Commissioning of the Lower Hybrid Current Drive System on Alcator C-Mod

Abstract

A lower hybrid current drive (LHCD) system has been developed for current profile control of advanced tokamak experiments on Alcator C-Mod. LHCD along with Ion Cyclotron Radio Frequency (ICRF) heating will be used to develop regimes with high confinement, high beta_n and high bootstrap fraction and extend them to quasi-steady-state conditions. This paper will describe the commissioning and initial operation of the LHCD system that includes a 50 kV, 208 A pulsed-power supply, twelve 250 kW Klystron transmitters, a 96 waveguide launcher and required control, protection and data acquisition systems

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Commissioning of the Lower Hybrid Current Drive
System on Alcator C-Mod
D. R. Terrya, R. Parkera, J. Liptaca, A. Kanojiaa, D. Johnsona, P. Koerta, G. Wallacea, D. Bealsa, R. Vieiraa,
N. Bassea, S. Wukitcha, W. Burkea, W. Becka, M. Grimesb, D. Gwinnc, S. Bernabeid, N. Greenoughd,
J.R. Wilsond
aMIT Plasma Science and Fusion Center, Cambridge, Massachusetts, USA
bMIT Lincoln Laboratory, Lexington, Massachusetts, USA
c Bagley Associates, Lowell, Massachusetts USA
dPPPL, Princeton, New Jersey USA
Abstract-A Lower Hybrid Current Drive (LHCD) system has
been developed for current profile control of advanced tokamak
experiments on Alcator C-Mod. LHCD along with Ion Cyclotron
Radio Frequency (ICRF) heating will be used to develop regimes
with high confinement, high f3n and high bootstrap fraction
and extend them to quasi-steady-state conditions. This paper
will describe the commissioning and initial operation of the
LHCD system that includes a 50kV, 208A pulsed-power supply,
twelve 250kW Klystron transmitters, a 96 waveguide launcher
and required control, protection and data acquisition systems.
I. INTRODUCTION
U NTIL high-performance high-bootstrap-fraction regimes
with fully non-inductive current drive have been pro-
duced for pulse durations significantly greater than the resistive
diffusion time, dedicated experiments will be required to verify
the assumptions used in advanced reactor designs. Lower
Hybrid Current Drive (LHCD) experiments on Alcator C-Mod
are planned to tailor the current and pressure profiles to reach
regimes of high bootstrap fraction (> 70%), high (3n (' 3) and
good confinement (HH
help to support the design basis for advanced tokamak reactor
designs, at least up to moderate bootstrap fractions (- 70%),
and provide a basis for advanced operation in ITER. Alcator C-
Mod is well suited for the development of advanced tokamak
(AT) scenarios due to its internal PF coils which enable the
strong shaping required for high 13n, sufficient installed ICRF
source power to reach high 13n at 5 T and cryogenically cooled
magnets which allow sustained 5 T pulse durations at up to 5 s
(several resistive diffusion times). The existing 8 MW (source)
ICRF system and the installed 3 MW (source) LHCD system
operating at 4.6 GHz into a single launcher are the tools for the
required experiments [2]. With waveguide losses and power
density limitations, the maximum delivered LHCD power is
expected to be 2 MW. A second phase is planned which will
add another launcher and 1 MW of source power. This paper
will briefly describe the LHCD system, required calibrations,
phase setting and initial commissioning and coupling studies
on Alcator C-Mod. Results and plans will also be discussed.
1-2) [1]. These experiments could
Work is funded by U.S. DOE Cooperative Grant No. DE-FC02-99ER54512.
II. LHCD SYSTEM
A. Transmitters and Power Supply
Three carts with four klystrons each operating at 4.6 GHz
and rated 250 kW CW provide 3 MW source power for
the LHCD system. A single 50 kV, 208 A power supply
is used to power all klystrons. Carts are semi-independently
controlled and have a fast Transmitter Protection System
(TPS) and programmable logic controller. Critical protection,
control and status information is shared between transmitter
carts and the high-voltage power supply for coordination of
overall control tasks. Klystron window optical arc detection
and reverse power fault detection circuits in the TPS provide
fast transmitter shutdown while klystron output circulators are
rated to handle full reflection at full power for a 5 s pulse.
Rectangular waveguide (WR187) operating in TE10 mode is
used to connect transmitters to the launcher. Waveguides are
pressurized with 10 psig nitrogen at the transmitter and the
launcher is separated from the transmitter by DC blocks and
pressure windows. The Coupler Protection System (CPS) is
designed to monitor 60 forward and 156 reverse power sample
points and recognize voltage standing wave ratio faults [3].
B. Launcher
Power is coupled to the plasma by 96 waveguides arranged
in 4 rows of 24 waveguides, with each klystron output being
split 8 ways into two waveguide columns with 4 waveguides
each [4]. Vacuum windows are positioned in front of the
cyclotron resonance at 4.6 GHz. The couplers, forward wave-
guide assembly (FWA) and rear waveguide assembly (RWA)
are major launcher subassemblies. (Fig.
RWA two-way splitter, magic Ts and directional couplers are
not shown. Four identical aluminum gaskets are used for the
RF seal between the RWA and FWA and between the FWA
and couplers. Tight control of gasket dimensions is required
and careful alignment of the mating waveguides is required
to provide adequate gasket compression and to avoid gasket
protrusion into the waveguides at the joint.
1) Rear Waveguide Assembly: The RWA is fabricated of 25
stacked plates with four waveguide slots milled into one side of
24 of the plates. Aluminum construction is used for the RWA
as it is located well away from the plasma and disruption loads
1). Note that the
1-4244-0150-X/06/$20.00 (C) IEEE

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Fig. 1.Cross section of LHCD system mounted in Alcator C-Mod port.
are reduced. A two-way splitter with magic Ts, high power
mechanical phase shifter, four directional couplers and four
E-plane transformers are used to split each klystron output
into four feeds for the RWA stacked plates. The final 3 dB
splitter is formed by slots in the wall between the top two and
bottom two stacked plate waveguides. Measured power split
at this splitter is -3.2 ±0.15 dB and loss in the stacked plate
waveguides is the same as loss per waveguide in the FWA.
2) Forward Waveguide Assembly: The FWA is similar in
construction to the RWA stacked plates except it is made
of stainless steel to reduce disruption loads. These plates
were copper-plated and polished before assembly. Transmis-
sion losses were measured to be about 0.33 dB/m except
for two with 0.5 dB/m. These waveguides also include H-
plane transformers that transform the 6 cm width of the
coupler waveguides to the 4.75 cm width of standard WR 187
waveguides. Flexibility in the location of these transformers
is taken advantage of to compensate for additional phase shift
caused by the poloidal curvature of the couplers.
3) Couplers: Each of four couplers consists of 24 wave-
guides measuring 5.5 x 60 mm in cross-section. They were
fabricated from titanium by wire electrical discharge machin-
ing and 24 ceramic windows were brazed into each of the
waveguides near the end of the coupler that joins to the FWA.
Viton seals located between the couplers and FWA form the
vacuum seal.
C. Control
Lower hybrid driven current location depends on the
launchednll spectrum, plasma temperature and density pro-
files. Dynamic control ofnll during the plasma pulse is thus
a key consideration, since such capability could eventually
be useful in feedback control of the total current profile
and in optimizing steady-state performance. Klystron output
amplitude and phase control can be realized at the low power
klystron input drive level. The LHCD Active Control System
(ACS) controls klystron output amplitude and phase, with key
components being in-phase and quadrature (I/Q) vector mod-
ulators (VM) and I/Q detectors [5]. A single master oscillator
split 12 ways provides drive for each klystron through the
computer-controlled VM, allowing nll to be varied from 1.5
to 3 [6]. The oscillator is also used as phase reference, or
local oscillator (LO) for I/Q detectors used to monitor the 50
dB intermediate directional coupler (IDC) forward outputs. In
open-loop mode the VM I/Q inputs are determined by operator
entry of demanded amplitude and phase setpoints to the control
computer. I/Q detectors monitor the in-phase and quadrature
phase amplitude and phase components at the IDC for each
of the 12 klystrons. The closed-loop control programs are
designed to compare operator amplitude and phase demand
requests in terms of I/Q to the I/Q detector outputs and to
calculate error signals used in determining VM setpoint inputs.
Closed-loop control should allow variations due to drift in
klystron output phase or amplitude, or waveguide heating, to
be reduced. Time response for modifying the nll spectrum
during a plasma pulse is designed to be less than 1 ms. Relative
phase of the two columns fed by the same klystron can be
varied with a high power mechanical phase shifter (MPS)
between plasma shots, but this can only be done between
plasma shots and requires a cell access. (Fig. 2).
III. CALIBRATIONS
Many calibrations are necessary to allow control, monitor-
ing and protection of the LHCD system. Twelve transmitter
forward and reverse power signals, 12 ACS klystron drive
signals, 48 RWA directional coupler forward and reverse
power signals and 96 rear and front RWA probe reverse
power signals must be calibrated. Also, the 12 IDC signals
located at the control phase plane are split (forwards) and
must be calibrated for control, protection and monitoring in
both the ACS and Coupler Protection System (CPS). Due to
time constraints, complete calibration of some signals was
not possible before commissioning. Combined measurements
and calculated estimates were used to provide signal scaling
during commissioning for all of the above except the drive and
monitor legs, which were carefully calibrated for amplitude
and phase using the network analyzer. To produce the spectra
at high power it is necessary to have an accurate mapping of
the amplitude and phase produced at each of the IDCs vs. the
demand settings requested by the operator.
A. Drive Leg Calibrations
Drive leg calibrations are carried out by replacing the master
oscillator feeding all VMs with network analyzer port 1. Port 2
is connected to the leg IDC forward outputs used to define the
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phase plane between RWA and FWA. The drive legs are non-
linear, and ACS programs generate setpoint scans of possible
I/Q values and apply them to the VM as amplitude and phase
measurements are made. Mapping the demand values vs. the
measured values produces a look up table used in calculations
that determine I/Q setpoint values. These values are required
to produce operator demanded amplitudes and phases at the
IDC for each leg over the operating range.
B. Monitor Leg Calibrations
Monitor legs include the IDC forward outputs and I/Q
detectors and provide the only system phase monitoring points.
A method similar to that used in calibrating the drive legs is
used, except that a single calibrated VM and low power test
amplifier drive leg is substituted for all drive leg klystrons. The
calibrated test drive leg is connected to each monitor leg at
the IDC forward output connection point and ACS programs
generate setpoint scans of possible test leg amplitudes and
phases as I/Q detector outputs are measured. Mapping of the
calibrated test drive leg output amplitude and phase values
vs. measured I/Q values produces a look up table used to
report and scale actual phase and amplitude values at the IDC
forward output.
C. Launcher calibrations
Ideally, the launcher would allow adjustment of amplitude
and phase of the 12 splitter inputs to produce a constant
amplitude and phase plane at the interface between the RWA
and FWA. This can only be approximated due to path length
variations in the RWA splitter network and any errors in the
RWA stacked plate 3 dB power splitter. To calibrate these as
closely as possible, shims were inserted in the splitter network
to reduce path length effects. Network analyzer measurements
were made from IDC input to the interface between RWA
and FWA and variations were typically found to be within
± 5 degrees and ± 0.5dB. Residual amplitude and phase
variation measurement results were used to calculate Fourier-
transformed launch spectra as a function of programmed
uniform phase shift progression. Measured residual errors give
spectra which are very similar to the spectra produced if a
perfectly constant amplitude and phase plane had been formed.
The above calibrations and measurements form the basis of
phase setting tables generated to simplify experimental setup.
IV. PHASE SETTING
To control thenll spectrum each waveguide's characteristic
phase shift must be considered in determining launcher phas-
ing. Klystron phasing required for a desired phase at the phase
plane is determined by subtracting the measured FWA front
end phase from the measured IDC phase and then adding to the
desired phase. Row A was chosen as the reference for phase
control so, for example, the calculated klystron
uses row A measurements to set the phase for columns
and 2, rows A-D. Phase differences between adjacent columns
driven by the same klystron are minimized by using row A
calibrations when setting the MPS. Ideal and approximate
phasing methods were tested during commissioning.
1 phasing
1
A. Ideal Phasing
With ideal phasing the MPS are set to match the phase
change from column to adjacent column, requiring adjustment
of both klystron phasing and MPS for desired phasing. The
wave is launched in either the current drive (CD) or counter
current drive (CCD) direction by changing the phase rotation.
B. Approximate phasing
Approximate phasing is used to avoid the 45 minute delay
required for experimental cell access to change the MPS. With
this method, ideal phasing is approximated by leaving the MPS
in a set position and adjusting the klystron phase demand. The
MPS are commonly set to 90 degrees with current drive phase
progression. Approximate phasing typically gives less power
in the primary peak with loss increasing as the deviation from
90 degrees increases. With the low losses expected for most
cases, the time saved using the approximate phasing method
makes its use acceptable [7].
V. COMMISSIONING
A. Installation and Initial Tests
The coupler was installed on Alcator C-Mod during Febru-
ary and early March of 2005. (Fig. 3). Initial testing included
successful checks of synchronization of the ACS, transmitter
and LHCD data system operation with Alcator C-Mod shot
cycles. Due to time constraints, drive leg calibrations were
only done at 35kV klystron beam voltage. Although this would
eventually limit going to higher power testing, other findings
proved to limit this as well. The CPS worked as designed, but
could not be used to protect the coupler since high reflected
power was observed on pulses without waveguide arcs and
low reflected power was observed on pulses with waveguide
arcs. Thus, for the remainder of the commissioning period the
CPS was bypassed and procedural limits were put on the test
power and pulse lengths to limit possible coupler damage due
to arcing.
B. First Measurements
First measurements of the reflection coefficient were made
as a function of phase progression and density at the launcher
mouth. Forward and reverse powers at 48 directional couplers
located just before the E-plane transformers feeding the RWA
were measured and their ratios were used to form reflection
coefficients. (Note that direct calibration of these 96 signals
during the initial commissioning was not accomplished and er-
ror bars could be as high as 20%). These measurements, made
at very low applied power in the range of 200 kW total, were
averaged to determine a single global reflection coefficient.
Dependence of the global reflection coefficient as a function
of phase and density at the launcher mouth as a parameter is
shown in Fig. 4. During measurements the MPS was fixed at ±
90 degrees, where the + sign corresponds to the CD direction,
the - sign to the CCD direction. A uniform phase progression
occurs only at 90 degrees. Densities measured by six Langmuir
probes embedded in the launcher face at launcher positions
1,3, and 5 mm (distance from launcher to limiter) vary over

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the launcher face and are in the ranges 2.5 -3.5 x 1018,
0.8 -1.3 x 1018 and 2-3 x 1017m-3 respectively. Good
coupling efficiency was obtained at relatively high density at
the couplers (-
phasing (90 degrees). Unfortunately, initial LHCD experiments
had to be terminated due to interaction of the titanium couplers
with hydrogen that led to loss of material from the couplers.
3 x 1018m-3) and at optimal current drive
VI. IMPROVEMENTS AND PLANS
During commissioning the ACS operator programs were
changed to allow more straightforward klystron amplitude
and phase setting. The new programs allow fast setup of
conditioning and experiment amplitude demand waveforms for
each klystron and include operator entry of shot length, start
and end power, pulse period and pulse duty cycle parameters.
The programs also allow the operator to easily set up the
phase demand settings for a range of experiments based on
the calibrations and calculated ideal or approximate phasing
requirements. A different approach to detecting arcs and
protecting the launcher is required, and work has started on a
method of detecting third harmonic signals during arcs which
will require only two protection circuits for all waveguides.
Other methods are being considered as well. Having the front
probe forward output signals and rear probe forward output
signals recorded was determined to be useful, so more moni-
toring channels are being added. To gain operating experience,
commissioning was started with shorts instead of loads at the
rear of the RWA. Loads were determined to be necessary for
optimal operation of the RWA stacked plate 3 dB splitters.
These loads have been designed and are being built. Adding
switched dummy loads after the IDC will save much time in
drive and monitor leg calibrations and system troubleshooting,
and these have been added and are to be remotely controlled.
Based on commissioning experience, programs to partially
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automate drive leg and monitorleg calibrations have been writ-
ten and are being tested. Since experiments require frequent
launcher position change, a system is being designed to allow
remote positioning capability. Most importantly, fabrication
of stainless steel couplers to replace the unplated titanium
couplers damaged during commissioning is well underway.
ACKNOWLEDGMENT
The authors wish to thank the MIT Alcator C-Mod and
PPPL technical staff for their hard work on the LHCD project.
D. Terry thanks George Mackay of the MIT technical staff for
his untiring efforts on the LHCD systems.
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[2] R.R. Parker,
unpublished.
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[6]
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[7]
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J. Liptac, "Lower hybrid antenna phase settings", unpublished (for MIT
Piobe anid
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Fig. 3.Launcher installed on Alcator C-Mod.