Beam current

Taking a closer look at LHC

Measurement of Beam Current

The beam current is the basic quantity of the beam, It is the first check of the accelerator functionality and is very important for transmission measurement and to prevent for beam losses.

Different devices are used for this goal, but the LHC circulating beam current measurement is provided by current transformers. They are based in the  Measurement of the beam’s magnetic field. This kind of devices are non-destructive, they have no dependence on beam energy and they have lower detection threshold.

For LHC beam are used two DC current transformers (DCCT) and two fast beam current transformers (FBCT) per ring (so, eight transformers in total).

DC current transformers (DCCT)

In a very simple approximation we can introduce how transformers work as follows:

Beam creates, such as a  current-carrying wire, a magnetic field B.

Transformer "feels" this magnetic field.

Torus guides this magnetic field which produces a secondary current Isec on the secondary winding on the torus. 

The crucial idea is that the beam acts as primary winding with Nbeam =1.


Ibeam/Isec= Ntorus/Nbeam ⇒ Ibeam = Ntorus · Isec

By using Ohm's Law,

V = R · Isec    ,   so   Isec = V / R

and finally,

Ibeam = Ntorus·V / R


Obviously, things are "a little bit" more complicated. For more information click here.


At full power, LHC beam current intensity is:

Ibeam = 0,58 Amperes

Considering 11245 laps we have:

0,58/11245 = 5,16·10-5 Culombs/lap

With  qproton = 1,602·10-19 C  

5,16·10-5 / 1,602·10-19 = 3,22·1014 protons/lap

Since there are 2808 bunches per ring, we have finally:

3,22·1014/2808 = 1,15·1011 protons / bunch


Xabier Cid Vidal, PhD in experimental Particle Physics for Santiago University (USC). Research Fellow in experimental Particle Physics at CERN from January 2013 to Decembre 2015. Currently, he is in USC Particle Physics Department (Spanish Postdoctoral Junior Grants Programme).

Ramon Cid Manzano, secondary school Physics Teacher at IES de SAR (Santiago - Spain), and part-time Lecturer (Profesor Asociado) in Faculty of Education at the University of Santiago (Spain). He has a Degree in Physics and in Chemistry, and is PhD for Santiago University (USC).



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