I would think an imposed alternating current would be like taking a transformer output and connecting it in series with a DC power supply output. The output terminals from a DC power supply are DC (Direct Current with a + terminal and - terminal). This DC power closely resembles a battery which supplies power in one direction only with a + terminal and a - terminal. A transformer, on the other hand, has output terminal or leads which are AC or Alternating Current. They are NOT considered as a + or - since they are AC.
The transformers I describe below have what is called a center tap. It means that when the transformer is being wound, one end of the wire is brought out and becomes one of the output terminals, lets call it terminal A. Halfway through the winding process, another wire is spliced to the winding wire and also brought out as a lead or terminal. This is the center tap and we'll call it terminal B then the remaining windings are wrapped onto the transformer and the other end of the winding is brought out as terminal C. Connect either one of the terminals (lets use terminal A just to try to keep things simple) to your "cathode". Then, connect terminal B from your transformer to the negative terminal of your DC power supply. We won't use terminal C in this example, so as a safety precaution, it should just be taped up to prevent it from touching anything else. The positive terminal from your DC power supply goes to the anode. Make sure the combined output voltages from the transformer and the DC power supply do not exceed the ratings for either one.
Don't do it yet, but when everything is turned on, the DC power supply will output DC and the AC power supply (transformer) will output AC. With everything still turned off, connect an oscilloscope to the output terminals of your DC power supply. Turn on the O-scope. You will see a straight line (called the trace) going across the screen from left to right. If the frequency adjusting knob on the O-scope is set to a very slow setting, you may see a dot (with a tail following behind it) going from left to right. This dot (or line) should be in the center of the screen. Now, with the voltage knob set to zero, you can turn on the DC power supply. (If you have a current control knob, it can be set anywhere because there will be "essentially" no current flow.) Watching the screen, you will note that nothing has happened. As you increase the output voltage, the trace will move up on the screen, but it should still be a fairly straight line. (If you were to connect the terminals from the power supply to the O-scope backwards, all that would happen is the trace would go down proportionately with the voltage setting.) The new location of the trace is an indication of how high the voltage has been set. The further up the screen, the higher the voltage. It is possible to "ZOOM" in on the trace and look at it in greater detail. Usually, there is still a little bit of waviness in the trace. This is called ripple (not the drinking type). Ripple is an end product of building a DC power supply which uses the AC line for its power source. Best quality power supplies have almost no ripple, and the output will most closely look like a battery. You will pay a lot for this grade of power supply.
Now, with everything shut down again, disconnect your DC power supply and connect the transformer terminals A and B to your O-scope input terminals. Radio Shack still sells transformers. They used to be called filament transformers because they supplied the power to light the filaments in vacuum tubes. Years ago, a common voltage was 6.3 VAC. Today, 12.6 VAC with a center tap at 6.3 volts is very common. With the O-scope turned on, apply power to the input of your transformer. Immediately, the trace changes from a straight line to what should appear as a sine wave (assuming the frequency of the trace is adjusted to replicate the frequency of your power line). Most likely it will be a really nasty looking sine wave because the power companies don't want to spend a fortune "conditioning" the power (for all their customers) any more than they have to. If the frequency knob is adjusted correctly, the trace should go from the zero line and curve upward to a maximum value. It will then reverse and start dropping. Once it reaches the zero line, the voltage will continue to drop even further. While it looks like the voltage is dropping, it is actually increasing in the negative direction. Once it has reached its negative limit, it will reverse again and will approach the zero line from the bottom. The amount of deflection in the positive direction and the negative direction should be opposite and pretty much equal.
Turn everything off, and connect the two power supplies as noted earlier. The two power supplies are connected in SERIES. When turned on, the output at your anode and cathode is a combination of AC and DC. Many people say the AC is riding on top of the DC. The actual voltage seen at the "anode" and "cathode" isn't actually DC OR AC. The actual voltage seen at the anode and cathode is the sum total of what the two power supplies are set at. If the DC power supply is outputting 10 VDC and the AC power supply is outputting 6.3 VAC, then at the instant the line power passes through the zero line, the voltage seen at the anode and cathode is pure DC with a voltage of 10 volts. The transformer is providing a path for the DC to make it from the DC power supply to the load through the anode to cathode, but isn't adding or subtracting anything. As the line power increases to its maximum value, the voltage seen at the anode and cathode is 16.3 volts (10 volts from the DC power supply and 6.3 volts from the transformer). The AC power has reached its peak and reverses. Along its way, it passes through the zero line once again. At that instant, the voltage at the anode and cathode once again becomes 10 volts with nothing being added to or subtracted from by the transformer. The AC power continues its downward journey becoming more and more negative until it reaches the maximum voltage in the negative direction. At this point, the voltage seen by the anode and cathode is 3.7 volts. (10 volts DC which is offset by the negative (-6.3) volts of the transformer = 3.7 volts). Once again, the line power reverses and the transformer voltage decreases until it once again passes through the zero line. In the U.S. this happens 60 times each second and is called 60 Hertz. I believe Europe is on a 50 cycle per second frequency and it is therefor called 50 Hertz. A Hertz is one complete cycle of the AC power, but is usually counted for a period of one second.
There, you have imposed an AC voltage on the output of your DC power supply. In the example above, the DC voltage is greater than the value of the AC voltage, so the anode is always positive to some degree and the cathode is always negative to some degree. If the power supply was set to 6.3 volts (and the transformer was still outputting 6.3 volts), the anode and cathode voltage would swing from 6.3 volts up to 12.6 volts and then down to zero and back to the starting point of 6.3 volts. This can get confusing. Even more so, if the transformer voltage (AC) is higher than the power supply voltage (DC). In that case, the anode and cathode actually change polarity from positive to negative (and vice verse) with every cycle of the line power.
This sounds complicated, and it can be. I have purposely not mentioned things like effective voltage just to keep it as simple as I could.