RE: Muon-On-Request

[Michael Barnes <barnes@triumf.ca>]

Dear Peter,

Thank you for your email.

Re the variation in plate voltage on a shot to shot basis, the following
is from my email of September 6th:
========================================================================
For your (MuLan) normal operating conditions (kicker on for say 22us and
off for 5us, repetitively), running at 12.5kV, each kicker will draw in
the region of 55mA DC from the HV DC supply. There is a 2500R resistor in
series with the HV DC supply, per MOSFET stack, which (together with high
quality capacitors) provide filtering to minimize RF getting out of the
cabinet. The normal voltage drop across this 2.5kR resistor is almost 140V
--- i.e. the 220nF capacitor nearest to the HV supply will be at 12.36kV
for 12.5kV DC supply voltage. For a given repetitive operating condition
(frequency and pulse width) this 12.36kV is more or less constant for
every pulse. If you suddenly switch the kicker to DC mode (one extreme)
the 55mA will not be drawn by the load and thus will result in the 220nF
capacitor closest to the HV DC supply being charged up (to at least
12.5kV, and possibly slightly higher, depending on the time response of
the current regulation circuit in the HV DC supplies internal controls):
thus when you next turn on the MOSFET stacks the 220nF will start at
approximately 12.5kV (i.e. 140V higher than immediately before you sent
into DC mode) and, again depending upon the response time of the HV DC
supplies internal controls, it could take tens of milli-seconds (or even
more) for the 220nF capacitor voltage to reach steady-state voltage. Hence
I can more or less guarantee that you would have a variation of at least
140V (in 12.5kV) of the deflector plate voltage over time, within each
92ms macro cycle.
==================================================================

To elaborate on the above:
The  drop in voltage across one of the 220nF capacitor when the deflector
plates are charged up to 12.5kV, is in the region of 6.25V. At the nominal
MuLan operating frequency (~1/27us = 37kHz), and during steady state
operation, the 6.25V is exactly "replaced" by the HVDC power supply during
the 27us interval (approx. 51mA average current*27us=1.377uC => 6.26V
recharge of 220nF [exact value of average current depends on spacing of
deflector plates too]). So that each pulse has the same magnitude
(~12360V).
Hence if the kicker were running at steady state at 37kHz and 12.5kV, and
then the period were suddenly doubled to 54us, the subsequent pulse
magnitude would be approximately 6.25V greater because of the longer
charge time, i.e.
last pulse at 37kHz = 12360V
first pulse at 18.5kHz is approx 12360+6.25V = 12366.3V
2nd pulse at 18.5kHz is approx 12366.3V+6.25V = 12372.5V
etc.

It is not quite as straight forward as this because of the RC
time-constant for recharging the 220nF and the action of the HVDC supply
controls -- but it gives you and idea of what will happen.

However the voltage pulse, during a 25us pulse, should remain constant.

Re your second question. Yes you may send the kicker a "KICKER ON"
continuously command without damaging the kicker. The plate voltage will
not be exactly flat during a very long pulse -- the flattop circuits are
optimized for MuLan operation -- the voltage on the plates may vary by up
to 1% in magnitude (depends upon the exact values of the DC grading
resistors on the MOSFET cards and the ~18k resistor in parallel with each
flattop card).

Hope the above answers your questions.

Cheers,
Mike

> Dear Francoise, dear Mike,
>
> Thanks for paying so much attention to the muon-on-request scheme.
> Francoise's short visit was extremely helpful.
> It started out extremely promising, we could increase our
> good muons (mu+) by a factor 2.5-3 and I could not observe any
> electronic cross talk thus far. The kicker operated stably
> over night.
>
> Let me try to define my understanding of our confusing language.
>
>  KICKER ON  = BEAM OFF
>  KICKER OFF = BEAM ON
>
> where KICKER ON is defined as HV ON on the kicker electrodes.
>
> After we received a muon within BEAM ON, we turn the BEAM OFF
> for 25 us. The we turn BEAM ON again. In this state we wait
> for the next muon signal.
> The average wait time can vary from ~10 us (for 100 kHz muon beam)
> up to seconds (when there is no beam).
>
> So we have
>
>    KICKER ON  = BEAM OFF        25 us fixed
>    KICKER OFF = BEAM ON         450 ns  - seconds
>
> (450ns are a hardware protection for the kicker).
>
>
> The second distinct question was simply whether I can send the
> kicker a KICKER ON control TTL level to keep it in a steady ON
> state, without manual operations at the kicker.
>
> With best regards
>
> Peter
>
>