**Safety Considerations for the HINS Proton Source and**

**Beam Transport System Assembly at Meson MS6 Lab**

August 22, 2005 (Revised: August 23, 2006)

Doug Moehs, Henryk Piekarz and Chuck Schmidt

**Technical support:** Phil Gallo and Walter Jaskierny

1.  **H+ ion source and beam transport**

> The ion source to be assembled and tested at MS6 is a Duoplasmatron
> proton source that was used previously at Fermilab. The schematic of
> the source is shown in figure below. The ultra high purity hydrogen
> gas is fed into the source chamber where it is ionized in a plasma
> arc. The H+ ion plasma is then compressed by a magnetic field of a
> solenoid magnet into a small orifice and expanded into a vacuum
> chamber where H+ ions are accelerated to a maximum energy of 50 keV
> (Fig. 1). The protons are
>
> ![](media/image1.png){width="5.302777777777778in"
> height="3.265972222222222in"}
>
> **Fig. 1 The Duoplasmatron Proton Source**

then focused and steered through two solenoids and trim steering magnets
to the entrance of the RFQ (Fig. 2). The proton beam current will be
0-40 mA with a duty factor 0f \~ 0.5% (0.1 -- 3 ms pulse length at \~
0.5-5 Hz repetition rate). At MS6, in place of RFQ the beam is dumped
into a small (6 incn dia.,

10 inch long) diagnostic chamber with a Faraday collector to measure the

> ![](media/image2.png){width="5.901388888888889in"
> height="3.5388888888888888in"}
>
> **Fig. 2 Proton beam transport system**

beam current or to a quartz window to view the beam size and basic
properties. A mid-transport vacuum chamber could be used for measuring
the beam profile or emittance.

2.  **Hydrogen gas flow arrangement and protection**

The arrangement of the hydrogen gas flow from the supply cylinder into
the ion source chamber is shown in figure 3. The H2 gas cylinder is 2
feet high

and 6 inch diameter. It contains 2160 liters of hydrogen gas under the
initial pressure of \~1700 psig. The VERIFLO-PARKER 316L stainless steel
regulator is used to reduce the output pressure down to \~10 psig. The
regulator is manufacturer tested to operate at 4000 psig inlet pressure,
and it provides the (1-30) psig outlet pressure. From the regulator to
the ion source all gas lines are ¼ inch stainless steel pipes, rated at
4000 psig pressure.

![](media/image3.jpeg){width="5.995833333333334in"
height="4.1819444444444445in"}

**Fig. 3 The H2 gas flow schematic**

Next to the regulator two safety devices are installed: (1) the Excess
Flow Valve (0.5 SLPM), and (2) the Relief Valve (Circle Seal-- 20 psig).
The hydrogen cylinder with both the Excess Flow and Relief valves is
placed outside the MS6 Enclosure in a secured cabinet. The gas line
enters the MS6 along its South-West pit wall, where the precision needle
valve (Nupro SS-4BMW-TW) and two flow meters (Dwyer--VFA--2" Scale)
allow for tuning of the H2 gas flow into the Ion Source chamber. At the
source the gas passes through a 6-inch long vacuum quality insulated
tube for HV isolation, and then through a fine throttling valve before
entering the source plasma chamber. A typical hydrogen gas flow is \~3
atm-cc/min producing a plasma chamber vacuum pressure \~100 mTorr. The
non-ionized gas (which for practical purpose is all the gas that was fed
into the Ion Source chamber) is pumped out with help of a 400 L/s
turbo-molecular pump into atmosphere via its Fore-pump (15 L/s). The
outlet of the Fore-pump (Welch-1397) is connected to a 2" diameter hose
allowing for the pumped-out hydrogen gas to exit safely outside the MS6
Enclosure. The gas-ballast valve will remain always closed (a tag is
attached to this valve).

3.  **HV power supply arrangement and protection**

The arrangement of the HV power supply is shown in figure 4. The ion

![](media/image4.jpeg){width="5.99375in" height="4.740277777777778in"}

**Fig. 4 The HV power supply and associated electronics**

source and its associated electronics operate at 50 kV potential above
the normal ground. Consequently, all source electronics is placed in an
isolated cabinet, and there is a ground shielding around the source, the
accelerating column, as well as a HV isolation for the cables from the
ion source to the electronic cabinet. From the output side of the
accelerating column through the beam transport including the vacuum
system there is no HV, and so all these test system components are on a
ground potential. The electronics cabinet is located inside a larger
grounded relay rack. The front and rear panel of the grounded rack have
magnetic switches interlocked to the HV power supply, and to a ground
arm which shuts off the supply and grounds the inner isolated HV if the
panels are moved slightly. The power for the electronics comes from a 50
kV isolation transformer located immediately above the isolated HV
cabinet. The source and the acceleration column are enclosed with a ½
inch thick Lexan box. All connecting cables are enclosed with double
walled PVC piping with an outer ground shielding. This protection system
and shielding has been tested to 75 kV. At the electronics rack a 5
micro-amps leakage current is observed at 50 kV and it rises to 30
micro-amps at 75 kV. No leakage current (\< 1 micro-amp) on all other
components of the Ion Source system was observed up to 75 kV. These test
data are logged in the MS6 Ion Source Logbook (page 13 -- 14).

We estimated (engineering calculations are enclosed in the MS6 Ion
Source Logbook, page 31-36) the upper limit of the capacitance of the HV
system (power supply -300 pf, equipment rack -350 pf, HV conductor line
-220 pf, Ion Source -20 pf) to be \<1000 pf. So, with a 50 KV maximum
operating voltage the stored energy is less than 1.25 J. As the limit
for the ground stick application is 10 J it is safe to use the ground
stick to discharge the MS6 ion source system.

The HV power supply is interlocked to 110 V power line that can be open
only following the LOTO procedure. In addition, the whole test area will
be fenced-off during the test with access to the authorized personnel
only.

4.  **X-ray radiation protection**

When the surfaces of the vacuum chamber, the Farady cup or of a viewing
window at the end of the test chamber are struck by 50 KeV protons a
direct X-ray hazard can be produced. These X-rays will have a very broad
energy spectrum reaching up to 50 KeV in the forward direction A
protective shielding of ¼" lead or ½" steel will be able to sufficiently
attenuate these X-rays so there will be no health hazard.

![](media/image5.jpeg){width="4.805555555555555in"
height="3.5569444444444445in"}

**Fig. 5 Arrangement for observation of the proton beam generated
light**

The Faraday cup inside the diagnostic chamber is normally placed in the
beam path, and so the protons are stopped in the cup. The x-rays
produced due to stopping protons are absorbed in the diagnostic chamber
walls.

The image of the proton beam can be observed, however, through a quartz
viewport installed at the end port of the beam diagnostic chamber. The
proton beam hitting the ½"-thick quartz window is stopped there and it
produces x-rays as well as light. Observation of this light allows one
to gather crude but very useful information about the beam spatial
parameters. In order to protect observer from the x-rays the light image
of the beam will be observed using its reflection in a mirror as shown
in figure 5. A 6" long and ½" wall thick SS collimator pipe mounted on
the quartz window flange provides x-ray shield for the observer. **It is
important, however, to have a soft X-ray meter available so it can be
used during the H+ source tests. In addition, the Radiation Safety will
be informed about the plan for a visual observation of the beam.**

**Finally, all H+ source tests at MS6 Lab will be always carried out by
qualified personnel, and never left unattended.**