Xray

pic taken from euronuclear.org
The spectrum of emitted radiation reaches from a maximum energy given by the kinetic energy of the generating particle down to zero energy. Bremsstrahlung only becomes easily noticeable when the energy of the particle is very high compared to its self-energy. This is mostly only valid for electrons (self-energy of the electron: 511 keV).

Why the K lines missing?

The electrons has insufficient energy to knock out the electrons in the inner shell (K), therefore no characteristic lines observed at lower accelerating voltages.

K lines

K alpha more likely than K beta , the electrons in the next higher shell has a greater probability of transiting downwards than the electrons in the next next higher shell, therefore the intensity is higher.

Xray tube

Looks like a light bulb and so small!

Xray production

The accelerating voltage of electron is in tens of KV. Hence if the KE of electrons is 100 kV the min wavelength of (continuous spectrum) bremstrahlung rad is 0.124 Armstrong.
For overview of X rays: www.ehs.unc.edu/training/self_study/xray/7.shtml

Characteristics line spectrum of Xrays

pic taken from ehs.unc.edu
If the incident electron energy is high enough, it may interact with an (inner) orbital electron, ejecting it from its orbit. As the vacancy is occupied by an electron from a higher orbit, the (quantized) excess energy is emitted as a characteristic photon .

Braking radiation(bremstrahlung rad)

pic taken from: www.rstp.uwaterloo.ca
A car with KE slow down resulting in heat generated. Similarly a electron with kinetic energy emits photon(Xray) when it slows down.The energy of the Xray photon has to be less than or equal to the incident KE of the electron. This accounts for maximum value for the photon hence a limiting minimum wavelength of the photon.

Bremstrahlung

To see animation go to this link below:
Bremstrahlung
Read more on how Xrays are produced at My book : Physics 2000 , look for Xrays.