a bi-exponential shape as shown in Fig. 2. The PMT detector generates a unipolar signal with amplitude changing from 0 V to negative values. Some digitizers, such as Teledyne ADQ14  have a configurable DC offset, which makes it possible to use the unipolar nature of PMT pulses. As a result, the input range can be changed from symmetrical (± 5 V) to asymmetrical (from − 10 to 0 V). Digitizer inputs are matched to 50 Ω impedance. To match the impedance of the digitizer and cable amplifier output also must have a 50 Ω matching circuit. The impedance matching resistors create a voltage divider, that will divide the voltage at digitizer input in half. Therefore, to fully utilize the input range of the digitizer the amplifier has to provide an output range of at least− 20 to 0 V. This voltage range is still reasonable and can be implemented. The signal level will be significantly higher than the noise and there are many operational amplifiers with a standard ± 15 V power supply, that can be also supplied asymmetrically (i.e. + 5…− 25 V). After taking into account cable losses the estimated amplifier gain of 40 dB will utilize the whole input range of the digitizer.
The PMT signal has fast falling edges as short as 2–5 ns. This results in the effective bandwidth of the signal reaching 200–500 MHz. However, it is still beneficial to limit signal bandwidth  to levels of around 100 MHz. With lower bandwidth, the high-frequency noise will be reduced. The pulse shape will be wider, but the pulse processing algorithms still can process it. Therefore, an amplifier with a bandwidth of at least 100 MHz bandwidth is desired. An amplifier should have a DC-coupled input to avoid variable bias. The DC offset can be cancelled using the baseline recovery algorithms.
The transmission of a PMT signal through the tokamak environment requires the use of a dedicated amplifier that fulfils the above-discussed criteria. In conclusion, the proposed guidelines for the PMT amplifier selection could be summarized as follows:
Low-noise (lower than 0.5 mV(RMS) at 100 MHz BW)
At least 100 MHz bandwidth
Gain of at least 40 dBV (100x)
High voltage output (10 Vpk-pk on 50 Ω load)
Ability to drive a 100 m long triaxial cable (capacitive load)
Additionally, complementary to the amplifier, a high-speed digitizer with a relatively high input range (higher than 10 Vpk-pk to match the amplifier output range) and a DC offset capability is needed. To further improve noise performance when using long signal cables, a triaxial cable should be used.