The problem is that you are using a MEMS digital accelerometer, and what you are reading is the SCK (serial clock) pin of the serial interface. In order to function, that sensor needs to be interfaced with a microcontroller, that sets it for the sampling frequency, the range and so forth.
So you don't have to expect a square wave with 100Hz frequency, but a fast (depending on the bus bitrate) spike, corresponding to a transmission. Expanding the spike, if the scope is fast enough, you should then see the clock square wave inside the spike.
Moreover, if you don't set the SPI interface correctly, the uC will not generate the clock (the sensor operates in slave mode), and you won't read any value.
If you want to see a 100Hz signal, you could probe the Int pin, which sends an interrupt to the microcontroller every time a measure is available. Then, if you handle the interrupt from the microcontroller properly, you wil see the pulse corresponding to the transmission every 10 ms (100Hz).
But make sure that you're not using motion detection; in that case, only when an acceleration is measured, it will generate the interrupt.
To read the data at the SPI port, the simplest thing is to configure the communication with the sensor; otherwise, it won't send data at all. Then, check if the microcontroller is getting the interrupts and if it's reading the data the sensor gives; you can use a timer to add a timestamp to values and check the frequency they come.
(still WIP)
You must be very careful when measuring voltages on the mains, especially in your country where the voltages are absolutely deadly.
The best way to approach this is to make a resistive divider box. This is a simple resistor divider housed in a safe non-conductive project box. Connect the top and bottom of the resistor divider to a line cord with a correctly polarized plug. Then bring the bottom of the divider and the center tap of the divider out to 5-way binding posts or banana jacks. Also route the Earth Ground lead of the line cord to another banana jack, or 5-way, on the enclosure.
Select a resistor divider ratio so you get an output voltage which is both safe to touch and suitable for your scope's input range. Also, select the resistor values so they have a low enough impedance to not affect your scope's accuracy, but are high enough that you are not burning up too much power in the upper resistor and creating a lot of unnecessary heat.
As you will be multiplying all of your scope readings by the inverse of this ratio, choose a ratio which is easy to manipulate mentally - e.g. 10:1, 15:1, 20:1 - but still provides a safe-to-touch voltage level on the output jacks. ( Not that you will be purpously & routinely touching the output terminals, but accidents and slip-ups do happen. )
Make sure you construct this box in such a way and seal it up so there is NO CHANCE of accidentally touching the Hot Wire. You might also include a pilot light to indicate the box is plugged into the mains. You can't be too careful when messing with the power mains!
Mark the resistor divider's ratio on the outside of the box. Multiply all of your scope readings by this factor to get the actual line voltage.
Best Answer
This is not easy to do well, especially safely, especially without excessively loading the circuit, and if you want a decent bandwidth. I have a Fluke High Voltage multimeter probe, but the bandwidth is only 150Hz, so it's useless for anything much above the 3rd harmonic of mains frequency.
A 20MHz Bandwidth 40kV Chinese probe is about $320 (from the usual online sources). This type has a 100M input resistance and an input capacitance of about 1.5pF (designed to work into a 1M scope impedance). There is an adjustment to compensate the probe, just like low voltage probes (but a typical square wave source might be down in the grass of your scope with the gain cranked up all the way).
Internally there are one or more high voltage (fairly) precision resistors paralleled with a similar number of high voltage capacitors and an capacitor and trimmer capacitor across the output, all with enough dielectric and physical size to make arcing-over relatively unlikely.
Here is a video of a dude playing around with one of these to measure flyback voltages and such like, and apparently surviving unscathed.
Of course any probe affects the point being measured, and if the "plasma" thing is a toy plasma globe, it will probably load the source severely since at (say) 40kHz, a 1.5pF capacitor has an impedance of under 3M ohms, which dominates over the 100M resistance. If it's a commercial plasma source for a sputtering chamber, you might have better luck.
Be sure to follow proper safety procedures and verify that all instrumentation meets safety certifications if you're dealing with potentially harmful voltages and currents.
The internal schematic of the probe probably looks much like the below, where the parts have quite unusually high voltage ratings (4kV) and the physical arrangement is also quite important to maintain safe creepage distances and to keep the capacitances similar.
simulate this circuit – Schematic created using CircuitLab
A similarly spec'd Tektronix probe is around $2,000.
Measuring the current would be easier if you did it on the low side (resistor and your 10:1 scope probe).