hans jostlein, lar workshop, november 4, 2004 1 liquid argon tpc high voltage issues

Hans Jostlein, Lar Workshop, November 4, 2004

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Liquid Argon TPC High Voltage Issues


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Voltage and current needs

Drift velocity sets the scale:


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Choice of Electric field Strength Choice of Electric field Strength

Based on:

“Reasonable drift time” Example: A field of 500 V/cm drifts electrons at

1.5 mm/microsec. It requires an electron lifetime of about 2 msec. This seems achievable

For a 3 m drift distance, the dwell time is 2 msec. High Voltage Technology gets hard quickly at

very large voltages.


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Choice of Drift Field -- 2

Technical limitations Our example requires HV = (500 v/cm) * (300 cm)

= 150 kV If we were to try and double this, the total voltage

would be 300 kV, which is quite challenging, while the dwell time would only go down to 1.5 msec.

In the other direction, if we were to halve the E-field to 250 V/cm, the dwell time would grow to 3 msec, not a big change from 2 msec,

but a huge voltage reduction, from 150kV to 75 kV, going from “achievable” to “routine”.


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Voltage Regulation Requirements

We pick an acceptable drift distance measurement tolerance of, say, 0.5 mm. For our example of 3m drift distance, this is 1/6000.

The drift velocity must be constant at this level.The velocity versus field curve is not linear.We find that at 250 V/cm we must regulate the Voltage to

1/4300;For 500 V/cm we need to regulate to 1/2800;And for 1000 V/cm we need to regulate to 1/2400

Not a big effect !


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Current Flow Issues Filter and Leakage Currents:

Leakage currents are not well predicted, usually. If we assume a leakage current of 100 microamp, as an

example, then for the 500 V/cm case we are allowed a voltage variation of (150 kV) / 2800 = 54 Volts

Any series resistor in the HV filter must then be 540 kilo-ohms or lower.

Electron drift current The Physics current, due to ionization in tracks is small: We assume about 55,000 electrons per cm of track. If

we take an example of 3 events per msec, each with a track length of 10 m, we get an ionization current of 1.6 E11 electrons/sec, which is 26 micro-amps.


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Noise Requirements Noise RequirementsNoise on the HV supplies (Main drift HV and ancillary wire

chamber bias supplies) couples capacitively into the wire chambers.

Main Drift HV Supply The noise of the main HV supply affects only the first

induction plane. The induced noise can be estimated from the noise fraction

on the main drift field, induced into one wire. For our prototype we estimate the capacitance from the

Main HV into one wire as about 1 pF. A 1 Volt pulse on the HV will induce a charge of 1E-12 C, which corresponds to 6250 electrons.

The induced charge from a real track ionization is about (55,000 e) *(1 cm / wire spacing) = 27,500 electrons.


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Signal to Noise

For a signal to HV noise ratio of 1%, we need to hold the HV ripple to 10 mV in the frequency region of interest.

Note that modern high frequency supplies often run in then 100 kHz range which is just about where we care the most.

A Filter is needed.


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HV Filter Parameters If we assume a supply with 10 Volt ripple, we need to reduce

the ripple by a factor of 1000x. Using an RC filter, we are further constrained by the maximum

series resistor , due to leakage currents, of 540 kOhms (see above).

An RC filter is a voltage divider. The noise is divided (approx) in the ratio (Series resistor /

Capacitor impedance). The capacitor impedance must be 1000 x smaller than the 450

kOhm series resistor, i.e. 450 Ohms. The capacitor’s impedance at the frequency of interest (100 kHz in our example) is R = C/(2*Pi*frequency).

W arrive at the required filter capacitance as C = 1/ (450 Ohm * 2*pi*100 kHz) =3.5 nF


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Multi-Stage Filter This is rather large and dangerous. If we design the filter as a 3-stage RC filter, we end up

with each stage having an 18 kOhm resistor and a 90 pF capacitor, which is much easier and safer.

The next slide shows is the filter box for the Flare

prototype. It is rather over-designed, but we had all the

components lying around, so it was free. We also want to be able to deal with unknown noise sources such as lower frequency noise.


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Flare Prototype Filter Box


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Wire Chamber Bias Supplies The field strength must increase after each wire

plane by about 50% to collect all drift electrons without allowing them to reach the wires.

For a plane spacing of about a cm, these supplies are in the 0.5 to 2 kV range.

Common supplies designed for phototubes are quite suitable.

They are well regulated and come with voltage and current readback.

Even though the noise coupling capacitances are somewhat larger here, no problems are expected.

Feedthroughs are not extreme, but require care.


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HV Feedthrough Issues

The main drift HV feedthrough into the cold volume poses some interesting challenges:

--high voltages --low breakdown voltage in Argon gas --cold end --gas seal --prevent oxygen diffusion into the liquid argon --Materials compatibility with argon purity


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High Voltage CapabilityFor the range up to 200 kV there is substantial

experience , e.g. from particle separators.Standard coaxial cables exist.At the outer cable end, the outer jacket and wire shield

is usually pulled back, while the polyethylene inner insulator continues for several inches.

The assembly dips into a dielectric cavity, and makes contact with some spring arrangement. The wire shield is tied to the outer shield of the feed through assembly.

At the Argon side, care must be taken not to expose any conductor to the gaseous Argon, which has a low electric field holding capability.

The liquid Argon, by contrast , can hold of as much as 2 mega-volts per cm.


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HV Feedthrough Design Following Icarus practice, we make the feedthrough dielectric from a

single HDPE rod that is long enough to dip well into the liquid Argon. The gas seal consists, as it must, of an inner conductor seal and an

outer dielectric seal. Both seals rely on O-rings. The inner conductor seal is located at the bottom of the outer cavity,

where it stays warm, and is spring loaded against the dielectric with a set of Bellville washers that reside on the cold end, inside of a corona ball.

The outer dielectric seal is warm also, and is incorporated into a pair of modified CF flanges.

Both O-rings are continuously flushed with dry nitrogen (or dry Argon—yet to be decided) to prevent oxygen diffusion into the TPC argon.. Oxygen diffusion through Viton (fluorocarbon) and most other suitable materials is significant.


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Flare HVFeed-

through


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Micro-Discharges The Icarus experiment had problems with HV micro

discharges inside cables and at connections. These discharges appear to be enabled by humidity.

Common cures include bathing the contacts in Fluorinert liquid and flushing with dry nitrogen.

We have adopted the latter. A fitting on the main feedthrough will be supplied

with dry nitrogen gas continuously. A small passage hole channels some of the gas into

the space surrounding outer O-ring, and the rest flushes the plug connection and flows into the outer jacket space of the HV cable.

A similar flush line will protect the HV filter and its supply and return cables.


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HV Monitoring DC monitoringFor value and stability of the DC voltages This is done by taking a small fraction of the voltage at the

bottom of divider chains.It is particularly important to monitor the Voltage inside the

TPC because it may have been reduced by unexpected leakage currents. One can imagine taking the test output from the cage divider and using it to regulate the DC supply.

AC MonitoringAc voltages are monitored from the last divider resistor.AC problems may include noise; drift; and micro discharges.

Use an oscilloscope or a discharge detector, a la Icarus.


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Current Monitoring

Leakage currents can be a problem and an early indictor of developing HV problems.

Currents are difficult to monitor in HV systems.The HV supply has often a ground-referenced

current output for that purpose.


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Electrical HV Safety These protective measures need to be taken:--- All devices are connected together with a good

grounding cable, e.g. of size 1/2inch x 1/8 inch woven copper

--- all HV devices have internal self-discharge resistors with a limited discharge time, e.g. one minute time constant or less

---all HV devices have a voltage divider output for monitoring their discharge status

--all HV devices are fed through a high value, low power, resistor which limits any spark currents and functions a s a fuse in abnormal current conditions. The fuse for the TPC should be located outside the TPC, e.g. inside the filter box, for easy access.


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Field Cage Design

The drift field needs to be uniform to preserve the reconstructed event topology accurately.

Typically the field needs to be shaped by a field cage, consisting of equipotential Loops.

Icarus has built those from Stainless steel tubing, held with PEEK insulators to the vessel structure. Chains of resistors divide the Main drift HV into appropriate steps.

They have also use sheets of printed circuit on fiberglass-epoxy for a smaller test chamber. For large detectors, stainless tubing seems to offer advantages.


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Electric Field Uniformity


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Electric Field Uniformity--2 Electrons drift along filed lines, at right angle to

equipotential surfaces. It is convenient to have electrons drift exactly along

straight paths. Very close to the cage hoops there will be

distortions. Yet the electrons’ drift path is still deterministic, and

can be calculated from the field map via the hoop geometry. If we know how far each electron has drifted, we can accurately determine its place of origin.


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Electric Field Uniformity--3

However, if we do not know the event time, we cannot do this and must reject the data very close to the hoops, except to gain topology information.

This is important information, e.g. when vetoing tracks entering from the outside.

In this case there is essentially no loss of fiducial volume.

We can use a very coarse cage structure, which is easier to build.


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Resistors must meet several requirements:

--be compatible with cooling--do not change resistance uncontrollably when

cold--do not suffer damage from thermal cycles--be able to hold off the step voltage without

internal discharges--be very reliable--be free of electronegative materials or

contaminants


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Resistors can be attached in different ways:

--solder to lugs on the SS tubing--plug into holes using proper electrical contactsFor redundancy on their larger detectors, Icarus

has used four parallel resistor chains.

hans jostlein, lar workshop, november 4, 2004 1 liquid argon tpc high voltage issues

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