ornl is managed by ut-battelle for the us department of energy recent performance of & plasma...
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ORNL is managed by UT-Battelle for the US Department of Energy
Recent Performance of & Plasma Outage Studies with the SNS H- Ion Source
Martin P. Stockli, B. Han, S.N. Murray, T.R. Pennisi,
C. Piller, M. Santana, R.F Welton
Oak Ridge National Laboratory
Oak Ridge, TN 37830, USA
16th International Conference on Ion Sources
New York, NY, USAAugust 25, 2015
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Abstract Early in 2014, after several years of producing neutrons with ~1 MW proton
beams, SNS started to ramp to higher power levels that can be sustained with high availability. Powers of up to 1.4 MW may be possible despite a compromised RFQ, which requires higher RF power than design levels to approach the nominal beam transmission. Unfortunately at higher power the RFQ often loses its thermal stability, a problem apparently enhanced by beam losses and high influxes of hydrogen. This led to the semi-retirement of the high-performing source #3. The apparently lower beam losses of the other two sources shifted the goal to delivering as much H- beam as possible with the least amount of hydrogen in the source, which led to plasma outages. Ongoing plasma outage studies show that the 13 MHz supply struggles with the ~90% power reflected by the 1-ms long 2-MHz plasma pulses. Possible mitigations are being tested, starting with a 4-ms RC filter for the reflected power signal.
Lowering the H2 pressure initially increases the H- beam current due to reduced losses, and since mid-2014 ~50 mA are routinely injected into the RFQ. Subsequent LEBT retuning improves the RFQ transmission by better matching the reduced-divergence beams. Accordingly ~35 mA H- beams exiting the RFQ have become routine.
To further support higher powers, under-performing sources are replaced after two weeks while well- performing sources are used for up to 8 weeks, frequently exceeding 3 Ah of H- without showing signs of aging.
These new approaches increased the average RFQ output peak current at the end of the pulse by ~2 mA while the standard deviation was reduced from 1.9 to 1.3 mA compared to the prior year, which included the high performing source #3.
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Content
This talk is all about delivering
more H - ions!
• Introduction to SNS• Brief Performance History• Recent Performance • How to make MORE beam
– Beam Current Limits– The RFQ– The 13 MHz system– Lowering the H2
– Plasma outages– A new tune for the 13 MHz!– The external antenna source
• Conclusions
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High Flux Isotope Reactor (HFIR) Intense steady-state neutron flux
and a high-brightness cold neutron source
Spallation Neutron Source (SNS) World’s most powerful accelerator-based neutron source
Basic Energy Sciences has created 2 powerful neutron sources at ORNL
Neutron scattering pioneer Clifford Shull
in 1946 at ORNL
SNS produces the Science with 18 state-of-the art Instruments. 2 more under development.
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The SNS and HIFR Science Output grows!
We aim for 400 neutron science publications by 2017!
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The Spallation Neutron Sourcesmashes a pulsed, 1 MW proton beam on to a Hg target to
produce ~21017 neutrons 60 times per second!
accumulator ring
Hg target
ion source
SNS was constructed by a collaboration of Lawrence Berkeley National Laboratory
Los Alamos National LaboratoryJefferson National Laboratory
Brookhaven National Laboratory Argonne National Laboratory andOak Ridge National Laboratory
SNS Accelerator Complex
Front-End:Produce a 1-msec long,
chopped, H- beam
1 GeV LINAC
Accumulator Ring: Compress 1 msec long
pulse to 700 nsec
2.5 MeV
LINACLINACFront-EndFront-End
RTBT
HEBT
Injection Extraction
RF
Collimators
945 ns
1 ms macropulse
Cur
rent
mini-pulse
Chopper system makes gaps
Cu
rren
t1ms
Liquid Hg Target
1000 MeV
1 ms macropulse1 ms
<1 sec
The front end produces a 60 Hz, ~1 ms long
chopped ~40 mA H− beam
The LINAC accelerates it to ~ 1 GeV
The ring accumulates it to ~40 A H− beam
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For the past 6 years SNS has been running near 1 MW except for cost- and target-issues! We aim for reliable 1.4 MW by 2017!
The Spallation Neutron Source
Much had to be learned to support ~1 MW operations with high availability!
1.34 MW1.4 MW
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Ion Source and LEBT Performance History
The 131th production source was started up early this August. We had 131 opportunities to learn, improve and perfect the operation of our H- ion source and drive up its performance to unprecedented levels. However, due to the nature of ion sources, especially
H- ion sources, many things are not fully understood and not exactly reproducible, and remain challenges!
Only source #3 makes ~38 mA
Year of ~40 mA MEBT currents
source leaks & contamination
Reduced RFQ transmission
large leak
e-dump fails
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Recent Ion Source and LEBT Performance
#3 preferred due to higher performance
RFQ input currents source #2 RFQ output source #3 RFQ output source #4 RFQ output
Source #3 was the best-performing workhorse until March 2014, when it became disliked due to its thermally loading the RFQ and limiting its power.
source leak suspected
#2 & #4 preferred for more stable RFQ
e-dump failure
Source #2 was benched in 2011 after a severe contamination. After being tuned up in 2014, it became the favored workhorse.
1.4 MW 1.4 MW
Source #4 serves as the alternate for the last 4 years. While its LEBT output may lack a few mAs, in the MEBT it is only ~1mA less than #2.
12 month Period 7/13-6/147/14-6/15 changeAverage RFQ input 45.4±2.4 51.8±1.2 +14%
Average RFQ output (end of cycle)
33.2±1.9 35.0±1.3 +5%
Despite benching star source #3, the average currents are up and the variations are down!
?
e-dump failure
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How to Make More H- Beam with lesser Sources?
Short Answer: Very carefully!
38 sccmplasma grows
28 sccmplasma steady
• After all high voltages have conditioned, the H2 flow can be lowered. This increases the H- beam as extraction-area losses are reduced.
18 sccmplasma starves
• After reaching the optimum H- beam current near ~28 sccm, the beam current decreases as the plasma is starved.
• The MEBT H- beam current is globally optimized during the final tuning using the
• This includes a LEBT retune to benefit from reduced-divergence beams obtained with smaller pressures.
• High-voltage upsets can cause plasma outages which are avoided by increasing the H2.
time-averaged charge, integrated over the full pulse-length. This is done during the power ramp-up to production.
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LEBT and MEBT Beam Current Limits60
54
48
42
36
30
= RFQ Output
Current
Increasing the LEBT beam current increases its emittance, which lowers the RFQ transmission. The trend of the transmissions suggest that we are close to the maximum RFQ output current.
Are we doomed to MEBT currents ≤36 mA?
TransmissionRFQ In
put Curre
nt
RFQ Output Current
Maybe not, if old dogs can learn new tricks!
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Andrei Shishlo pointed out one way!
0.25 0.30 0.35 0.400.0
0.2
0.4
0.6
0.8
1.0
1.2
1.4
New Set Point
N
orm
ilize
d C
harg
e, a
.u.
RFQ Amplitude, a.u.
MEBT BCM11 Integrated Charge vs. RFQ Amplitude
Old Set Point
Courtesy of A. Shishlo
Increasing the RFQ power can drastically increase the MEBT beam!
However, to remain thermally stable, less H2 influx is needed!
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Gas inlet
Window
Cusp magnets
Dumping magnets
Cesium collarPlasma
Extractor
E-dump Lens 2RFQ entrance flange
Ground electrode
Lens 1
Outlet electrode
Filter magnets
RF antenna
Gas inlet
Window
Cusp magnets
Dumping magnets
Cesium collarPlasma
Extractor
E-dump Lens 2RFQ entrance flange
Ground electrode
Lens 1
Outlet electrode
Filter magnets
RF antenna
The SNS Baseline Ion Source and LEBT•LBNL developed the SNS H- ion source, a cesium-enhanced, RF driven, multicusp ion source. •Typically 300 W from a 600-W, 13-MHz supply generates a continuous low-power plasma. •It is tuned by minimizing the reflected power.
•The high current beam pulses are generated by superimposing 50-70 kW from a pulsed 80-kW, 2-MHz amplifier. It is tuned for maximum H- beam current.
2 MHz
13 MHz
CPCS
But the H2 needs to be lowered, which caused plasma outages that needed to be understood!
There remain issues, but this injector keeps breaking records!
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Ramping the 2 MHz with 1.96 MHz
forward amplitude
reflected amplitude
•With 13 MHz plasma the LCR circuit rapidly builds up oscillations before drifting off resonance due the evolving plasma inductance. •Only little RF power is reflected because the plasma absorbs the RF readily. •However, without plasma the LCR circuit builds up to 76 kW, absorbing up to 71 kW. The electric fields generated by the very large antenna current are unable to break down the pure H2 gas.
The 80 kW QEI cannot provide reliable ignition!
•RF build-up was studied with 4-week-old source #2 using a 2 MHz directional coupler on ground.
13 MHz
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How to reliably ignite the Pulsed SNS Source? Unable to reliably ignite a
clean H2 source with the 2 MHz RF, the only option is to:
1. Ignite continuous 13 MHz plasma with a pressure bump (PUFF).
2. Use the 13 MHz plasma to absorb the 2 MHz plasma.3. After a plasma outage go back to 1).
However, until July 2007 we operated without 13 MHz because the 2 MHz self-ignited the plasma of the poorly-conditioned, dirty sources which delivered decaying beams.
It is that simple! And we do for >7 years!
Persistent operation of old sources depends on continuously maintaining some plasma!
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A Plasma Outage
PlasmaNo plasma
2 MHz detuned
More 13 MHz
Seen by the 2 MHz directional coupler on ground
2 MHz forward
13 MHz
2 MHz reflected
Plasma outages start at the end of a 2 MHz pulse!
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In the past the average reflected power shown by the 13 MHz supply was used!
Our tunes turned out to be inconsistent.
The biggest surprise was that the 2 MHz plasma reflects practically all 13 MHz RF, which triggered foldbacks of the output power!
Time Resolved 13 MHz Measurements
300W 400W 450W
In 2014 FE scope#1 was used to show forward & reflected 13 MHz RF.
COMDEL said time averaging the reflected power is difficult!
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1 1Tues day, Apr il 20, 2004
Title
S ize Document Number Rev
Date: S heet of
COMDELThe Competetive Edge in RF Technology
Comdel Inc.11 K ondelin Rd.Gloucester, MA 01930P hone: (978) 282-0620Fax: (978) 282 4980
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The CX600C73 from 0.1 to 1
R19 from 1.2k to 3.9k
from 0.12 to 4 ms
The CX600 was modified to average the reflected power over 4 ms!
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300 W; 25 sccm 400 W; 25 sccm 450 W; 25 sccm
The 4-ms Filter works!
1.0 1.1 ms 1.0 1.1 ms 1.0 1.1 ms
Lower Hydrogen pressures are possible.Plasma outages are preceded by a reflected power bulb after the 13 MHz reflected power recovers!
No more foldbacks of the output power!
0.9 1.0 1.1
300 W; 25 sccm 300 W; 20 sccm 300 W; 19.3 sccm
0.9 1.0 1.1 ms0.9 1.0 1.1
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Tuning the 13 MHz Matcher• In the past the 13 MHz
matcher was tuned with plastic slot drivers until the reflected power from the COMDEL CX600 indicated a minimum.
2 MHz
13 MHz
Old matcher
+2 dials
= reproducible tunes!
• Adding 2 dials this spring enabled reproducible tunes!
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An outage resistant 13 MHz tune The two 13-MHz tuning capacitors are
strongly counter-correlated! The resonance is less than 1 turn wide!
Measuring the outage flow has shown that tunes with higher reflected power and less bright plasma allow for lower hydrogen pressure.
That has enabled neutron production with ~10% less hydrogen which increased the beam current by 5% due to a higher transmission through the RFQ!
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Baoxi will be around for the rest off ICIS!
MonPS35
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Ion Source & LEBT Performance
For the last 4 years the ion source and LEBT were ~99.5% available.
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Source Replacements
This is infant mortality!The diameter of the most plasma exposed “tapered bend” of the antenna was measured before and after service.Since 2014 the average measured wear was 40 µm or ~10% of the coating.• There is no correlation between
the measured wear and the service duration: r = 0.12.
Service cycle is increased to 10 weeks for 5 A· h of H-!
• Since late 2011, 6 and 6+ service weeks are customary for high performing sources to deliver more H- ions when desired.
• 33 production sources were replaced in the past 2.6 years.
• 30 at the end of their service cycle.• 2 prematurely after e-dump failed.• 1 prematurely after antenna failed.
• Plasma light emission data suggest that most wear happens during conditioning!
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SNS External antenna source RF-driven (pulsed 2
MHz, 30-50 kW), Cs-enhanced, multicusp H- sources capable of delivering ~50 mA.
Similar to baseline source except for a water cooled AlN plasma chamber, antenna external to the vacuum and a plasma gun for ignition.
Few issues remain after a 40-day run with stable beam and e-dump after a single cesiation!
See Rob Welton’s Poster TuePE36
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Summary and Conclusions• We have operated for over one year with source #2 and
#4 which yield a lower thermal load to the RFQ. • Paying more attention and spending more time tuning the
source and LEBT have increased the H- beam current.• However, the compromised RFQ transmission reduces
this gain to 1-2 mA in the MEBT. • Plasma outages appear to occur when the 2 MHz plasma
decays before enough of the 13 MHz can be absorbed to restore the 13 MHz plasma.
• Breakthroughs allow for lower H2 pressures with higher H- beam currents without frequent plasma outages.
• Matching the 13 MHz with more reflected power and less light has enabled operation with lower hydrogen pressures, which decreases the beam emittance and increases the transmission through the RFQ.
Thank you for your attention!