Electrical Hazards and Safety with Ion Sources

1
Electrical Hazards and Safety with Ion Sources

• Martin P. Stockli
• Spallation Neutron Source
• Oak Ridge National Lab

Oak Ridge, TN 37830
2
Prologue

• 40 mA minimum current during a 1.23 ms long
  pulses!
• gt50 mA peak current
• 340 A antenna current!

This are exiting times! Thanks to new controls we
are now able to fine tune the source on the hot
spare stand to the parameters needed for full
operations in 2009!
3
Content

• High Voltage Requirements
• High Voltage Problems
• High Voltage Platforms
• Very High Voltage Platforms
• Small High Voltage Platforms
• Intermediate High Voltage Platforms
• The SNS High Voltage Platform
• The SNS Ion Source Hazards
• The SNS Ion Source Safety Measures

4
Introduction

• Ion sources are not a consumer product.
• Only a few are produced in small series for
  industrial equipment such as ion implanters, leak
  detectors, etc.
• Most ion sources are prototypes, designed and
  built to meet very specific applications.
• Ion sources are not specifically regulated, but
  as electrical equipment, they are subject to
  electrical safety regulations. They regulate

Voltages gt 50 V and currents gt 5 mA or stored
energies gt 10 J
True for practically all ion sources True for
high current sources True for high current
sources
This presentation focuses on electrical
hazards and safety for ion sources,
especially for high current ion sources (gt 5mA).
5
Electric Hazards

• 1,000,000
• 100,000
• 10,000
• 1,000
• 100
• 10
• 1

• Electric hazards increase rapidly with voltage.
  For facility power, the voltage classifies the
  hazard.

• However, what happens when things go wrong,
  depends on the current, over which we have only
  limited control.
• Whenever possible, the current shall be limited
  to safe levels by fuses, circuit breakers,
  resistors, GFCI, etc.
• RD devices often do not allow for safe limits.
  Their hazard classification depends on voltage
  and current capability.

gt500 V very dangerous (skin puncture)
Voltage V
safe lt 50V
6
Effects of Electric Shock
for 60 Hz Body Effects
? 1 mA Harmless, may not be noticed
? 5 mA Painful shock (limit of GFCI)
? 10 mA Muscle contraction, which can prevent the release of the circuit cause serious reflex reaction resulting in bodily injury
? 50 mA Breathing stops due to paralysis of the chest muscle nerve damage
? 100 mA Heart malfunction fibrillation (very difficult to restore normal beating) complete stoppage
? 1 A Internal burns cause tissue and bone damage due to heating
? 250 V ? 500 A External burns due to the electrical explosion
7
Effects of Electric Shock
Frequency current duration Body Effects
10 A 1 ?s Harmless carpet shock
1 MHz 200 mA few s tolerable
60 Hz 100 mA 3 s lethal
DC 500 mA 3 s lethal
8
RD Electric Hazard Classifications
Current A
Voltage V
9
Ion Beam Formation
Ion sources require an electric field to
accelerate the ions in an uniform fashion that
forms a laminar beam. In the absence of charged
particles the electric field is uniform EV/d,
with the ion source voltage V and the extraction
gap d.
When charged particles are present, their
space charge modifies the electric field up to
E (z/d)1/3 ?V. The low field for small z
limits the the number of particles that are
accelerated. This limits the ion beam current
density j to j V3/2/d2. High voltages
enable the production of high current ion
beams! The extraction gap has to be operated
near the breakdown limit. Occasional arcing is
unavoidable for high current ion beams.
10
Low Energy Beam Transport
Ionized residual gas neutralizes the ion beam.
This neutralization can be difficult to control
(e.g. in magnetic fields, choppers, etc.). When
partly neutralized ion beams are transported, the
space charge of the (non-neutralized) beam pushes
the particles towards larger radii, causing an
undesirable growth of the ion beam diameter.
The SNS accelerator has a short, electrostatic
LEBT to minimize the ion beam neutralization and
the undesirable growth!
The space charge of the ion beam is proportional
to the density of the particles, which is
indirectly proportional to the ion velocity. By
increasing the beam energy by a factor of 4, the
space charge can be reduced by a factor of 2.
High current ion beams need high voltage!
11
SNS Requirements for the Ion Source and LEBT

• Ion Specie H
• Pulse Current at source 50 mA
• Beam Energy 65 keV
• Rep Rate 60 Hz
• Pulse length 1.3 ms
• trimmed to 1.0 ms
• RMS Emittance 0.2 mm?mrad
• Droop Noise during single pulse minimal
• Pulse to Pulse Variations minimal
• Operation time between m
• maintenance several weeks
• Mean time between failures many months

gtgt 5 mA gtgt 50 V
The SNS ion source is a significant electrical
hazard!
12
The SNS Ion Source

• Plasma generation
• 15-40 sccm Hydrogen gas
• 13.56 MHz RF, typically 150 W continuous (supply
  600 W)
• 2 MHz 50 kW RF pulsed at 60 Hz / 1.3 ms (supply
  80 kW peak)
• Plasma confinement
• Multicusp 0.25T
• radial 20 magnets
• axial 4 magnets
• Negative ion formation
• 200 Gauss transverse filter field
• 10 0.5 mg Cesium cartridges that need to be
  heated to 500?C to release Cs

Ions
The SNS ion source contains several hazards, but
we focus on HV
13
The SNS Ion Source High Voltage Platforms
SNS has two operational ion sources The
Front-End ion source feeds the SNS accelerator.
The Ion Source Hot Spare Stand is an (almost)
identical copy of the Front End ion source and
LEBT, and is fully functional. Besides being a
complete set of hot spares, the stand is used to
test and develop the SNS ion source.
Front-End
Hot Spare Stand
Front-End Building
14
Low Energy Beam Transport
10 keV protons V -20 kV
Electrostatic focusing depends on a
significant change of the ion energy.
Decel-accel lenses have significant aberrations
due to beam size.
10 keV protons V 7 kV
Technical high voltage problems limit the use of
accel-decel lenses to low voltage ion sources
(few 10s of kV). The compact SNS LEBT uses
decel-accel lenses that reduce the beam energy by
a factor of 3!
15
Low Energy Beam Transport - HV Problems
Electrostatic focusing of high voltage beams
requires the same polarity voltage as the beam.
HV supplies are designed to source current. They
can sink only a very small current through the
feedback resistor. When a part of the beam hits a
decel-accel lens, the voltage will rise quickly
according to I?Ri.
Adding a significantly lower ohm load resistor
(ReltltRi) to the output extends the range where
the supply can sink current up to Imax V/Re.
16
Low Energy Beam Transport - HV Problems
However, for lens 2 of the SNS LEBT, preloading
is not an option because of the integrated
chopper.
The high voltage of the 4 lens segments are
modulated by the HV pulsers through the HV
blocking filters RcCc .
Warning The voltage of beam transport elements
may be higher than you think!
17
The advantage of RF sources
The 1st Maxwell Equation ??E ?/?o states
that electric fields are generated by any free
net charges and the easy controllable surface
charges ? on electrodes. This explains the need
for the surface charges to generate the ionizing
electric field. As the negative surface charges
attract positive ions the sputtering problem
appears unavoidable.
E
-

The 2nd Maxwell Equation, however, ?xE -
?B/?t describes a curling E field generated by a
changing magnetic field in the absence of any
charge! A changing magnetic field B can be
produced in N windings with radius ro with an
alternating current i iocos(?t) B
½µoNi/ro (Biot-Savart). Now integrate
Maxwells 2nd equation for Faradays law
?Eds -dFB/dt -d/dt ?BdA and solve
for E E(rltro,t) ¼r/roµo?Niosin(?t)
Sputtering greatly reduced!
This is a circular electric field that bites its
own tail rather than a poor electrode!
18
Matching an RF driven ion source
The antenna current is maximized by matching the
RF amplifier output impedance with the impedance
of the antenna circuit by adjusting the
transformer ratio NS/NP.
The antenna current is maximized by tuning the
RLC circuit to its resonance. The resonant
frequency is ?2 (LC)-1 and the impedance is
Z?o/io (R2(?L-(?C)-1)2)½ With L 10 µH and
? 2?p?2 MHz, C needs to be
tuned to 0.6 nF to obtain the maximum current
io ?o/R.

• Experience lead to increasing C to 2 nF, while
  reducing L to 3 µH.

19
Matching an RF driven ion source
The antenna current is maximized by tuning the
variable capacitor CVDP-2300-15S by Jennings.
A 40 mA MEBT beam requires 45 kW RF, yielding a
300 A peak antenna current. This corresponds to
212 A RMS, near the caps current limit!

• For a resonant 300A peak current the peak voltage
  over the components is V0 Z?i0 i0/(??C)
  i0???L

CVDP-2300-15S
Max capacitance 2300 pF
Min capacitance 50 pF
Test Voltage 15 kV
Working Voltage 9 kV
Max RMS current 200 A
Length 9.80 in.
Diameter 5.56 in.
Max torque 8 in. lbs.
Capacitance (pF) 2300 1500 600 50
Inductance (?H) 2.8 4.2 10.6 127
Peak Voltage (kV) 10 16 40 480

• The 2.3 nF capcitance on the hot spare stand are
  safe, while the 1.5 nF capcitance on the front
  end tests the high voltage limit of the capacitor!

20
Low Energy Beam Transport - HV Problems
Ion optical elements, such as lenses, and
diagnostic elements, such as Faraday cups, can be
charged up by the ion beam, up to the voltage of
the ion beam. The stored energy increases with
the voltage. It is E C?U2/2. The (stray)
capacitance of disconnected elements is small,
few 10s of pF. Therefore the maximum stored
energy is always less than 10 J. The average SNS
beam current is lt 50 mA?8 d.c. 4 mA and hence
not a life endangering electrical hazard.
Faraday cups and/or disconnected ion optical
elements can give you an unpleasant jolt, but you
will stay around to figure out what happened.
21
Low Energy Beam Transport - HV Problems
This, however, is not the case when elements are
connected to the high voltage supplies!
Model Voltage kV Current mA Capacitance nF Stored Energy J
FE IS DTI 70 20 500 1225
HSS IS SH 70 110 10 28
E-dump LT 8 250 lt15
Extractor /EW 20 30 lt1.5
Lens 1 EQ 60 19 4.1
Lens 2 EQ 60 19 4.1
Steerers EH 3 33 lt0.2
Chopper EQ 3 400 1.1
High current ion beams require high current
supplies that are dangerous!
22
RD Electric Hazard Classifications
Ion source platform
matching network
Current A
Voltage V
23
Effects of Stored Electrical Energy
Energy Example Result
0.01 J a carpet shock harmless
0.1 J stored in the tube of a tv reflex reaction
1 J a 3 kV cap, size of a C-cell battery hurts a lot!
10 J in a microwave oven will incapacitate you
100 J delivered by a defibrillator can stop and start the heart
1000 J energy storage caps, or utility arc blows off body parts
When failures occur, the high voltage platform
inside the Big Blue Boxes can be very dangerous.
Entry requires training. LOTO is equipment
specific and requires training and certification.
24
A spare for the SNS 65 kV supply

• The Front End features a custom built DTI 150 mA,
  70 kV supply
• ?V/V ? 0.3 achieved with a 0.5 ?F output
  capacitor (75mA,1.3ms)
• an arc detector triggered a fast HV switch to
  decouple 1000 J stored energy
• this fast HV switch became permanently conductive
  at LBNL
• DTI estimate for more reliable switch k 50
• DTI estimate for replacing failed switch k 20
• Paid DTI to remove failed switch k 10
• Off-the-shelf 80 kV fast switch from Behlke k
  2.7
• Some RD is required to integrate the Behlke
  switch. Operation without a fast switch is likely
  to contribute to sparks that upset control units.
  We have beefed up the high voltage robustness,
  which dramatically decreased the spark rate.
• But a spare supply is needed!

DTI estimate for a spare supply k 150
25
A Spare for the 65 kV Supply

• The Hot Spare Stand features a k 15.2
  off-the-shelf Glassman 110 mA, 70 kV supply
• its 10 nF output capacitor stores only about 20 J
  
• voltage drops 750 V/?s at the start of each
  extraction (75 mA)
• voltage recovers after 0.5 ms

• Possible solutions
• Start source 0.5 ms early
• (increases source duty cycle)
• Add 0.5 ?F fast switch
• (requires some RD)
• Use feed forward technique to reduce voltage
  excursions
• Speed up feedback circuit

The latter solutions provide a significantly
safer system!
26
High Voltage Platforms
The generation of ions requires the operation of
power supplies to heat filaments and/or to
generate a plasma, to control flow valves,
etc. In addition, proper operations requires
monitoring voltages, currents, temperatures,
etc. All this equipment has to be connected to
the ion source that is at high voltage, requiring
a high voltage platform.

• Depending on the high voltage and the size of
  components, high voltage platforms can be
  categorized into 3 groups
• Small high voltage platforms lt 100 kV small
  components
• Intermediate high voltage platforms lt 300 kV
• and/or large components
• Very high voltage platforms gt 500 kV

27
High Voltage Platforms
25 kV/cm in 1 atm air
28
Very High Voltage Platforms
Very high voltage platforms (gt 500 kV) require a
full metal enclosure of the platform and full
metal coverage of the ground. Intermediate
corona rings are needed to control and limit
variations of the high voltage gradient.
The radius of the corona guards need to be large
enough to keep the fields significantly below 25
kV/cm. Bare metal walls drain the corona charges.
29
Small High Voltage Platforms
Small components subject to lt100 kV, can be
enclosed in a plastic box containing a isolation
transformer and the power supplies needed to
operate an ion source. The box should also
contain all load resistors, limiting resistors,
and monitors that need to operate at high
voltage. The box has to be finger safe, meaning
that it is safe to touch the box and all
connections at any place, at any time! The cables
connecting the box to the source need to be rated
for the maximum voltage, e.g. Vmax
Vplatform Varc
Axial PIG ion source developed at IFK, Frankfurt
U., and sold by NEC
Small ion sources are normally of little concern
because they employ lt5 mA supplies !
30
Small High Voltage Platforms

• To be finger safe, one needs to prevent under all
  conditions all
• Arcing through the enclosure
• Arcing along the surface of the enclosure

Spellman High Voltage recommends lt 3.3 mm
(0.13) per kV air gap and/or creepage distance
or 8.5 for 65kV
Spellman High Voltage further recommends lt 30
kV ¼ polycarbonate enclosure lt 100 kV ¾
polycarbonate enclosure gt 100 kV grounded metal
enclosure
31
Intermediate High Voltage Platforms
Intermediate HV platforms (lt300 kV) can be open,
mounted on top of HV insulators. HV supply and
transformers can fit underneath.
In open platforms metallic parts couple
capacitively to ground. All metallic parts need
to be resistively connected to the platform. High
voltage limiters prevent secondary arcing of high
resistance components.
32
The SNS Ion Source High Voltage Platforms
Due to the number and size of the components
needed on high voltage, the SNS high voltage
platforms are mounted on HV insulators inside a
full metal enclosures, and intermediate high
voltage platform.
Timing adapter
Lens 2 (45 kV) platform
E-dump supply
13 MHz RF
2 MHz controls
2 MHz amplifier
H2 flow control
Power adaptation and distribution
IS (65 kV) platform
33
The SNS Ion Source High Voltage Platforms
The SNS high voltage platform has three
components

• The Big Blue Box contains the electronics

• The Golden Matcher contains the 2 MHz matching
  network

• The Ion source cage contains the ion source and
  the 13 MHz matching network

34
The SNS Ion Source High Voltage Platforms
The SNS ion source is operated 65 kV, and
contains open 2 MHz and 13 MHz RF!
35
The SNS Ion Source High Voltage Platforms
Accordingly, the SNS ion source cage is
interlocked with both RF supplies and the 65 kV
supply.
36
The SNS Ion Source High Voltage Platforms
The SNS 2MHz matching network with open RF, is
operated at 65 kV.
37
The SNS Ion Source High Voltage Platforms
Accordingly, the SNS golden matcher is
interlocked with the RF supplies and the 65 kV
supply.
38
The SNS Ion Source High Voltage Platforms
The SNS Big Blue Box has two interlocked access
doors in front.
39
The SNS Ion Source High Voltage Platforms
The SNS Big Blue Box has also one interlocked
access door in the back. The BBB contains no
longer open RF. Therefore it should permit RF
work without bypassing any interlocks. However,
we still need to reprogram the BBB PLC and EPICS
(Johnny Tang volunteered).
40
The SNS Ion Source High Voltage Platforms
Grounding sticks are the ultimate safety measure
(and life insurance) of any ion source staff. The
issue is complicated by having 5 different access
ports from 3 different locations! To assure
applied grounding sticks whenever an access is
open, we apply grounding sticks whenever we open
an access port, even if grounding hooks have been
previously applied from an other access port. The
grounding sticks remain in place until the
corresponding access port is closed!
41
The SNS Ion Source High Voltage Platforms

• The main concern about the SNS (and many other)
  high voltage platforms are the fact that they are
  not finger safe unless they are guaranteed to be
  grounded. In addition, the SNS ion source
  platforms are fed from 3 different circuit
  breakers in 2 different panels.
• There is no specific regulations for high voltage
  platforms. As electric equipment they are subject
  to the electric safety regulations. Consistent
  with the SNS LOTO procedure, we use a JHA to
  access the high voltage platform for minor,
  routine, and repetitive tasks, that are integral
  to the operation, such as resetting circuit
  breakers, rebooting scopes, and opening the
  hydrogen bottle.
• To guarantee the grounding of the high voltage
  platform, we
• Switch off all relevant supplies with EPICS
• Disable all HV supplies and grounding both
  platforms by disabling them with the BBB safety
  chain
• Check the engagement of the grounding relays
• Disable all HV supplies and ground both HV
  platforms by breaking the interlock when opening
  any of the access ports
• Wear safety glasses and ground all relevant high
  voltage platforms by applying the grounding
  hook(s)

42
The SNS Ion Source High Voltage Platforms

• For non-repetitive and non-routine tasks, we use
  our equipment specific LOTO procedure.
• To guarantee the grounding of the high voltage
  platform, we
• Switch off all relevant supplies with EPICS
• Disable all HV supplies and grounding both
  platforms by disabling them with the BBB safety
  chain
• Lock out and tag out the AC to all relevant
  supplies
• Check the engagement of the grounding relays
• Disable all HV supplies and ground both HV
  platforms by breaking the interlock when opening
  any of the access ports
• Wear safety glasses and ground all relevant high
  voltage platforms by applying the grounding
  hook(s)

5 out of 6 measures are administrative controls
that are at risk of being ignored by staff that
is tired and anxious to restore operations! We
plan to add one additional, independent
engineering control!
One of the complicating issues is the
verification of LOTO. We plan to integrate the
LOTO verification in an additional, independent
engineering control that we plan to add the BBB!
43
The SNS Ion Source High Voltage Platforms
The SNS high voltage platforms features creepage
distances of 3.5 to 5, significantly more than
the 1 for the breakdown of dry air, but
significantly less than the 8.5 recommended by
Spellman High Voltage. In addition the enclosure
features insulators in high field gradients.
Accordingly breakdown are rare, but regular!
44
The SNS Ion Source High Voltage Platforms
The grounding relays are properly rated, but not
sufficiently overrated. The (painted) walls near
the grounding relays show many marks of
discharges! No marks can be found in the BBB of
the IS HSS!
45
The SNS Ion Source High Voltage Platforms
The grounding relays for the LEBT have been
covered with a Lucite sheet/metal screen for
additional protection.
Check 45 kV deck grounding relay before applying
grounding hook.
46
High Voltage Safety

• We are modifying the Big Blue Box interlock
  system to allow for RF tuning without having to
  bypass interlocks.
• All BBB interlocks have been checked. When any
  interlocked access port is opened, all high
  voltage supplies are switched off and disabled,
  and all outputs are grounded with relays.
• We have developed a BBB-specific LOTO procedure
  and a JHA for BBB entries for minor tasks that
  are routine, repetitive, and integral to the
  operation.
• In a single incident, a lower level relay hang-up
  caused the 65 kV grounding relay to remain open.
  Safety remained guaranteed by 3 complementary
  safety measures.

We plan to collaborate with Lloyd Gordon to
further improve the BBB safety system. We expect
to add captured keys and an independent 2nd
grounding chain.