J. Plasma
J.
Plasma Fusion
Fusion Res.
Res. SERIES,SERIES,Vol.
Vol.6 6(2004)(2004)417–420
000–000

Application of Ultrashort-Pulse Reflectometerto a HYPER-I Device Plasma
KANEBA Takahiro, TOKUZAWA Tokihiko1, OKAMOTO Atsushi2, YOSHIMURA Shinji1,
TANAKA Masayoshi Y.1 and KAWAHATA Kazuo1
Graduate University for Advanced Studies, Hayama, 240-0193, Japan
1
National Institute for Fusion Science, Toki, 509-5292, Japan
2
Graduate School of Science, Nagoya University, Nagoya, 464-8603, Japan(Received: 9 December 2003 / Accepted: 6 August 2004)

Abstract
The ultrashort-pulse reflectometry is considered as a tool for determining the electron density profile in a large
magnetic fusion device. We have been developing an ultrashort-pulse reflectometer. This device has higher spatial andtemporal resolution than other conventional diagnostics to measure electron density profiles and fluctuations. We havesucceeded to obtain the desired range of the millimeter wave from an ultrashort-pulse for a plasma measurement. We
have installed the reflectometer on HYPER-I device and operated using a real plasma. In this paper we show thereflectometer configuration and the preliminary experimental results..
Keywords:
plasma diagnostics, electron density, microwave, ultrashort-pulse, time-of-flight

1. Introduction

2. Experimental apparatus

A plane electromagnetic wave, which is launchedperpendicularly to the external magnetic field into a plasma,is distinguished between the ordinary mode (O-mode) and theextraordinary mode (X-mode) for the direction of the electricfield component. When these waves arrive at the criticaldensity regions, they can not propagate and then reflect. In
the case of O-mode operation, the critical layer is determined
only by the electron density profile. We have aimed to develop
an Ultrashort-Pulse Reflectometer (USPR) that has higher
spatial and temporal resolution than other conventional
diagnostics. Ultrashort-pulse reflectometry experiments were
initially carried out on CCT tokamak [1], subsequently on
GAMMA 10 tandem mirror [2], also on SSPX [3]. Because
an ultrashort-pulse has broad band frequency components ina Fourier-space, an USPR can take a place of multichannel
pulsed radar reflectometers [4]. When an ultrashort-pulse islaunched into a plasma, each frequency component reflectsfrom the corresponding cut-off layer. If the time-of-flightmeasurement of each frequency component pulse is done, theelectron density profile and the fluctuations simultaneously
can be determined. In this paper, the USPR system isdescribed in Sec. 2. Section 3 gives the calibration experimentand preliminary results on the HYPER-I device [5] arepresented in Sec. 4. We summarize the present results in Sec.
5.

2.1 Transmitter design
Our ultrashort-pulse reflectometer uses an impulse of–2.2 V, 23 ps full-width half-maximum. To extract the desired
frequency range from this impulse, a 50 cm R-band
rectangular waveguide is used. When an ultrashort-pulse islaunched into the waveguide, it is transformed into
electromagnetic waves with a broad frequency spectrum. Thistransformation is due to the dispersion effect in thewaveguide. The group velocity vg in the waveguide is givenby2⎛ c ⎞vg = c 1 − ⎜⎟ ,⎝ 2 af ⎠where c is the velocity of light, a is the long side length of
the rectangular waveguide, and f is the frequency of theelectromagnetic wave in vacuum. The output from the 50 cm
R-band waveguide, which is fed by the driver pulse, is shownin Fig. 1(a). The resultant frequency component is shown inFig. 1(b). Consequently the frequency component of theoutput wave gradually changes from high to low. Then we
obtain the chirped millimeter wave that has the frequency
components between 26 and 40 GHz. The USPR system has
been installed on the HYPER-I device is shown in Fig. 2.
©2004 by The Japan Society of Plasma
Science and Nuclear Fusion Research

Corresponding author’s e-mail: tkcha8@nifs.ac.jp1417

Kaneba T. et al., Application of Ultrashort-Pulse Reflectometer to a HYPER-I Device Plasma

The chirped wave (26 – 40 GHz) is amplified by a power
amplifier and divided from the incident wave to produce thereference wave by a directional coupler. The reference wave
is detected by a Schottky barrier diode detector to obtain thereference pulse. The reference pulse is transmitted to a
constant fraction discriminator (CFD).

2.2 Receiver design
The reflected wave is mixed with continuous wave froma 42 GHz local oscillator. The output of the mixer is amplified
by an intermediate frequency (IF) amplifier (2 – 18 GHz) andthen divided to six channels. Each IF signal is filtered bybandpass filters with the center frequencies of 3, 5, 7, 9, 11and 13 GHz and with the band width is ± 0.5 GHz. The six
signals are detected by Schottky barrier diode detectors toobtain the reflected pulses. The detected pulses are amplified
by pulse amplifiers and leaded to CFDs.

2.3 Data acquisition system
Positions of cut-off layers are estimated from time-offlight (TOF) measurement technique. The reference pulse isused as the start signal and the reflected pulse is used as thestop signal for a time-to-amplitude converter (TAC). As thisTAC can be operated every 2.5 µs, the fluctuation
measurement is available in this time scale. The TAC outputs
an analog voltage proportional to the time difference between
the start signal and the stop signal. The TAC output istransmitted to an analog to digital converter and the digital
data is stored by a personal computer.

Fig. 1(a) 40 – 26 GHz chirped waveform propagating througha 50 cm R-band waveguide. (b) Open circle are time
evolution of the frequency component of the chirped
waveform from 62.5 ns to 63.5 ns, and the solid line isthe calculated value by using the theoretical equation.

Fig. 2

Schematic view of USPR, which is used for plasma
experiments with HYPER-I device.

3. Calibration experiment
In the calibration experiment, we use a metallic plate atdifferent positions to simulate the reflection from a plasma.
We measure the reflected pulse and the TAC output voltage
with changing the position of the plate. The TAC output
voltage is proportional to the delay time. The detected pulses,
which IF is 13 GHz, are shown in Fig. 3(a). Here, the distanceis changed from 50 cm to 170 cm with the increment of 20cm. Figure 3(b) shows the value of the delay time. It is foundthat the linearity of this TOF measurement is good and thevalue is also reliable. When the distance is changed from 50cm to 66 cm, the value of the delay time is obtained, whichis shown in Fig. 3(c). The spatial ambiguity of USPR isestimated from the maximum deviation width of the delaytime. The maximum width of the deviation is 80 ps and it
corresponds to 6 mm in a real space.

4. Experimental result
HYPER-I device consists of a cylindrical vacuum
chamber (30 cm in diameter and 200 cm in axial length) and
10 magnetic coils to produce magnetic field of 1 kG along
the chamber axis. The plasma is generated by electron
cyclotron resonance heating using a microwave of frequency
2.45 GHz. We use the O-mode operation in HYPER-I argon
plasma experiments. At first we use one channel system. In
the condition (condition (A)) that the operation pressure is2418

Kaneba T. et al., Application of Ultrashort-Pulse Reflectometer to a HYPER-I Device Plasma

Fig. 4Fig. 3(a), (b) Time evolutions of the delay time of thereflected pulse in condition (A) and (B), respectively. (c)Time evolution of the estimated central electron density
estimated in condition (B). (d) Time evolution of the ion
saturation current measured by the Langmuir probe.

density at a fixed spatial point. In condition (B), the plasma
density seems to be lower than that in condition (A). Whenthe plasma density is below the critical one, the launchedpulse is not reflected from the plasma and reflected back fromthe opposite wall. In condition (B), the USPR is considered
to be working as a delayometer [6]. Because the groupvelocity of the microwave pulse in a plasma is lower thanthat in the vacuum, the time of flight increases. The delay
time of the plasma is given by(a) The detected pulses amplitude (IF = 13 GHz)
observed at different positions. (b) The change of thedelay time as a function of the distance between theantenna and the plate. (c) Enlargement of the upper
figure in the case that the distance is changed from 50
cm to 66 cm.

τ pe =

5.3 Pa and the heating power is 12 kW, the time evolution of
the delay time of reflected pulse is shown in Fig. 4(a). The
frequency of this signal is 35 GHz. Here, the delay time issubtracted by the delay time of the pulse reflected from theopposite inner wall of the chamber in the vacuum. When thevalue of the measured delay time becomes negative, it meansthat the existence of the cut off density corresponding to theprobing frequency is established in the plasma. In anothercondition (condition (B)) that the operation pressure is 2.5 Pa
and the heating power is 10 kW, the result is shown in Fig.
4(b). The value of the delay time is larger than that from thewall of the vacuum chamber. Time traces of ion saturation
current measured with a langmuir probe are shown in Fig.
4(d). As is shown in Figs. 4(b) and 4(d), both the time
behaviors of the delay time and the ion saturation current aresimilar. This signal represents the time evolution of electron

2cl ⎛∫− l ⎜⎜⎝12

1 − f pe (x ) f2⎞− 1⎟ dx ,⎟⎠where fpe is the plasma frequency, f is the probing frequency
and l is the radius of the plasma. By using an assumption of
the parabolic density profile such as
2ne (r) = ne ( 0) ⎛⎝1 − ( r l ) ⎠⎞ ,we can determine the electron density. Where r is the distancefrom the center of the plasma. The estimated central electrondensity is also shown in Fig. 4(c). The central electron density
is estimated to be about 0.7 × 1019 m-3.
Next, we show multichannel experiment results. The
experiment is performed in the condition that the operationpressure is 5.3 Pa and the heating power is 32 kW. The TOF
3419

Kaneba T. et al., Application of Ultrashort-Pulse Reflectometer to a HYPER-I Device Plasmameasurements for each channel are shown in Fig. 5. All pulses
reflected from the plasma are observed until t = 1.6 second.
Subsequently, the reflected signal, corresponding to the higher
density, can not be observed and then that appears with large
delay. Because the plasma density gradually decreases withtime, the critical layer of this frequency disappears and thenthis channel becomes a delayometer.

5. Summary
We have been developing an USPR that can measure anelectron density profile and fluctuations simultaneously. We
have succeeded to obtain the desired range of millimeter wave
from an ultrashort-pulse for a plasma measurement. We havedeveloped the receiver system with high sensibility. The
spatial ambiguity of 6 mm has been achieved in calibration
experiments. The USPR experiment was performed in theHYPER-I device plasmas. Multi channel system has been
constructed for accurate TOF measurements.

Acknowledgment
This work was partially supported by a Grand-in-Aid forYoung Scientists (B) (No. 15760625, 2003) from the Japan
Society for the Promotion of Science to one of theauthors(TT).

References

Fig. 5[1] C.W. Domier et al., Rev. Sci. Instrum. 66, 399 (1995).
[2] S. Kubota et al., Jpn. J. Appl. Phys. 37, L300 (1998).
[3] Y. Roh, C.W. Domier and N.C. Luhmann, Jr., Rev. Sci.
Instrum. 72, 332 (2001).
[4] T. Tokuzawa et al., Rev. Sci. Instrum. 74, 1506 (2003).
[5] M.Y. Tanaka et al., Rev. Sci. Instrum. 69, 980 (1998).
[6] R. Pavlichenko et al., J. Plasma Fusion Res. SERIES 1,378 (1998).

Time evolutions of the delay time for the four channel
system. Each IF is 7, 9, 11, 13 GHz, which correspond to
the electron density of 1.5, 1.4, 1.2, 1.0 × 1019 m-3,
respectively.

4420