Electrostatic particle accelerator: Difference between revisions

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Electrostatic accelerators are often confused with [[linear accelerator]]s (linacs) simply because they can (but do not always) accelerate particles in a straight line, as a linear accelerator does. The difference between them is that an electrostatic accelerator accelerates a charged particle by passing it through a single DC potential difference between two electrodes, while a linear accelerator accelerates a particle by passing it successively through multiple voltage drops created betweem multiple accelerating electrodes with an oscillating voltage. The output particle energy of a linear accelerator can be much higher than an electrostatic accelerator, because in the electrostatic accelerator the particles are only accelerated once by the voltage, so the particle energy is limited to the accelerating voltage, while in a linear accelerator the particles receive multiple pushes from the voltage, so the particle energy can be much higher than the accelerating voltage. The oscillating fields in a linear accelerator can only accelerate particles that enter the beamline at a particular time in each cycle, so they don't produce a continuous beam of particles but a series of pulses of particles, while the constant acceleration voltage of an electrostatic machine can produce a continuous beam of particles.

== Particle energy ==

== Understanding the origin of the electron volt ==

Since the charged particle is accelerated through a single potential difference between two electrodes, the output particle energy <math>E</math> of all electrostatic accelerators is equal to the charge on the particle <math>q</math> multiplied by the accelerating voltage <math>V</math>

The relation E=qV also indicates very simply why the [[electronvolt]] (eV) was invented for use in accelerator-based sciences, because if q is entered in integer units of the [[elementary charge]] and V in [[volt]]s, the energy is given in eV; if we wanted to convert the energy into [[joule]]s, we need to multiply by the elementary charge in [[Coulomb]]s on both sides of the equation, yielding a very small number. Usually, the high voltage is quoted in MV and the beam energy then in MeV. With more sophisticated, higher energy machines, those working in [[particle physics]] are just accustomed to discussing mass and energy in units MeV or GeV, but the relationship of the accelerator state to the beam energy is not as simple as merely knowing the terminal voltage and the particle species accelerated.

:<math>E = qV</math>

If the charge is in conventional units of [[coulomb]]s and the potential is in [[volt]]s the particle energy will be in [[joule]]s. However because the charge on elementary particles is so small (the charge on the electron is <math>1.6(10^{-19}</math> coulombs), the energy in joules is a very small number.

Since all elementary particles have charges which are multiples of the [[elementary charge]] on the electron, <math>e = 1.6(10^{-19}</math>, particle physicists use a different unit to express particle energies, the ''[[electronvolt]]'' (eV) which makes it easier to calculate. The electronvolt is equal to the energy a particle with a charge of 1''e'' gains passing through a potential difference of one volt. In the above equation, if <math>q</math> is measured in elementary charges ''e'' and <math>V</math> is in volts, the particle energy <math>E</math> is given in eV. For example, if an an [[alpha particle]] which has a charge of 2''e'' is accelerated through a voltage difference of one million volts (1&nbsp;MV), it will have an energy of two million electron volts, abbreviated 2&nbsp;MeV. The accelerating voltage on electrostatic machines is in the range 0.1 to 25&nbsp;MV and the charge on particles is a few elementary charges, so the particle energy is in the low MeV range. More powerful accelerators can produce energies in the giga electron volt (GeV) range.

== References ==