- #1
Enthalpy
- 667
- 4
Hello everybody!
Neurology uses Transcranial Magnetic Stimulation (TMS) for research and sometimes diagnostic and treatment.
Introduction there: http://en.wikipedia.org/wiki/Transcranial_magnetic_stimulation
Some documentation there, but other manufacturers exist: http://www.magstim.com/
As I understand it (but beware I'm bad on biology and neurology), not the magnetic field, but the gradient of electric potential and current it induces, creates the desired effect on the brains. The action resembles partially an electro-shock, but:
A pulse can have 100µs rise time and 1ms fall time. Series of pulses are also used, sometimes of alternate polarity, but always with pulses of asymmetric transition times.
A good gradient of electric potential needs a strong magnetic induction, and in these non-permeable materials, this means kA in a coil, many kW pulse power, and as a result, the coil is very loud and gets warm, which limits the duration of a session or demands active cooling.
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I propose to split each pulse into N shorter ones, scaling like that for understanding:
Provided the desired effect is still present, splitting the pulse provides big advantages to the apparatus:
Coils should be made of so-called Litz wire (probably a wrong translation for "braided wire"). In fact, if this isn't already done with 100µs pulses, it should have been: this limits the heat in the coil but may demand a current braker. The current braker could even be a series resistor, easier to cool than the coil. Shorter pulses demand wire braided more finely, which reduce the wire's section filled by copper, so the energy lost in the coil is divided not quite as quickly as N2.
Marc Schaefer, aka Enthalpy
Neurology uses Transcranial Magnetic Stimulation (TMS) for research and sometimes diagnostic and treatment.
Introduction there: http://en.wikipedia.org/wiki/Transcranial_magnetic_stimulation
Some documentation there, but other manufacturers exist: http://www.magstim.com/
As I understand it (but beware I'm bad on biology and neurology), not the magnetic field, but the gradient of electric potential and current it induces, creates the desired effect on the brains. The action resembles partially an electro-shock, but:
- The skull is tranparent to the magnetic field, while it hampers the electro-shock's current;
- Hence the skin isn't as brutalized by TMS;
- The path of the current and the intensity are better controlled;
- This let's concentrate the effect to some cm2.
A pulse can have 100µs rise time and 1ms fall time. Series of pulses are also used, sometimes of alternate polarity, but always with pulses of asymmetric transition times.
A good gradient of electric potential needs a strong magnetic induction, and in these non-permeable materials, this means kA in a coil, many kW pulse power, and as a result, the coil is very loud and gets warm, which limits the duration of a session or demands active cooling.
-----
I propose to split each pulse into N shorter ones, scaling like that for understanding:
- Individual pulses are N times shorter, both at rise and fall time;
- The reactive volts per coil turn are kept the same;
- Hence the rate: amps*turns per time unit is unchanged;
- And the amps*turns are divided by N.
- The electric potential gradient, which results from the rate of change of the magnetic potential, is unchanged; (if you prefer, it varies as the volts per coil turn, which are unchanged)
- The cumulated duration of the electric potential gradient is unchanged;
- The proportion between rise and fall time, hence the ratio of electric potential gradient is unchanged.
Provided the desired effect is still present, splitting the pulse provides big advantages to the apparatus:
- The current is divided by N, but the duration remains;
- Consequently, forces in the coil are divided by N2;
- The coil's deformation speed after N shorter pulses is divided by N, and the noise power by N2;
- The energy dissipated in the coil is lowered. By less than N2 because losses increase with frequency;
- Electronics can deliver N times less current if the voltage is kept, and the energy is N times smaller.
Coils should be made of so-called Litz wire (probably a wrong translation for "braided wire"). In fact, if this isn't already done with 100µs pulses, it should have been: this limits the heat in the coil but may demand a current braker. The current braker could even be a series resistor, easier to cool than the coil. Shorter pulses demand wire braided more finely, which reduce the wire's section filled by copper, so the energy lost in the coil is divided not quite as quickly as N2.
Marc Schaefer, aka Enthalpy