2. Reflectometry Inject mm waves into plasma reflection at wpe
optical wavelength
determines position
of reflection
A) interferometry
B) Pulse radar
Also for fluctuation
studies!
From: P. de Vries

3. Plasma Radar Reflectometry
Wave Reflection in Ionospheric Plasma *
O-mode wave cutoff:
X-mode wave cutoffs:
*Edward V. Appleton, “The Ionosphere”, Nobel Lecture, 1947

4. Basic Principle of Plasma Reflectometry
Electromagnetic wave incident on plasmas will reflect from plasma cutoff layers, similar to how microwaves reflect from waveguide cutoffs within tapered rectangular waveguides.
Tapered Waveguide Analogy
Plasma Reflectometry

5. Reflectometry
Reflectometry exploits the reflection of electromagnetic waves from plasma cut-offs to either measure electron density profiles (similar to ionospheric sounding) or spatially resolve density fluctuations.

6. Reflectometry

8. New MHD physics by pulsed radar reflectometry
Peaked density in m/n = 2/1 islands also seen by multi-pulse Thomson scattering
ECE
Magnetic Field: 2.25 T
ECE : HFS, Pulse radar : LFS
m=2 islands in NBI-fueled TEXTOR shots often have peaked density profiles
P.C. de Vries et al., Nucl. Fusion 37 (1997) 1641.

9. Reflectometry
Fluctuations

10. Schematic Diagram of FM Reflectometry

12. CW FM Reflectometry

13. Schematic diagram of an X-mode swept FM reflectometry
system for the ‘Day One’ KSTAR plasmas.

14. Day One-Case II KSTAR Plasmas

16. Schematic diagram of the FM system installed
in KSTAR for the calculation of the IF phase
change

17. Complex Demodulation Method

18. Signals of the CDM procedure
in time and frequency domains
raw IF signal (b) shifted signal
by –f0 in the frequency
domain (c) filtered signal
using a low pass filter

21. JET FMCW Reflectometer

22. Amplitude Modulated Reflectometry
The incident EM wave possesses a low frequency sideband (<< 0)
resulting in two near-frequency signals, f0f
The two signals acquire a different phase (u and l, respectively)
The output is the difference = u-l
This is important as can be seen starting with the formula for the radial
position of the cutoff frequency:
We can make the following approximation for the phase delay

26. TJ-II AM Reflectometer

27. What is USPR? r n e
f=f2
f=f1 f f
A’(f)
A(t)
f2 > f1 t t f1 f2
USPR uses an extremely short pulse (or a chirped waveform) which contains Fourier frequency components spanning the full range of the plasma density.
When an ultrashort pulse is launched into the plasma, each frequency component of the pulse will be reflected from the corresponding cut-off layer of the plasma.
By separating different frequency components of the reflected wavepacket and performing time-of-flight measurements for each frequency component, it is possible to obtain the electron density profile with a single ultrashort pulse source or a chirped waveform.

28. What is USPR? (Cont.)
Potential Advantages of USPR
The data analysis does not depend on the time history of the reflectometer signals.
USPR is capable of easily eliminating spurious reflections in the time domain.
USPR data analysis is extremely fast and consequently real time monitoring may be possible.

29. How to Achieve High Frequency Operation with USPR
A Single USPR Source
An impulse generator/up-converting mixer
Mixer I R
t = 2 ps for f = 160 GHz (for SSPX)
Calculation of the required pulse power launched into the plasma. L
SBF
(optional)
Assumption: NF=3.5 dB at the first receiver amplifier.
Local
Oscillator
for 400 MHz BW
Assumption: S/N=20 dB, LI=10 dB, LP=20 dB.
A commercially available impulse generator can be utilized.
No down-converting mixer is necessary.
Limited mixer IF signals.
for a 2 ps pulse duration
Difficulty to commercially obtain such a short duration and high voltage source.
A down-converting mixer is necessary.

30. Upconverting Mixer Approach in USPR
-100
-75
-50
-25 25 50 75
100
0.2
0.4
0.6
0.8 1
1.2
1.4
1.6
Time (ns) 2 4 6 8 10
0.05
0.1
0.15
0.25
0.3
0.35
-200
-150
150
200
0.5
1.5
2.5 3
3.5 20 30 40 60 70 80 5 -6 -4 -2
Frequency Range:
GHz
Bandpass
Filters
Detector
9.5 +
0.2 GHz
Post-Bandpass Filter
Signal
Post-Detector Signal
9.5 GHz
27 GHz
Gunn Osc.
Amplifier
Horn
Return Signal
Frequency Range: 33 – 41 GHz
Balanced
Mixer
Impulse Generator Signal
Signal from Waveguide
Launched Signal
Plasma
27 GHz Highpass Filter
Dispersive Waveguide
Time-of-flight
Measurement
Electronics

31. Ultrashort Pulse Reflectometry for Density Profile Measurements on SSPX
Ultrashort Pulse Reflectometry involves the propagation of a short duration (2-3 ns) chirped waveform which contains a broad range of frequency components, with each component reflecting from a different spatial location (density layer) in the plasma.
On the Sustained Spheromak Experiment (SSPX) device, wideband mixers up- and down-convert 6-18 GHz chirp signals to millimeter-wave frequencies ( GHz).
Double-pass time delay data simultaneously collected at many frequencies provide detail information of density fluctuation, and may be inverted to generate plasma density profiles.

32. USPR Millimeter-wave Subsystem
36" Overmoded
(WR-28) W/G
Single Sideband Mixer
Assemblies
GHz
GHz
GHz
GHz
57 GHz
44 GHz
27 GHz
140 GHz
121 GHz
GHz
GHz
3 dB
To USPR
Tx/Rx
Subsystem
89 GHz
Vacuum
Windows
& DC Breaks
6 dB
12"
5"
6"
7"
cables
SP6T Switch
Schematic diagram of the millimeter subsystem
Photograph of millimeter enclosure box
A microwave switch matrix directs a low frequency (6-18 GHz) waveform to one of six millimeter-wave mixers for frequency up- and down-conversion.
A single waveguide/horn assembly is utilized for both transmitting and receiving USPR signals.

33. USPR Horn and Waveguide Assembly
SSPX vessel
Plasma
Horn support
8” O.D. flange assembly
Millimeter-wave
enclosure box
~5 cm
~8 cm

34. LHD Pulsed Radar Reflectometer
for density profile and fluctuation measurements
4ch(33,39,60,65GHz) heterodyne system
＃34676 (35s discharge)
Green : Launched Pulses
Yellow : 39GHz
Blue : 33GHz
Red : 60GHz
Reflected Pulse
Pulse width : 2 ns (typical)
Repetition rate : 200 kHz (typical)
TOF resolution : 50 ps (=7.5mm)

35. Ultra-short Pulsed Radar Reflectometer
6ch (29, 31, 33, 35, 37, 39 GHz) filter bank system

36. Reflectometry Inject mm waves into plasma reflection at wpe
optical wavelength
determines position
of reflection
A) Interferometry
B) Pulse radar
Also for fluctuation
studies!
From: P. de Vries

37. Measurement of Turbulent Fluctuations in Fusion Plasmas: Correlation Reflectometry
Cutoff Layers
Hardware Elements
Data Analysis
1. Specularity ~
Ref E1 w1 m1 ~
Source
2. Correlation m2 ~
Source w2 E2 ~
Ref
• Tune relative frequency to produce correlation vs cutoff layer separation.
• Compare with simulation using 1-D and 2-D full wave analysis.
- density fluctuation level
- radial density correlation length

38. Correlation Reflectometry

41. Doppler Reflectometry

42. ASDEX Doppler Radars

43. Microwave Reflectometry Basics
In the presence of density fluctuations, the reflected electromagnetic wave spectrum is broadened with a strong weighting by those fluctuations in the vicinity of the cutoff layer.
For small amplitude fluctuations and a 1-D plane stratified plasma permittivity, the fluctuating component of the measured phase is given by the 1-D geometric optics approximation
The reflected phase fluctuation power spectrum is related to that of the density fluctuations
where is the density scale length,
and it was assumed that where

44. New MHD physics by pulsed radar reflectometry
Peaked density in m/n = 2/1 islands also seen by multi-pulse Thomson scattering
ECE
Magnetic Field: 2.25 T
ECE : HFS, Pulse radar : LFS
m=2 islands in NBI-fueled TEXTOR shots often have peaked density profiles
P.C. de Vries et al., Nucl. Fusion 37 (1997) 1641.

45. Reflectometry
Fluctuations

47. What is Microwave Imaging Reflectometry?
Microwave reflectometry is a radar technique similar to ionospheric sounding, employed here for density fluctuation detection
1-D fluctuations: straightforward interpretation
when
2-D fluctuations: Interference when observing beyond the diffraction distance
Imaging can restore phase front!

48. Microwave Imaging Reflectometry (MIR)
An EM wave propagating through a plasma can be reflected from plasma cutoff layers. The reflected wave contains the phase information about the cutoff layer
Microwave reflectometry: A radar technique used to infer the electron density characteristics by probing the reflected waves from density-dependent cutoff layers
Plasma
Isodensity Surface
illumination
Detection
In MIR, imaging optics are employed to recover phase information from waves reflected from localized region of 2D density fluctuations

49. Effect of Fluctuations on 1-D Reflectometry
Reflectometer signals (here TFTR) corrupted by interference from reflected wave components
Power spectrum and amplitude distribution verify randomized interference pattern
TFTR microwave signal phase plots
Spectral information lost for strong turbulence case
Weak turbulence
(clean signal)
Strong turbulence burst
(distorted signal)
E. Mazzucato, et al., Phys. Rev. Lett. 77, 15 (1996)

50. (a) Microwave intensity, |E|2, for a conventional reflectometer
system and (b) for an imaging system

51. Need for Microwave Imaging Reflectometry (MIR)

52. Microwave Imaging Reflectometry (MIR)
Cutoff layer imaged onto detector array (3 example points shown), eliminating the interference effects of multiple reflections
Detection system shares the same plasma-facing optics
Probing beam illuminates extended region of cutoff layer
Curvature of the illuminating beam matched to that of the cutoff surface (toroidal and poloidal) for optical robustness

53. Principle of microwave imaging
Passive diagnostics (radiometry) collect radiation from a quasi-isotropic source
Active diagnostics (reflectometry) rely on coupling incident and reflected waves
‘curvature matching’ at the reflection layer

54. Integration of ECEI and MIR optics
Integrating diagnostics means accommodating different sets of constraints
ECEI: minimize distortion of the image plane
MIR: match the curvature of density cutoff layers (4 frequencies on DIII-D, 8 on EAST)
MIR focal plane is curved
curved detector arrays (DIII-D and EAST)
focal curvature adjusting (FCA) lenses (J-TEXT)
electronic/digital beamforming w/ FPGA

55. Integration of ECEI and MIR optics
Integrating diagnostics means accommodating different sets of constraints
ECEI: minimize distortion of the image plane
MIR: match the curvature of density cutoff layers (4 frequencies on DIII-D, 8 on EAST)
MIR focal plane is curved
curved detector arrays (DIII-D and EAST)
focal curvature adjusting (FCA) lenses (J-TEXT)
electronic/digital beamforming w/ FPGA
shared optics

56. 3rd Generation MIR System Employing Beam Shaping PAAs
Window
Beam Splitter
...
Beam Shaping PAAs
Detector Array
RF Source
Computer
Plasma
The curvature of the cutoff layer varies
Mirror
Imaging Lens
Changes of Focal Points

57. Doppler Reflectometry - principle

58. Doppler Reflectometry – rotation and Er-profiles

59. Doppler Reflectometry – Er fluctuations

60. Millimeter-Wave Reflectometry
Plasmas are dispersive media whose refractive index is a function of plasma density; higher frequency radiation reflects from higher density plasma layers. Reflectometry, as its name suggests, exploits the reflection of electromagnetic waves from plasma cut-offs to either measure electron density profiles (similar to ionospheric sounding) or spatially resolve density fluctuations. The UCD Plasma Diagnostics Group is researching various aspects of microwave and millimeter-wave reflectometry for plasma density measurements (both density profiles and fluctuations) in current tokmaks as well as next generation devices such as ITER. Theoretical, computational, basic experimental and technological development efforts comprise a concerted research program designed to elucidate the physics of fluctuations reflectometry and produce innovative instruments. As shown in the characteristic frequency profiles for TPX and ITER, the frequencies of interest for reflectometry range up to nearly 250 GHz.

61. Density Profile Diagnostic Systems
For measurement of plasma profiles, the major reflectometry techniques are swept frequency modulation (FM), amplitude modulation (AM), moderate-pulse and ultrashort-pulse reflectometry. Discussion of the advantages and disadvantages of each of these methods follows.
FM Reflctometry
FM reflectometry systems utilize one or more swept frequency sources to cover the frequency range of interest. Mixing a portion of the transmitted radiation with that reflected from the plasma produces an IF signal whose frequency is proportional to the double-pass phase delay. Generally, a system of this type suffers from poor time resolution due to long sweep times, although fast-sweep solid state oscillator/ multiplier combinations are available in the lower frequency bands. The high cost, slow sweep times and limited frequency range associated with high frequency millimeter-wave sweep oscillators make this technique not well suited for use on next generation tokamaks such as TPX and ITER.

62. Pulsed Reflectometry
Time-of-flight radar system function by reflecting short pulse of electromagnetic radiation. By collecting double-pass time delay data at many distinct frequencies, it is then possible to invert the time delay data to generate plasma density profiles. Moderate pulse reflectometry is a time-of-flight radar technique which propagates a short duration pulse of microwave or millimeter-wave radiation with a relatively well defined frequency (i.e. &Delta, f << f). Complete density profile data are obtained by either sweeping the frequency of the pulsed millimeter-wave source, or by using a series of distinct frequency sources. Ultrashort pulse reflectometry is similar to moderate pulse reflectometry, except that an extremely short pulse (or chirped waveform) containing frequency components that span the desired plasma density profile. Both pulsed reflectometry techniques are under investigation by the UC Davis Plasma Diagnostics Group. In addition, technological advances in state-of-the-art high speed millimeter-wave switches (for moderate pulse reflectometry) and nonlinear transmission lines (for ultrashort pulse reflectometry) are also under development by the Millimeter Wave Technology Group at UC Davis.
AM Reflectometry
AM reflectometry shares many of the advantages listed for moderate-pulse reflectometry, except that the complicated time-delay measurements are replaces with much simpler phase-delay measurements. Unlike moderate-pulse reflectometry, however, it is nearly impossible to distinguish between false and real reflections in an AM system. Extreme care must therefore be taken to eliminate false reflections from entering the detector/mixer before the system can produce significant, believable profiles. The high speed millimeter-wave switching technology required to implement AM reflectometry on next generation tokamaks such as KSTAR and ITER, however, is identical to that required for moderate pulse reflectometry and is under development by the Millimeter Wave Technology Group at UC Davis.

63. Density Fluctuation Diagnostic Systems
In addition to determining electron density profiles, reflectometry is known to be extraordinary sensitive to density fluctuations. Reflectometry offers advantages over other density fluctuation diagnostic under consideration for use on TPX and ITER class machines such as beam emission spectroscopy (BES) and conventional FIR laser scattering. To conduct electron density fluctuation measurements, the frequency of the probing reflectometer beam is kept constant (rather than being swept of modulated), so that small density fluctuations near the reflecting layer can both phase and amplitude modulate the probing beam. The actual mechanism by which fluctuations modulate the reflected beam, however, and issues of localization and wavenumber sensitivity, remain poorly understood. The UCD Plasma Diagnostics Group is therefore conducting a multifront approach to investigate the physics of reflectometry, utilizing both theoretical and computational modeling as well as a series of basic plasma experiments.

64. REFLECTOMETRY
Reflectometry, as its name suggests, exploits the reflection of electromagnets waves from plasma cut-offs to either measure electron density profiles or spatially resolve density fluctuations. Additional possible applications include measurement of the plasma current profile, magnetic field fluctuations, and wave damping. In Figs. 3.1 and 3.2, we have plotted the characteristic plasma frequencies for TPX and ITER, respectively, in a number of density regimes. The reflectometer signal in a tokamak usually propagates from the outside (low field side) of the machine (from the right as shown in Figs. 3.1 and 3.2). These figures show that utilization of the right hand cut-off for X-mode propagation requires sources in the GHz range for TPX, and in the GHz range for ITER (assuming core densities of 41020m-3 for both machines). In order for the reflected signal to be detectable, the path from the reflecting layers (where strong absorption occurs). This criterion may limit the range of electron densities which can be probed via reflectometry. Fortunately, for X-mode propagation on TPX and ITER, this only occurs for plasmas with central densities > 21020m-3, where the second harmonic layer obscures portions of the plasma.
For measurement of plasma profiles, the major reflectometry techniques are wideband swept frequency modulation (FM), narrowband swept FM, amplitude modulation (AM), moderate-pulse and ultrashort-pulse reflectometry. Discussion of the advantages and disadvantages of each of these methods follows.

65. The simplest of the above techniques is wideband swept frequency modulation, which utilizes one or more wideband swept sources to cover the frequency range of interest. In this scheme, the reflected wave is mixed with a portion of the outgoing wave to produce a Doppler-shifted signal whose frequency is a function of the group delay (defined as the time taken for the probing beam to propagate into the plasma, reflect, and propagate back out) of the reflected wave. Generally, a system of this type suffers from poor time resolution due to long sweep times, although fast-swept solid state oscillator/multiplier combinations are available in the lower frequency bands. The narrowband swept FM system is similar to the broadband swept FM system, except that the wideband source(s) is replaced with a number of narrowband swept sources. Although the sweep times can be significantly shorter than those of wideband systems, this technique requires specialized hardware for multiplexing the sources and distinguishing the reflected signals.
In moderate-pulse reflectometry, the sources of the narrowband swept FM system are replaced with an equal number of lower cost, fixed frequency, gated sources. In this techniques, a wavepacket with a given frequency and moderate time duration propagates into plasma from a transmitting horn, reflects, and then returns to be collected by a receiving horn. Through double-pass time-of-flight measurements of the reflected wave, the group delay is obtained and hence the distance to the reflecting layer can be computed. Although there is still a need to multiplex the sources, separation of the various reflected signals is greatly simplified by sequentially switching the input sources. This allows a number of reflected channels to be received by a single, wideband millimeter-wave mixer. An additional advantage of moderate-pulse reflectometry over the FM techniques is that false reflections will not influence the group delay measurements, as the echoes fall in non-interesting time windows.

66. Characteristic frequencies for ECE and Reflectometry on TPX.
In Figs. 3.1 and 3.2, we have plotted the characteristic plasma frequencies for TPX and ITER, respectively, in a number of density regimes. The reflectometer signal in a tokamak usually propagates from the outside (low field side) of the machine (from the right as shown in Figs. 3.1 and 3.2). These figures show that utilization of the right hand cut-off for X-mode propagation requires sources in the GHz range for TPX, and in the GHz range for ITER (assuming core densities of 41020m-3 for both machines). In order for the reflected signal to be detectable, the path from the reflecting layers (where strong absorption occurs).

67. Characteristic frequencies for ECE and Reflectometry on ITER.
In Figs. 3.1 and 3.2, we have plotted the characteristic plasma frequencies for TPX and ITER, respectively, in a number of density regimes. The reflectometer signal in a tokamak usually propagates from the outside (low field side) of the machine (from the right as shown in Figs. 3.1 and 3.2). These figures show that utilization of the right hand cut-off for X-mode propagation requires sources in the GHz range for TPX, and in the GHz range for ITER (assuming core densities of 41020m-3 for both machines). In order for the reflected signal to be detectable, the path from the reflecting layers (where strong absorption occurs).

68. Figure 3.3 shows the arrangement employed to test this approach in the laboratory where a full mock-up of a DIII-D vacuum port was utilized.
Schematic illustration of the moderate-pulse reflectometry laboratory test setup.

69. As can be seen in Figure 3.4, extremely good spatial resolution is possible (< 3 mm).
Laboratory test results of a moderate-pulse reflectometry system.

70. Amplitude modulation (AM) reflectometry shares many of the advantages listed for moderate-pulse reflectometry, except that the complicated time-delay measurements are replaced with much simpler phase-delay measurements. Unlike moderate-pulse reflectometry, however, it is nearly impossible to distinguish between false and real reflections in an AM system. Extreme care must therefore be taken to eliminate false reflections from entering the detector/mixer before the system can produce significant, believable profiles.
Ultrashort-pulse reflectometry for density profile measurement is, in principle, quite similar to moderate-pulse reflectometry. The essential difference is that rather than launching a number of different frequency wavepackets of millimeter-waves with a moderate time duration, a single ultrashort pulse is propagated into plasma. The duration of this ultrashort pulse is designed such that the wavepacket contains Fourier components that span the entire accessible plasma cut-off profile. Each Fourier component of the incident wave packet reflects from a different spatial location (density) in the plasma. This causes the duration of the reflected pulse to be considerably longer than that of the input pulse. By separating different frequency components of the reflected wavepacket and obtaining time-of-flight measurements for each subpacket, the density profile can be determined using just one source and a single set measurements. Figure 3.5 schematically illustrates a heterodyne implementation of ultrashort-pulse reflectometry on ITER in which separation of reflected subpackets is accomplished by mixing the received signals with a small number of discrete sources, then amplifying and bandpass filtering the downcoverted signals. The Investigator’s group has tested this concept in the laboratory using a tapered waveguide to simulate a spatially varying cut-off frequency. In addition, they have performed preliminary tests on CCT using a two channel system which further provided confidence in the approach (currently, an eight channel system is being fabricated).

71. Possible implementation of ultrashort-pulse reflectometry on ITER.
Figure 3.5 schematically illustrates a heterodyne implementation of ultrashort-pulse reflectometry on ITER in which separation of reflected subpackets is accomplished by mixing the received signals with a small number of discrete sources, then amplifying and bandpass filtering the downcoverted signals.
Possible implementation of ultrashort-pulse reflectometry on ITER.

72. Ultrashort radar reflectometry on (a) DIII-D at 80 GHz, and (b) ITER at 180 GHz due to input pulses of width 3.0 psec and 1.5 psec respectively. A detector bandwidth of 2 GHz is assumed.

73. In addition to determining electron density profiles, reflectometry is known to be extraordinarily sensitive to density fluctuations. Another density fluctuation diagnostic under consideration for use on TPX and ITER class machines is beam emission spectroscopy (BES). Whereas reflectometry is sensitive to a wide range of fluctuations wavenumbers, BES is generally sensitive only to fluctuations with small wavenumbers (k < 2 cm-1). In addition, BES requires the presence of a high intensity, low energy neutral beam to operate, hence profiles may not be obtained when the neutral beam system is off-line. It should be noted, however, that performing both measurements simultaneously is highly desirable, as has been demonstrated on TFTR [3]. Reflectometry also appears to have advantages over conventional FIR laser scattering, especially on large machines where port access is limited. In addition, the sensitivity of reflectometry to low k fluctuations appears to be superb.
To conduct electron density fluctuation measurements, the frequency of the probing reflectometry beam is kept constant (rather than being swept of modulated as in the case of FM and AM reflectometry), so that small density fluctuations near the reflecting layer can both phase and amplitude modulate the probing beam. Traditional reflectometer systems operate preferentially in one of two regimes: density profile measurements of density fluctuation measurements. In contrast, the pulsed reflectometry techniques (both moderate-pulse and ultrashort-pulse) offering the exciting possibility of acquiring both profile and fluctuation data simultaneously by increasing the pulse repetition rate above that of the fluctuations one wishes to observe.

74. Evidence of poloidal asymmetries in H-mode transition from inside and outside launch reflectometer measurements on the CCT tokamak.