Hi Sameer,
I agree with Clemens about the pulse duration - there are several parameters
at play here which are relevant for both probe (arcing) as well as sample
(RF heating): Maximum pulse peak power (should be handled by PICS values/probefile
values for Varian) but also pulse energy (product of intensity/power and duration),
then there is also what we call duty cycle (percentage of pulse time versus delay
time) which can be relevant in experiments like T2 relaxation/echo trains:
- maximum peak power for short pulses (typically 30/90/180 degree pulses) -
PICS/probefile
- maximum power for short-term decoupling (100 ms or so) - PICS/probefile,
but attention if you change your acquisition time in a HSQC for example -
PICS/probefile values are only valid for default (aq/at)
- maximum power for long-term decoupling (seconds or indefinite like for 13C
1D where the decoupler is always on - this is usually around max. 0.5-1 W
permanently) - PICS/probefile
- maximum duty cycle (pulse duration - usually at full power - versus delay
time). This is rather complex and should be handled by the pulse sequence. When
you try to execute a rather long train of 180-delay-180 etc. echos the delay
time is VERY important. Also for acquisition of spectra (often quadrupolar
nuclei) you may run into this when having very short acquisition and relaxation
times, trying to acquire many spectra per second. Similar issues can arise when
acquiring fast-acquisition/real-time acquisition experiments like ASAP, SOFAST
etc., not to mention MRI experiments which can be pretty crazy as well.
This is not easy to handle and may dramatically depend on the properties of
the probe - solids and MRI probes can take a lot more punishment than liquids
probes…
Cryoprobes have quite a good handle to determine RF duty cycle - their
temperature will rise by RF heating, hence the required counter-heating power
goes down (which keeps the probe at a constant 16-20 K). As soon as that heater
value starts going down you know that you should punch your probe a bit less.
Speaking of duty cycle the probe is not the only thing which may get killed
by too much pulsing - preamplifiers are also affected by this and have been
observed to die as well.
Clemens - I don't quite understand why you say that an attenuator before the
amplifier may render the pulse response less linear. Normally power amplifiers
start getting a bit non-linear only near their peak power output. When avoiding
that regime by lowering the input signal a bit I would think that the amplifier
is rather more linear than less? Can you maybe shed some light?
Cheers
Bert
> Am 28.02.2025 um 01:25 schrieb Anklin, Clemens via groups.io:
>=
> Sameer
>
> my first question would be, why do you want to limit the power to below what
> PICS allows. The PICS power levels are safe for the probe. These entries
> define the highest power the probe can tolerate. Of course there are durations
> that might be unsafe to your samples. Setting a pulse to several seconds at
> maximum power will also not be healthy for the system.
>
> I would not recommend to put an attenuator between frequency source and
> amplifier. This would no longer guarantee a linear behavior over the power range.
>
> Best regards
>
> Clemens
>
>
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Received on Sun Mar 02 2025 - 06:42:07 MST